;'-" .,

.,,, .

t'" , . ,." .. ", I. " '

GEOLOGIC INTERPRETATION

FROM WELL LOGS

STANDARD OIL COMPANY OF CALIFORNIA

SAN FRANC I SCO

1967

1"'). ~ .

'--~-- .-----.------... - ' . . . •. "--- .. _ .. ' -' I

GEOLOGIC INTERPHETATION FHOM WELL LOGS

_INTHODUCTION

The purpose of this manual is to compile under one cover the principles of well log correlation, together with examples of lithologic interpretation and expressions of stratigraphic and structural features which can sometimes be recognized on well logs.

For the past 35 years, geologists and engineers have correlated electric and radioactivity logs. During the first 20 years, this correlation consisted of a comparison of log curves checked by paleontologic dating and lithologic infonnation from cores and cuttings. Very little consideration was given to the electrochemical and petrophysical fundamentals and relationships that determine the amplitudes and shapes of the various curves. Quantitative analysis was relegated to the log analyst, usually an engineer, whose pri­mary interests \~ere porosity and hydrocarbon saturation.

Advancing technology during the last 15 years has provided a multiplicity of electric, acoustic, and radioactivity logs which have greatly enhanced both qualitative and quantitative interpretation, but for all except the expert log analyst, correlation and geologic interpretation seem to be more diffi­cult than with the simpler appearing logs of the past. Modern drilling fluids and drilling practices have also contributed to the confusion. Geologists and petroleum engineers must therefore acquire a basic under­standing of petrophysics and quantitative log interpretation. These subjects are now well covered by Company manuals by Mr. J. E. Walstrom, by Chevron Research reports, and by several published texts. Geologic interpretation and correlation of modern logs are not adequately covered; hence the need for a supplemental manual designed primari1y for the exploration geologist.

About half of the log examples and accompanying descriptions for this compilation were taken from the 1959 W.O.I. manual, "Stratigraphic and Structural Interpretations from Hell Logs," the 1959 Chevron West manual, "Qualitative Use of Well Logs," Chevron Research memoranda and reports, and Schlumberger Interpretation Techniques. The balance of the examples were contributed by the editor and the following individuals from the Standard of California family:

C. A. Bengston Chevron Research F. L. Campbell Chevron Research L. R. Litsey Chevron Calco C. D. Jones Chevron Exploration B. B. Cooley Chevron Sotex C. F. Lamb Chevron Sotex C. V. Moore Chevron Sotex R. E. Murphy Chevron Sotex H. L. Turner Chevron Sotex J. R. Lishman Chevron Standard D. H. Organ Chevron Standard E. C. Bowman Chevron West

F J. E. Dryden lYestern Operations, Inc. D. N. Helmuth lYestern Operations, Inc. R. R. Johnston lYestern Operations, Inc. R. L. Manly lYestern Operations, Inc. H. E. Nagle lYestern Operations, Inc.

Many other examples were not included because, in the editors' judgment, the interpretation was unclear or ambiguous, or a better example of the same feature was available. Because of the differences in background and exper­ience in local areas of the various contributors, some may prefer local examples to those selected for the compilation. The manual is in loose-leaf form for that reason and also so that additional material may be added in each section.

As research progresses in computer applications to well log analysis and lithologie interpretation, computer logs and plots should be included in this manual. The experimental logs now available were not included at this time, because they could be misleading. At the present state of the art, the geologist should be working toward a better understanding of the influ­ence of various rock properties on the basic measurements made by logging tools. He will then be better equipped to interpret and analyze the computer logs derived from these measurements.

J. C. lYells Editor May 1967

(

CONTENTS

PART 1. CORRELATION

Types of Logs Most Suitable for Correlation Lithological Correlation Time Line Correlation Adverse Physical and Chemical Effects Logarithmic versus Linear Scales

PART 2. INTERPRETATION OF DEPOSITIONAL ENVIRONMENT FROM LOGS

Interpretation of Marine Sand Environments from Electric Logs Regressive Beach Sand Pattern Transgressive Beach Sand Pattern Sand Bar Pattern Channel Sand Pattern

Lagoonal Depositional Pattern Continental Cycles Type of Clay in Clayey Sands Stream Channel Cut and Fill Salt Solution and Sand Infill Carbonate Patterns

Recognition of Depositional Environments in Carbonates Reef Carbonate

Direction of Sediment Transport

PART 3. LITHOLOGY FROM LOGS

Conglomerates Siliceous Shales

Diatomite Opaline Shale Fractured Shale Reservoirs

Siltstone Bentonites Sandwich Beds Glauconitic Sands Organic Shales

Nodular Shale Carbonaceous Shale

Coal Beds Sandstone Reef Carbonates and Evaporites

Limestone versus Dolomite Dolomite Cavern Vertical Fractures Anhydrite Salt and Anhydrite Salt (NaCI) Identification Potash

Volcanics Tar Sands and Volcanics Shallow, Unconsolidated Tar and Gas Sands Mineralized Zones Basement Rocks

I PART 4. STRUCTURAL INTERPRETATION FROM LOGS

Steep Dips Unconformities Normal Faults Reverse Faulting and Overturning Structural Interpretations from Dipmeter Data

PART 5. HYDRODYNAMICS AND ABNORMAL PRESSURE EFFECTS

Transition from Fresh Water to Salt Water Hydrodynamic Effects Abnormal Pressure Effects on Well Logs

PART 1

CORRELATION

)

---- ------ - ----~-----

, '"

I I

. srlllUEml mt SI,I,I,£l1H fllPII1TI1I

1.

~tOHU.HfOUS fOHNlfAl , I,)~ mitlivolh i

RESISTIVITY ohml. mtlm

h' ........ "

Rm-I.IOI17-

,- ... "

.

+

I'

11

i i_

.. i .1

!

-tt,: • \_ i . :

,~ I i \

r,

r> , :

.' " .. . . -

- ---,- ',-

i

-"" -'--- I

/I

1,.­

., F-' V ~. c c

i- i-

-_ .. ,-,....-.-

'"

S~OHl.lHfOu~ rO!fMllu ~ I,~ millivolt, j

R[~ISIlYln

ohml m'lm

,

ELECTRIC LOG

IN FRESH

WATER MUD

COMPARED

WITH

LATEROLOG

IN SALT MUD

( REDRILL)

Note positive S. P. opposite sands in salt mUd.

KERN

COUNTY,

CALIFORNIA

Figure 1-1

•

COMPARISON OF

ELECTRIC LOG

WITH

INTERVAL VELOCITY LOG

GRAPEVINE AREA CALIFORNIA

VElOCITY IN F t 11 PER HeDNO

~DOO--E:ffiillJj L'LLU:tj

_-----1

1QOO---H

- ,

lSQII--

Figure 1-2

LITHOLOGIC CORRELATION

The first step in sny correlstion problem is to assemble all the electric logs, core and cuttings descriptions, and paleontological data that are available. In some areas, radioactivity logs and velocity logs may also be quite helpful. Then, before attempting any correlation, the logs should be studied critically and compared with the lithology, as shown in the descrip­tions of cores and cuttings. If the geologist is not familiar with the section, core samples where available should also be examined and compared with the written descriptions and electric logs because descriptions of the same type of material by different geologists often vary considerably.

After the geologist has become familiar with the material available, the regional perspective should be established. This is usually done by choos­ing the most complete core records, and in some cases measured outcrop sections, as starting points in several parts of the area and comparing paleontological data to establish time relations. The geologist is then ready to begin correlating logs.

Regional correlation is usually best accomplished by first comparing large stratigraphic units and correlating by general over-all character because individual marker beds may not be continuous and facies changes and struc­tural complications may cause confusion if smaller units are used. Confusion may also result from using electric logs which do not properly reflect the lithology or which were recorded under different conditions. (See Figures 1-3 to 1-11.)

Large stratigraphic units which can be correlated over broad areas may be predominantly sand, or shale, or carbonates, or they may be quite variable with one or more persistent beds that are easily recognizable. The S.P. curve is normally used for correlating sand and silicous shale intervals although sometimes the resistivity curves must be used in conjunction. Large shale intervals usually show more detail on the conductivity curve or amplified short normal of the induction-electric or standard electric logs, and the Laterolog-8 on the logarithmic scale of the DIL system also shows excellent detail in low resistivity intervals. In SOme areas, the ohm-meter value of shale resistivity in a particular interval or a sequence of intervals may be correlated. When sttempting to correlate shale resis­tivities quantitatively it may be necessary to correct the short normal curve for the size of the hole and the mud resistivity. In carbonate sections, the gsmma ray curve is used for detailed correlation because the high resistivity effect on the S.P. produces a rounded and sluggish-looking curve. (See Figure 2-110).

When the broad regional stratigraphic correlations have been established, the next problem becomes one of breaking down the larger stratigraphic units into environment of deposition as illustrated in Part 2 of this manual. The S.P. curve can be very helpful for this purpose in sand-shale sections because its pattern normally reflects ratios of shale or argillaceous mater­ial to clean sands. This does not mean a decrease in self-potential is necessarily a measure of decreased permeability. It means a decrease in self-potential indicates an increase in argillaceous material provided there

2

-. , is no chsnge in formation water salinity. Dipmeter plots used with electric logs may indicate the direction of sand transport snd may help to identify individual sands. This strstigraphic applicstion of the dipmeter has proven to be very helpful in California and South Louisiana.

Some specific rock types or groups show more or less definite character­istics or log patterns as illuatrated in Part 3 of this manual. In most casea, however, these patterns are not sufficiently diagnostic to be used without corroborating evidence from cuttings, cores, or sidewall samples.

3 .•

Type of Log

SP

Short Normal

Lateral

Induct lon

Laterologs (Guard electrode)

Microlog

Gamma Ray

Formations to Which Aop1icable

Any except those of very high resis­tivity.

Lov and moderate resistivities.

Any

Lov and mod.era te reslstivlties~

Any

Any

Any

Type of Mud

Any conductive* mud so long as Rmf is not equal to Rv.

Any conductive* mud except saturated or nearly saturated salt muds.

Same

Same

CORRELATION

Advantages and Special Merits

Sharply defines contacts be­tween shales and porous nonsha1es.

wide selection of scales provides adequate corre­lation character over wide range of resistivities.

Always gives marked "kicks" in right direction for ~~ln resistive streaks with characteristic shadow zone and false peak below.

Conductivity curVe is par­ticularly useful through long intervals of low resi.stivity.

Any conductive* mud Sharp bed delineation. but best with muds of lower resistivity.

Any conductive* mud except saturated or nearly saturated salt muds.

Any

Extremely detailed. Excellent supplement to amplified short normal through long intervals of negligible resistivity contrast and substitute for long lateral for delineating thin resistive markers.

Reflects only natural radio­activity which for most formations e~'ibits less lateral variation than do electrical char'

)-istiCS.

hole Not affected by cond:tions CT :or:natton

Limitations

Affected by lateral changes in formation water salinity. loses character as Rmf/Rw approaches unity, as e/d. becomes quite small and as Rt/Rm and Rs/Rm become quite large.

Reflects variations in Rmf, ROS and Di and extremely sensitive to e/spacing as e approaches spacing; reverses but has characteristic shape when e equals or is less ~~an the spacing.

Poor definition of thin conductive beds surrounded by thick reSistiVe forma-tions. •

Does not always indicate thin resistive streaks.

Low radioactivity contrasts cannot always be overcome simply by changing scale as can low electrical contrasts.

Precautions

Proper scale must be chosen to provide desired definition.

Care must be taken in selecting proper scale for correlation realizing that this generally is not the same as the desired scale for formation evalUation WOrk. Amplified scale best in long shale sections.

Effect of adjacent beds on shape of curve must always be cons idered.

Care must be taken in selecting proper scale.

Hyperbolic scale generally best for correlation work.

Quality of log severely affected by choice of logging speed, time constant, instru­ment sensitivity and scale.

______ .c~·_~ _____ . __ ._._. __ . ____ . .-------- ----- --------- - - ._--

I f f r

! I

I ! i

Sonic (Interval Velocity) (Acoustic)

Dens i ty (Camma -Cammal

SP

Gamma. Ray

Sonic

Density

Any

Any

Any except those of extreme resis­tivity.

Any

Consoltdated

Any

Any. (Res ponse in empty holes unknown)

Any

Any conductive mud so long as Rmf is not equal to Rw.

Any

Any fluid

Any

*Cond.uctive is used under ~ of~, in the general sense and does not refer to degree: Includes all drilling fluids with a continuous water phase.

.,

Measures a basic charac­teristic of the formations. It is not materially af­fected by changes in Rm, Rw, Sw, or (within limits) d.

Records basic parameter, density, of the formations. Two receiver, compensated log effective through casing.

LITHOLOGY

Distinguishes between shales and nonshales.

Not affected by bore hole condition:i.

Records basic par&me~er, velocity, of the formations. Where adequate knowledge of porosity and fluid content 1s available, formation velocity can be related to Lithology.

Records basic parameter, density, of the formations. Where porosity and fluid content are known, rock type may be identified.

Gives abnormal readings 1n high porosity, unconsolidated, non invaded gas (and some­times oil) bearing sands and through zones of extreme fracturing.

Same as for correlation work.

Local relationship be~~een radioactivity and lithology must be known. Always same limitation as for correlation work.

Same as for correlation work.

Sensitive to sharp irregu­larities in the bore wall.

Optimum blas setting must be selected to avoid excessive cycle skipping or electrical noise.

Same as for correlation work.

Same as for correlation work.

Same as for correlation work.

Proper calibration essential. Should be checked against core analysis if possible.

.~

,.

TIME LINE CORRELATION

The electric log markers most frequently used as time lines are bentonite beds because of their distinctive pattern, as shown on Figures 3-8 to 3-10, and their widespread occurrence. In some areas where volcanic activity was restricted to short time intervals, dense basalt flows which usually show a distinctive pattern (Figure 3-29) may be used. Basalts are particularly useful in continental sequences where bentonites might suffer more from differential erosion.

Sedimentary deposits often cross time lines, but there are some blanket type deposits thst may be recognized, particularly on gamma ray logs. Glauconitic sands, often associated with rapid transgression following'a perIod of erosion, are quite distinctive particularly in aress where the sediments ordinarily show low radioactivity (Figure 3-13). Thin radioactive shales may be good time markers particularly in pre-tertiary rocks. Potash beds may be correlatable in saline basins (Figure 3-28), and coal beds in continental basins (Figure 3-16).

In some areas easily recognizable necks or shoulders have been observed on both the S.P. and short spacing resistivity curves at formation boundaries and sometimes within a particular formation. The lithologic differences reflected by these distinctive changes in electrical characteristics are not always apparent and may be immaterial provided they check with paleontological data.

4

ADVEIlSE PHYSICAL AND CHEMICAL EFFECTS

The following examples (Figure 1-3 to 1-11) show some of the correlation and interpretation problems caused by the drilling fluid and the electrical cir­cuitry. Fortunately, effects as severe as those illustrated are rare in most areas, but geologists should be alert for unusual appearing logs.

Despite quality control checks, poorly calibrated logs and mechanical effects which could have been corrected do slip by occasionally. These effects may be severe enough to hamper correlation as well as quantitative interpretation. All questions regarding log quality should be brought to the attention of an experienced development geologist or a Company log anslyst.

5

I­--H- -

- ,. -

- I- ..

LL..l....

-

..

..

S.P. AND WATER SALINITY

-

-

'-_.

~ _.:>

. r.-,- "~_I-_: "-F-'I;- ... . ·t-h

. ". .. . ,~, .. : I L. ....

.. S::;;. ." "

.. "

• I

I .. <.

.. .. .. ..

.1 -I

,

The SP log is a good indicator of sands. However, the SP deflection is dependent on the contrast between the mud and formation water salinity and may not always show sands. Other factors such as streaming potential and Ca or Mg ion can affect SP deflection.

Typical SP deflection when mud is much fresher than formation water •

Formation water and mud have approximately the same salini­ty. resulting in almost no SP deflection. The mud salinity is approximately the same as the upper example; therefore. the formation water is much fresher than above •

Reversed SP resulting from the formation water being fresher than the mud filtrate •

NORTH PARK BASIN Figure 1-3

RELATIONSHIP OF MUD TYPE

TO IDENTIFICATION AND CORRELATION

OF SANDS

LIGHT RED (CHEMICAL-GEL)

-.~~: 1:­f..- .

8 '~-~~~'~-~~~'~~'.~.~U~~~ 1-- --~. 1-...

. h - . .- 1--1- -I- I-

:1-::: =:t:-tit:1~1 -1-->-

":-'_::.

, PIERRE. FM. r.:

I-..... t­

I-I-+-IH--IH-I g I~~- - t--~ : .= - . ..1- 1- -, 1

1- . _. .-i-

"-1--

-:-'r,' ... +-.-

1-

i 1_

.f-< - - l- F

!t-SAND . .. , ~t-

1 ~I)~- . -. : 1-,,- -_l-

. 1-1- .-j-- .

I--~

-. 'r:- .

GYP BASE

.1- - .. -

1-

1-·

L- __ _ ---I- _

. I-I-·

-- I-I­i\ --

- I- ..

--I-

- 1--1- ..

.- --

- _ .. _- - -~-I==I-

..1-- --------;..---

~. _ ..

t Ul-I-..I-4I'=....Jf-:Wl.... '.1..-...L' ..J-_. LL.ll.:...L....L.L.L . ..l

PIERCE FIELD

DENVER BASIN COLORADO

}lud nature is very important to S.P. development opposite sands. Although low contrast between mud and formation resistivities will result in a log as sho'-lIl on the right, the depressed S.P. in example on the right is caused by the nature of the Gyp Base drilling fluid. Care must be taken when correlating or making sand counts to account for changes in drilling fluid

Figure 1-4

I '.

i 1 ;

l i , j

I j j '"'l J .... ! 00

" " .. -I I.n

; I (

! ,

ORI GINAL HOLE INHIBITOX MUD

MUD RESIST. BHT. (l45°F)=O.14

I .' -:- 1-

> .

~'.

• I~"" § • <.~

~

"

,

- --~

/ 1),

r--i' , i-I-- l-" ~: · < g .

" .. ~; .

~ -:.

• ~ . . .

1->. I- t- ., F'- .

.. ,,'

• .. 8 .

v -"

~ ..... , . . ..

• , , § 1--. .

II?-I .'

---.-----.--.----

-

,

, I

,

~ ~ ,

, I

. ..

-

, ,

I

. -

SIDETRACK HOLE GYP.-DIESEL MljD

~rn ""'I'F'::. ,;"0,,.( , - - .

" - . j'= R .'

i 1-- \-.

f': , -, ., l-I- ..

1-0

i t- f-- 'Ii' +.

" ..

I- t- 1>- I- t- +- -Iff- t- - -

i .. :>:

~ ~" ..

. ,- . - , -

'if • .. !

. . .. - I-- - -

t-

• - -§ - '.jS \-' ~. -

. -- . ' .. : . i-1-= -" r:.' ....

• §

" . ' ..

:+- -+-: EFFECT OF DRILLING FLUID ON E-LOG OIARACTER

+- "'~

-~

--

l-I ' ,

i5 - 't-

1

IHIJ~.lhl'Ollllllt _.-.. -.-. .

:at.

;Jt· , .. ~

I·: 'c

'UUIII'JIY ""- .... , ...

:1'

. 1

~-

'lllllll'lI' ...~ . .... , ...

ffi_~"" IE,l _ _ -U. :± -Tt- .

DOWNIIOLE GROUND

EFFECT ON TilE

S.P. CURVE

The shifting shale base line is caused by a long ground electrode moving with the logging sonde and being affected by the average potential of the 100'-200' interval which it straddles. Downhole grounds are sometimes necessary to avoid adverse surface effects par­ticularly in metropolitan areas. This S.P. curve cannot be inter­preted quantitatively, and the curving base line must be csrefully considered in qualitative interpre­tation and correlstion •

Figure 1-6

... --

--.- -

-l-

I- ..

RESITIVITY REVERSAL

FROM CRITICAL BED THICKNESS

-I--t-. -I--'I--t--

1--

+1-"1--

l-

I---

I- -

I- ,I

i r

i ,

, "

0

§

"

.. . ..

_~ _ t-::: _ .1\ . -~ \

.'

I---- ".'

\

-.. -

!-

DENVER BASIN COLORADO Resistive bed between

16" and 64" thick.

Interval 910-920 feet on log to left shows. a resistive bed on all three curves. Log on right side shows a well defined resistive bed only on the lateral curve. The long normal on the right hand log is affected by critical bed thickness and actually shows resistivity reversal. Normal curves always show resistivity re­versal in beds of high resistivity when the bed thickness is less than the log spacing. When correlating by normal curve character, the bed thickness should be considered.

Figure 1-7

/-, }

NOTE INt:REASED GAMMA RAY SENSITIVITY

NOTE SCALE CHANGE

EFFECT OF WELL - BORE FLUID

ON RADIOACTIVITY LOGGING

Galllllll Radiation Nautron 8

- 1---

~- _.. . --

I

-

~ i

i

J= I-

• §

~

I -

GYPSUM BASIN

COLORADO

-- -

•

-- -

,

~

~

Well bore fluid depresses gamma ray and neutron activity. Unmarked fluid levels may resemble formational boundaries,

$t

MAXIMUM NEUTRON READING OUT OF FLUID IS ABOUT 880 COUNTS

------FLUID ~VEL

MAXIMUM NEUTRON READING IN FLUID IS ABOUT 540 COUNTS

Figure 1-8

(

I '---" POROSIT Y INDUCED FROM

WASHOUT OF MATRIX

Gamma Radiation MicroLaterolog

.J: ... . i"'- .. .

.. ~. ;.~ • .. F- ..

. . , .. "

Laterolog

_ ... - - ~ . :,.. .

-.

:' I- E:: ... :~ ; _ . t: .; .,

K

KIBBEY LS. I :.FF ~~~Wbt=t~:1jiittt+t1 ~ .. - i-o . -H POROSITY u.f -U-f-t--i'-\'i;::H-tt-t-H

NOTE HIGH RESISTIVITY

I-~ .. 1-+-· - 1'-", .I; . .. .. . . -

( . - - -"k::. :I-+-f:::l r:.::.- -

I'­Irl-

.. - --f--t-:-. -l'.'t-",.J=l.; ·1< ...

k: •.•

WILLISTON BASIN

NORTH DAKOTA

Porosity as shown by MicroLaterolog has been caused by washout of salt cement from fra,gmental limestone. The Laterolog with a deeper investigation, reads beyond the washout zone and the high resis­tivity is indicative of low porosity •. This particular circumstance could be easily confused with oil saturation. Sample examination is necessary for interpretation.

Figure 1-9

~ i I

I

SEVERE HOLE WASHOUT

IN ABNORMALLY HIOII PRESSURE SAND

IVAN RI VER,ALASKA

In the interval 8700' - 8750', the S.P. curve is reversed, mud travel time is recorded on the Sonic log, mud density on the density log, and even the deep induction curve is affected primarily by mud resistivity. Fortunately, cavities such as this are rare in permeable sands.

Figure 1-10

I ) HIGH PRESSURE SALT WATER INFLUX

~-

5 rr <: Il!:' 0 ~ .

~~- -I-0 7-.. >

-. I 1"-

- ..... ~

. ;" t r..... -I- -

- -'" g ~ ~ -0 0 , -::

7'" - -0-, l- f-

T ... ~ ~ C SP reduced oy t by aalt 'Water ~ _ 1'low.

V I-'( 00- -. • r:> - - - -

§ I~dr~g g 11,571'-rnud level In pita r_ --1-.. rising (gained 50 bbls.). Increated -" I~

po mud wt. to 16.8 lb/gall chlorides -. /- -

I? ~ increased i'rca 6200 ppm to 14,500 -. .. ppm. .. . ... .. ~ ....

--f . ~ - - p

( c h.. .. t I":: ~ ~ 0 '"'" ~-

~ I-' 1'< -~ e:.

1<;; '-FR

11754

• j -

0 A'16"-M (AMP) 2 6 FF40

0 SHORT NORMAL 10 2000 INDUCTION 0

0 100 4000 2000

INDUCTION 20 ~ •••••••••••••• __ •••••••• _ ••••• JJ!

-1-/ + Q ................................. _.lD.Q

SPONTANEOUS· POTENTIAL " RESISTIVITY CONDUCTIVITV ~ millivolts ohms-maIm '/I' h I 1900 '" ml 1m as m ~ .tIm,"",,!_

Figure 1-11

LOGARITHMIC VERSUS LINEAR SCALES

The re-introduction of logarithmic scales by Schlumberger in 1962 caused some consternation among geologists unfamiliar with that type of format. The older generation will remember that Schlumberger recorded resistivity curves on a logarithmic scale in the middle thirties, but that was a non-focused two curve systemand.there was no particular advantage to the logarithmic scale except for greater detail in low resistivity intervals. This detail was later restored on the linear scale with the expanded normal curve.

The Dual Induction-Laterolog 8 Survey (DIL) provides, in a single surveying operation, three focused resistivity measurements and an S.P. curve. It was designed to determine depth of invasion and provide a good value of true resistivity where the deep induction curve was affect~d by invasion. The logarithmic scale was chosen primarily to facilitate the quantitative evalu­ation of invasion from the ratios of the resistivity values recorded by the three curves. On this scale, the horizontal separation between any two curves is the ratio of the resistivity values.

Contrary to early experience with non-focused systems, the logarithmic scale now improves the readability of a three-curve focused system. It provides greater detail in the low resistivity range and eliminates confusing off scale traces and scale changes. All resistivity curves are on the same scale and the expanded and ten times scales are eliminated.

Detailed correlation is facilitated by the logarithmic scale, particularly between wells where the resistivity contrsst is large, as shown on Figure 1-12. Two of the logs are linear recordings of 16" normal and lLd induction curves; the other two are logarithmic records of the LL8 and lLd curves. Major features are readily correlated between any two of the four logs. Further­more, the few minor features that are apparent on the 16" normal curve are readily correlated with the LL8 recordings. However, as is frequently the case, the normal curves are quite rounded and afford little detail for precise correlations, particularly when the hole diameter is large and/or there is a high contrast between formation and mud resistivities. The LL8 curves provide a much sharper bed delineation and more detailed information.

6

WELL SP

-20+ H

\ · 8

) '--- · ·

( 0 0

· · 0

) 0

/ (

COMPARISON OF CORRELATIONS - LOGARITHMIC SCALE OF LLS-ILD VS LINEAR SCALE OF 16 IN. NORMAL-ILD

A WELL B WELL C WELL 0 RESISTIVITY RESISTiVITY RESISTIVITY RESISTIVITY

0 .( g .e 10 100 IOOC 10 100 1000 0 .oe we ,2: . 12. $ ,~ . '

1-7- - .-- - ---. (~

. -- - -- ...... 1-" -- -- • -- 8 ---.-;,:~: .. _\. ... , ·"':·S r--

• • ~ ~_ .. ":s_ 8

'~t;~ .. -:::

~ •.... ~ :- .. .... _ .. "" --.. =-------t- -- . - f- -~" :=,'--

"'y_.,- ,;~ ~ .-- ->J, ___ " J! ....... -.. --- . I.. ~~ - f- -- -- 1...--- "

l~---- .. , C~, ,.iJ - .' .

"..::;- ---- .. -I .. ... _'"

g _H :".= __ ;-~.t. 1":' .?"""

-. --f-''' ... :--- - I- "'. ..:.. - -- ---- -- ./ '. • · "l~ 'l J.~?6FF;jQ

(~ - - ---- 8 . -- 7· .. ' , ... , ;~~

-• .

(f£ - (.' .' .:' .. 8 -.

(\16- NORMAL ~RMAL 'E . .

-- iE' ILd--...,. .- '--- :::::: q ""::::-LL8 ,

I Ld-fJ- LL8 . .,:,1 r--·- .... -- ..' > .- --

~'" ~:S: .. ·· " -

, . '~F40 , •

-~~ 1-, '. g ;r '~'\ .. .. .. - ::; .-_ .. ..

- -.,.~,--~ .---:.0;: ~:--::: ;.':.'= .-......... ~ .... --- - . '- J!W. ... :.~-."... · -t·· " • ~ .---- ;?; 8 ~ -=' .

I-:-:..d=~ .. '" 2::-- --r-~ ;.;.:--',' · ::::::.-• ~:::'.--

0 , _.,:< .,":;; 0 =- _Ji -_ .. -(.---==-- ) <--. -.. ----- -- ;s: -----_.. ....... ~:" ___ : .. ~::" ;z

- . ~: ... '.

--~ --f-_<; ~:----

f:~ "':4;: -- .::: ::---4. -'- .' --? ~ . - .. :'. - ---- ' -• -i

" : ~:' '"'" .:; > ..

.; ~ -

,> '\ " § c:: ~ _1 ..... -. \ ........ -:?: =-

'.~

Figure 1-12

,~ 1.,",. -:

·fr':If)i:~ PART 2

.~~ 'or ,

INTERPRETATION OF DEPOS ITiONAL

ENVIRONMENT FROM LOGS

/,

"

-.J' ,\r •. ,. "

. '>

I !

>, "

";;',: :,' '/

/

I II '1

r

, I /

I I

!

. I "----"'

The accompanying examples are excellent illustrations of the four electric log patterns described by Mr. Nadler. However, before using such patterns, particularly in a new area, the following points should be considered:

First, the S.P. curve on the electric log should be used exclusively, as was done by Mr. Nadler, as the resistivity curves are more affected by formstion fluid. Also, finely interbedded ssnds and shsles may approximate the appear­ance of both transgressive and regressive environments on three-electrode resistivity logs because of masking and false resistiVity effects.

Second, the type of mud used in the hole and the relation of its salinity to that of the formation water should be checked. Lime-treated mud is particu­larly detrimental to the S.P. log csusing erratic behavior in many cases, and in fresh water areas, the S.P. curve is considerably dampened and may even be reversed if the salinity of the mud is greater.than that of the formation· water. (See Figures 1-3 to 1-5). In such cases, the gamma ray curve may be substituted for the S.P. curve provided that the natural radioactivity is primarily a function of clay or shale content.

Third, the relationship between the S.P. curve and sand permeability is not always as clear-cut as might be implied from the examples shown. Siliceous shales, like those of the upper Miocene in California, often show electric log patterns similar to those shown by chsnnel sands. (See Figure 3-5). This is partly a result of fracturing and partly the high concentration of silica, as compared with clay minerals, in the cherty or diatomaceous beds.

Fourth, electrical disturbances (both natural and man-made) may cause erratic behavior of the S.P. curve. A scratched electrode effect, ordin­arily caused by pulling through a tight spot in the hole, may look very much like a transgressive ssnd pattern, and drift of the S.P. curve, if not properly evaluated, may also cause an erroneous interpretation.

2

REGRESSIVE

POSO CREEK

CALIFORNIA

SAND

Figure 2-l

, V

SANDSTONE CYCLIC REGRESSIVE SANDSTONES OF THE

CRETACEOUS FRONTIER FM OF CENTRAL WYOMING WERTZ FIELD, WYOMING

-1-1- -

-I-

. - .I\.'~" -1-1- - - _. . . -H~-····· .. -H~ - ...... , .

-1- ,. ,. - 1-,

....

fl )( 0 . ?> .. ' .. . ..

,=:P- ... . . - .- .

. .'- f.. .+ . r;;.... ... .. . . 1'<: .,.. - . i"" ~. ".1- : '~,

.. ' . .

- .,. :: " .~ -; ,cI:-::·· . '_ K

.-~ .,. . . _. ~: ... ... . 1.,1-. , +_.

12'. ·1- : ':I~P

I~ 1- ..

tttlBf-:' .. -- I +1- .

p. ..

[ , 1\ I--1-. ++ ...... .

+-1- I-

'f-'I- ..

50

. I 50

Figure 2-2

TRANSGRESSIVE SAND

CASTAIC FIELD

CALI FORNIA

- I-I-f-

100 MV

H-

tot: j-~-

I ..

--

H:':: .• -

-. 1- '-'

/-I-.

.' . 1-+ - -I-

f- -I

I-I-

- I-e- .. I-I--

':-

i

~.

--- 1--1--I-

I- I-

- -.~ -..

1- - 1- - -- ~~ -~ I-I-

'. " -• .' -

+ 0 ohm, ""1m eo Figure 2-3

'--

OFFSHORE BAR

COALI NGA NOSE

CALIFORNIA

Note Uniform Character Of S. P. Curve.

'100

1200

, ., .., i:!

Kft' 1051

KRf. '018

,,00 • 1--------+-+-1~-----~ 17 ..

1400

1100

",o0

"00

,aoo

"00

8000

8ol~

71

94

33

~ ()

~ C\. , .... ()

'"

• ~

'" ... ., ., Ot ..

100 MV

0-1505,.1.

0-500 s.e,df..

+ 6015TD

GS ''!~4

EGA 1313

....

[ G Z 1001

Figure 2-4

,-

l II)

~ II)

i \....-.-

) !

BAR SAND 8

§

• ~

• §

GREEN RIVER BASIN WYOMING

ALMY FM. ~.

Bar sands are characterized by the lack of thin shale interbeds, massive sand character and blocky S.P. shape.

Figure 2-5

CHANNEL SAND

ELK HILLS

CALIFORNIA

4'10

.000

ItoD

100 Coarae,Grltty 5

Sand Interbedded With Fine Sand And Brown Shal ••

0

• '00

600

• , ..

• .'0

4-14. J :':nd - r:; ~"

-~~fri~: , _ 1 "mp"

-

-- t ~

nlif"io~ -~ ~j

TI

-

-i;j - ~ . , , ,.1' ,

=~ -

_:4 .

--

•• 'oiI 100 MV

l

~ '::'1-

~1

.'

~ ~ ':;"" -

:~ ~~~

f - -~

~

'f

.;;' a:

1--.: 15$

~-

~ i= -

~ iJ

~ ~ I .. ~ ~ 1'1

~ -j. :., '0'

+ 0 ohm' m1/", sell. 'to.

100 Figure 2-6

I

CHANNEL SANDS

.'

....

. ,

<.

RED WASH FIELD UINTA BASIN

UTAH

The channel sand environment cannot be conclusively identified from electric logs. The thin sand beds reflecting frequent depositional variation are well shown by electric logs. However, such a pattern characterizes not only channel environments but others as well. Lithologic descriptions of the sands and associated beds plus detailed mapping must support identification here.

Figure 2-7

\ I ~

",i

. ) '--.c.

18'8" Lateral

-I-... - . - ~.

~

LAGOONAL DEPOSITIONAL PATTERN

1\

....... 16" Normal

...

,: . - .

'. '. ,.

_.4-... (

\'" .

, .-

'-.

, 1:' .. ,,:

GREEN RIVER BASIN

WYOMING

...,.-64" Normal

T-~

; .-r:r: ~.+ .• ....&..-l­.- . .:......: ~

. ·.·-t-"i

_ ...... , • ...J .. , .. "

, '

~.

The sedimentation patte.rn of a lagoonal environment is characterized by relatively low percentage of coarscr clastics, thin beds and coals. The resistivity contrast between thin coal or siltstone beds and shale or sand CSUSOG a rapidly alternatina high to low resistiVity character. The thin resistive beds may reach critical thickness for resistivity reversal in the • normal curves, however, the lateral curves reflect the resistive beds except in the shadow zones •

Figure 2-8

CONTINENTAL CYCLES

Cyclical deposition can sometimes be recognized in continental fresh or brackish water sediments. Figure 2-9 is an example from the west side of the Cook Inlet in Alaska where the Tertiary topography was probably similar to that of today, but the climate was warmer.

The log example shows two cycles which began with the dumping of coarse sand and gravel into coastal marshes at or near sea level. These flood deposits were followed by finer sediments and periods of little or no sedimentstion during which thick peat deposits accumulated. The peat has since been converted to soft, lignitic coal beds by burial and compaction.

3

•.... ---,----, 'r

__ ~_: '_. ~_. ~: :~-'~' ~-4{~: '.+-' _. :~~-o;~~l~i_ti+i;.+! li~!,~. _ ... +1 -ii-!j~r~: i~ii~~2J±j =~j=;-;~j,~l!~~~j:.r~--~~~.~~. ~ .:l I I 1 ~ ..... j

.. J ... L .-.. Cl8AY ,ijll',. . '''~~I''; . l' ! ; i I!i:. . .... ~ . . L;· ._. t' _'. ii' . . I • I , I:, ' .- .. ; .+ i .... 1.. s171 f it"I'i ,I!I .. I( ~ ;; :1 l H. jIll .. ~.j .~: :: :::1 .......... T' ····I,···~ j" ~.I' ··!··Ilt:.....·' l-t·llt •.. - ••..•.. :

. ~!"::t' I , I ._~. COAL I '1' 'j': ! I ,"~<:::;"'~ ,-.- ... ,. ,

...••. - ....... _ .• ---,. c' Ii 'ill -... "1' '" .' i-!""'~"'"'' .:

':: -;-"1 .... -_. ,.tj·i:"·-"I··t·il"l:·-~~'''::··';·· .. : .. · . 'i 'II I...· ,.;, , I •.• ~ __ ...... _ •• .0- _,,_ ' I I I. .. . ~f.,... II j •• •• J.; • d .. .o... .'

.•... ~-- .. -~- -1--' , j I. . t' ~,' . ! I I : I ....,'" - .... -., "-'1 .. - ,. I :,',;-::; --~.-. .-.J • . • i ! ~ , I . 1" •

• •• .j .

~ ... :.-~ . ...j-:-;.~i-. ...j .. .J-- Cl.~Y " .. _.i .. !k>!ll _.i .. ,.~l.:., .----:-:,. ''::':~~1:1= -', ·~.·-rt· And! .;., T I( "'j .. -. " .. ;:~: .::~. __ I I ·I{·o, ... r- -I-~' ll ___ +·····~ .-h-~ •. __ !... . •..•.. ~-

. I' , .. ! SILT, +-tlii1' -+ 1',:1 i .... ~ .. ,' ;or",./... ....... I ___ .;.......:.....+~l ... ( I' . "

.• __ ~ I • r Willi' ' I': " '~. I _ .~-i...J. L.sr--r +- THIN 11' 11~'- ":'; :~ .: _. ...;-;-.;_. __ ... - ..... , I' c::::', -1 to', --'1'" .~,":t..:.: - ':, ----'--.- ... -'" .

_~.T-i·+--··C;,..,,; §~1f,slRI1· -. + 'll~~:" ,.:" ":i:;"~-·"···"·· !' 1/, ~ ITI,;---:·TI):;i·-·-t-·;~;~;i-i-· .... I .•

. :. __ , i.....!c;o:i-"". L . 0 ,!, '11J" I;, !'il , I I ::::So I, .;." --.. '--t~·1 ... -.- •..• - ....... ---........ -.-

.. :-. ill s; , ~ ..... J - .: .. :'{""t]"1 "_±j'-:J-ULi --,.-......... 1-.-.-

.~:=:. : . \.: I ' FINERij ':.; .'~'.: :~\ iii. 1 1\ W ......... m_. , i-I SAND '1 ~ ,:.r- ~" .-l'ilTii+i --:~ .. -... - --.

. ~.....l....J, ,'\....:. ffit' '\: :.~ " i ; I'll , I __ , .. t,!·-. ;',"1' .. 1, ..• --.'--.

____ -J --". I I,' • ' iii' , • _ ': <c --:' i COARSE .. 1 '-I'~; :=- .. ~ ,::. >: i '--'j i TtJl ,----. :---r-;-<::::: I I SAND I r!il . ·~:I---l-:-m,,~~-"·"-·----·

. ! ~ I I '" 'j" i" ,., ~,.... . ':C ": .- .-._.-.'-j-; -,--_. ---.- ---, ,__ '" i! • .. i, : ' 'I iii , t I J---g!" : ~. ::.-...:.' I ; +d. I -.-----:-.. : + I If,. COAL li~.':,::~·~ {-Ft' r.~:; --':'~:'~~ll,""1 .-~--- .. j--

.. -........,-1 I ! :~... iII~'''I--'' -~.... -. '.-- .---.--; 'I I • ':: .. - .I-.J.~ !.~. ,. '';~i I -1-.--

1 ___ -'-...;.....+++~~I......j._1 .. l~ , ill, , ; I ' CLAY' ,. · ir 'II ,. I'" ' , , I '\.. AND! f-:-; :. -: '-j 'I ( :.:. j i[ I - -'-7~t: iii' ....... .

'-, ... , SILT 1-11" - ., .. -~.~, •. """7' --I-·j--:-"TIt"hM .. ·· .---. ___ +_+ .,.~ ,- ·'·I_,--.+·t·,.-~r-:-~.··'i L ..... ___ --. ! I ' " 1,1' , j'~'" I '." I i

1-:+: +' :+1 ++=~,~ -,-",.~, 2:=.Ig---j.' 8:: I-- -; ''1'-1 ',!: .. -:--r-~ -1_ <. 0: i I . , " I.;, I '

- .. -'-:"1' I ~ , g'1 w ~ .- i . fllTI·...;-1.."'r·J·nl'r-.L.- . '" .---. _____ • ..~~ - ,'.. I ",,...!Ai J

-,-', 1 :?--.. FINER' .. : -- " ~ .-I.-I-l- .j-l.t...LUU. ........ 1----'i"+,--r-r=:s"~' SAND .,.j: .. ",l~ ·····[-1-1'::!if·· .. -' ..... -.. - ...

~ .~-.~ .<.. .. ··1 ~- ~~-·I·- l_ll:- ! C-~ --" I ~; ~1:1::_~_.; .;: -,

. . I I !

. -to +-t-::±:.. +-. i·i ,- l.l~'l-l··H~' ;i' I.... "'r-'-" - -l-. ~ (i' I . COARSE,' i l' .. .-. ,RF;' . i .... ·ft· j.q n -l-~""· ..... - .-. . - T'" I H-- SAND "j .': ," -. ,.~.[t 4. " .. - - t ~(1t '.-' .... 1.--.. ' r' I I I A"'O C"L'I . .w;.. . 'f['" , I .' I ' ; i ! , 1\, 1 I I"''''' .. '',·r,,:''- -' :, .. ---" .4 .... ,j.-- ............... ,.--.

I ,I - " I' ," I ".!. I o . . ,.' .'

: ; i i . l il,l " 1-0 I::; :;. ': ~.~''',. ~, : ; . : .. T ·-n-i+'. ~ ;..- - 1';H';'~I,j '--l·tll/·,~ 'i-rHH .. --~ .. -: -....... . ~~1-:-1 i ! i

l i ~Il' :1-"~~ .' 'lIIU' .. - ·-t-t +I~ ,·--t-· -...... I ~.~~.

, ,- VOI+ .' o.lii 1 I j:ro I - C{1 !l ~ --;-: -j'T;, 11; 10-:- :." ")0116'" :_I.J I 1\ 1/1:1,1 1 c..1l1 1 1 ':11 I !

Figure 2-9

' ...... _.

TYPE OF CLAY IN CLAYEY SANDS

The type of interstitial material in sands can often be used to interpret the depositional environment because it is indicative of the source of the sediments. Figure 2-10 shows a continuous sand and conglomerate section with a sharp break in electrical properties and permeability at 12,154'. The only apparent difference from visual examination of the cores was a slight change from gray to white in the pore fillings, but the x-ray spec­trograph identified montmorillonite and chlorite in the upper sands and kaolinite and clay-size white mica in the lower. The former identifies a volcanic source whereas the latter is indicative of plutonic rocks.

The "b" factor is proportional to the cation exchange capacity and therefore the surface conductivity of the interstitial material. A "b" factor greater than -0.2 will cause a rock to react like a shale electrically. For further detail on electrical properties of clayey sands refer to C.R.C. Research Report 781 by A. E. Worthington, dated November 9, 1962 •

. The presence of swelling clays in the upper sands is also indicated by the micro-caliper curve on Figure 2-10. Permeable sands normally stay to gauge because of the protective mud cake whereas impermeable sands are subject to physical and chemical erosion by the drilling fluid.

Another example of sands with electrical properties eimilar to shale is shown on Figure 2-11. The sands above 2120' are relatively clean, reworked marine sands whereas the lower sands are plugged with montmorillonite clsys indicating a continental volcanic source. In this area, the tight, clayey sands are easily distinguished in cores by their characteristic green or brown coloring.

4

.. 115 -IE'

[,,-1- ~ 1=1= ~ ~

°0

' •

r.c . . . 1- .I- .' .

, -- " . - 1- _.

.... "

. '. SI : .=.:" ~:: . I=§

=1-- 11' I 1= I, -'-".., lu 1-

~~ 1= , ,Ie

I- I- 1:- =~ I- e:) = .... :::~ ! -- " ~ " ....

~ ---- - ---

,-

I~ -=+--~,

'" I..! "~~ -: ~(

_._.

1~1f =::

i :: :'.:', 1=

: - 1= I -ItC;j -",

_-= :c,: ,. __ = _, ':': ~e = _ r~'::' , .'

-,-.-

:-c ,--c \( ',':' :'. .-: :':::: 1"- ." .,i' -'" -, 1'- '- ,

':: -,--

=-- "- W,'. ::= ~ r~ ..... !!~ ICC 0.:;:. :..:. ,

=~

f I;'·): 1= ~ --= I-

-, ..... r: ,',:. 1--~ 7..

- I- ' '"

·~l.,:

I» , .... '.'

,. ---- -- --- :. '.: , : ;

.-:=: .L i ~ ~ " ~ ---

=- '--- -- -- -"'1,- '-'-' =.:. ::!

- - -:- -, ~

- - ::: ' :';':: => ::.t: = ~ -"

= ) -- '-~ ,

Figure 2-10

[

Figure 2-11

7j , '~

.\ vi

~ ·1

I . I ,

J ~

'1 ( , l

~ ~ . I

j

1

j

I I I j

"

S11

"1 .... DO

v' l

KlWRY

. UPPElt THERKlPOLIS

SHALE

MUDDY SILT

. LOWER mERKlPOLIS

SHALE

i. ii'ideapread t "Markers"

!; .DAKOTA SILT .. 'I' .... '"

I

i

~

( ..../

SANDSTONE

( \ )

STREAM CHANNEL CUT AND FILL OF mE CRETACEOUS MUDDY ("GRIEVE") SANDSTONE IN THE WIND RIVER BASIN, WYOMING

This is a good example of ~ into a pre-existing marine shale and silt interval followed by ss fill. This appears to be stream channel depOSition on a nearly flat, slightly emergent surface. The width of the channel is less than 2 miles at the line of the cross section. This sand produces from an upstructure meander in the Grieve field. That this is not a marine bar sand may be readily seen from the marine shale and silt found on both sides of the sand development, with widespread silt "markers" found in every well in which the Grieve sandstone is absent. Differential compaction of the shale-sand interval is readily apparent.

=c:t:: i I .. ~~E r- 'c .. c:. ::Et:: I--~~E r-::::Et:: ~ . ~~E - ~ t::Et:: E~E I~ .. t::Et:: ~~E t:: t:: .

EE~ ~ E~E - r-E j::: .. . .

t:: ,ifF Ie

~ ~§ I I C;::. ~ ~~ I I t ~ - - I , E ~~ E E~ / r/ .

"" ~ ~§ ..

E . t:::1o' t::~ . -

~ §~ ~ E t==_ .. . . Ie - ..

E ~~ -- - Ir"-~ t::E EE - -- - - C :I -t:: -~ . .. - ...

1< ,..f--. "-

i

-

SALT SOLUTION AND SAND INFILL POWDER RIVER BASIN, L ... l<tf~etl LOVht-y V(J....tlcy rce(d-)

WYOMING ( PlgpJ"'lI.t-

Detailed log correlations reveal anomalous stratigraphic and structural conditions created by differential solution of salt, and compensating thickening of overlying beds. The time of collapse is dsted by the com­pensating thickening in >the part salt beds. In this example, the bulk of the Goose Egg salt was removed from the left well post-Morrison, with the resulting collapse low being infi11ed with lower Lakota ssnd. These two wells are 80 acres apart.

5

,r

'--....--

ADDITION SAND HER E

SOLUTION __ SALT HERE

I

I

I

I

I

~ -~

I

I

:l:!=t.=mtit1 i

11111 . I

• ~

I

MUDDY

GOOSE EGG

= . I

MINNEKAHTA I ~~i

MINNELUSA . .i-

! ~ I

~ H ii Figure 2-13

;

CARBONATE PATTERNS

A carbonate section is characteristically highly resistive as compared with a sand-shale section, and because of the high resistivity, which causes a high potential drop in the formations with respect to the bore hole, the S.P. curve is sluggish or rounded in appearance and formation discontin­uities are poorly defined.

Porous intervals in, carbonates are recognized by abrupt decreases in resis­tivity opposite intervals on the S.P. curve which are convex in the negative direction. If the S.P. curve is concave in the negative direction, the interval is shaley. For the recognition of thin porous intervals, wall resistivity, induction, and focused resistivity logs used with the S.P. or gamma ray curve have been highly successful. For quantitative evaluation of porosity the neutron, Sonic, or density log is used with the gamma ray curve.

Radioactivity logs are preferred for.detailed correlation in most carbonate areas. These are supplemented by focused electric logs and continuous velocity logs like the Schlumberger Sonic Log.

6 . '

,--...

CARBONATE ROCKS

SOUTHEASTERN IDAHO

NOTE SLUGGISH S.R CURVE AND HIGH RESISTIVITIES.

i

i

------ --+

i

i

.-.. . po.. - H-

-;

iJl ::\=f --t

I=~:..:~~ .~~:-. -t _~> ~; ~ -ttO~'~V _1+. 0: o:;:~~ . --.1:: 0·; Oh~' m~~' .o~·

Figure 2-14

'""--,,,, I

EVAPORITE

CLASTIC

BASINAL WITH THIN CLASTIC INTERBEDS

RECOGNITION OF DEPOSITIONAL ENVIRONMENTS IN CARBONATES

-... - -- - - .... . ..• •.. _. ____ ...... _. J.

::'''' ",,'"'' I

~~=::.~~ -=;~~,~.y .. - .. __ ....... - .... - .. =:-:::.~.:. -:::~,-~::,:: i ." ,.._1-1',--",--1 . ~- ;=~r-~:..~::t - ,- " -------+ -- .,-t- ----... ,,~

- - t-I_--.~~· ~ ---_. -----;- -- f- ---_ .. ! '- >--".

, r-----~ ..

--'-

'T" ,~.;;:::. :qsrrrr~T+r 1. I .<;<~.: . iii ?' j'.' _. _', ,~~t

. f· J ,. P. 1 1

D .. ,il:~·,I-~."'!',.·. 0;:;. ++-+-!-1r~4.'--<1-. l-_+_-I-"l-I_+-f-

. ' , ... '!-'f-. . I~~.. I-I-t ( ;c;;r: . j • :.' I '\ ' , i /-l\-j-.~ .. -"'1""t+-t-t-+-t""""t-t-t-t-t-t-t­"'!-'I-I-

- .:.r:1= .1-+ -

• ,< - 4:=:" t4=1 P=fJ _ ~ [ ,. , , :', :1::. %:r-=r=ri:j' 1.:::I~1=t= "I' , ,.' .'. , ,/.. -.

~::: ... :' , I, • i. . ... , -","" . ,1 .i"~.-=

. ." I···. ~ : '

I ~' .j'" U-lI-H~l ~:=.: . (""i'~' ~: i I )) 1'""-1-..I.. ,_I--.. I .. _ ,. '1-1_1-. .. . . ,..l--f-"": I ..r

8 >1Y.5>' !!I ~11? ! II K" I ;. -l=::: Jt::tJ

• I I If;, .

< ( "

.. of- I-I­I· '" r--'

. ',,---'

~ )~ I " ---

= = RED RIVER! 'lYi d:: - r-e--= = FM. 1, . i I . . '- . '" " '~~-4~~'~~++'~'~~~

-,

~ -+ r--'::1 ~ : :';;,;.. / -",l ;t .~ : i >' >

8 '-tIM~~, --t-.··.,r-l'-<r>++++-t-+cl-Il-t-t-

NOTE CONTRAsTING HIGH AND LOW RESISTIVITY

NOTE LOW UNIFORM RESISTIVITY

:(11' :1<; r' I II -i+t+ ___ NOTE IUGH UNIFORM

-Ht--t--+x+1

(-t-H---t-1!r-t'.rl-1-t--f- RESISTIVITY

~. T ,. .. I·

- !~, i '-..,=

: I ~l:i I I. \."

;" 'I . ".:.....-" : ,\' I'"

=t=~~A"+ , /; ~! .!: • ~ . ..L-"_L-", :

1\ ! : , I i I !..:

WILLISTON BASIN NORTH DAKOTA

Three broad carbonate depositional environments can sometimes be recognized by electric logs. The evaporite association i. characterized by alternation of very high and low resistivity. The clastic phase may have low resistivity if the porosity is high and formation fluid is saline. The basinal limestones, if not argillaceous, usually show high uniform resistivity.

Figure 2-15

REEF CARBONATE

The log character that distinguishes a reef from other porous carbonates is a complete lack of bedding indication on the gamma ray and S.P. curves. Both curves have a uniform, blocky appearance similar to that shown by thick bar or beach sands. Figure 2-16 shows a true biohermal reef mass from 7234' -7388'.

Depending upon the post-depositional structural history, back-reef sands may enhance or dissipate stratigraphie entrapments. A typical example of the electric log character of back-reef sandstones is shown on Figure 2-17. In this case the reef was built on granite wash covering a granitic basement high. The high radioactivity shown on the gamma ray log indicates these sands were derived from the underlying granite wash which apparently con­tains a high percentage of potassium feldspar (orthoclase). The radioactive isotope of potassium, K-40, probably accounts for most of the radioactivity.

Proximity to a reef may be recognized from a comparison of well logs by an increase in the calcareous content of shales. Also, a small incr~ase in dip may indicate draping against the side of a reef.

7

T .,

" ' , I \~ , "I' ',i ! I " ' I ti! ~ \ j .

tt "'I I , l ' , , I! ,

~ -.l;'~! j i . L1!

; (t I

·t' . ±

. __ . __ ~_. ____ ItUImm-.u ._/ • . e....------..: •

REEF CARnONATE

tum ... ... __ ",n

,.1''-'

ita.W I j

Figure 2-16

I

IPOMr'MtOUI·POrtMIIAl I mlllf¥ottt

-_._+. ~.--... --;_.

RUI5T1VITY ohtnl.m'/m

BACK REEF SANDSTONE

I

i

i

I

I

•

Figure 2-17

WfLl COO!: 1051

SOUTH LOUISIANA

CROSS-BEDDING IN UNIT '0' INDICATING THE SAND TRANSPORT DIRECTION

DIP UAGNIJUOf DIP AIINUTH

tY K1" 20" W 40" roo 60" 2ro- tY 90" 1110" I i I I I i I I iii

SUJoIMAR'I Of DIP

OIRECTlONS

--=+-l~:-~~_Jllllrlmml ij'

I I I I rr 10" 20" 30-

+ HOLE DE .... IATION

• GOOD .. FAIR • POOR

, I , ..... , I

I

tt-I

The dips in Sand Unit "B" range in magnitude from about 4° to 20° and form a prominent southerly directional pattern. In contrast, the dips in Sand Unit "A" are generally lower (except for 3), and they form a less consistent directional trend. The structural dip of about 2° southeast at this loca­tion was determined from subsurface mapping.

The dips in Unit "B" are interpreted to be cross-bedding, and the direc­tional pattern indicates that the approximate direction of sand transport was S 10° E. The dips in Unit "A" conform more closely to structural dip than do the dips in Unit "B", and cross-bedding appears to be less prominent.

This stratigraphie section is part of a deltaic sequence in the Miocene of South Louisiana. Subsurface mapping has shown that the sand body, composed of both units "A" and "B", is lenticular, and that its axis is oriented ppproximately north-south. The well in this example is located in the thicker part of the sand body. The combined data indicate that the sand was deposited by paleocurrents fiowing southward.

Figure 2-18

"

CROSS-DEDDING IN UNIT '8' INDICATING THE SAND TRANSPORT DIRECTION

WELL CODE 1240

SOUTH LOUISIANA

----+-1 ~ 1-/-----I',

<----~f\ll\. :c

--~-~~I-~----

: .. .. I ,,'

DIP MAGNITUDE DIP AZIMUTH

I!: . '" I. .. .. ' ...... ' • I~ •• "1 ltIj .

>. " , I"'~ ,>: ',' •

+ HOLE OEVIATlO~

• fAIR a POOR

•' , , • . , I

In Sand Unit "B" dips range from about 4° to 32° and are relatively consis­tent in direction. Structural dip in the vicinity of this well is no more than 1°, bssed on subsurface mapping. The dips in the shsle, Unit "A", and in the sand and shale below Unit "B" indicate that locally the structural dip direction is slightly north of west. The dips in Sand Unit "B" are interpreted as cross-bedding and indicate an average sand transport direction of about S 25° W. •

This stratigraphic section is part of a deltaic sequence in the Miocene of South Louisiana. Sand Unit "B" is a lenticular sand body that trends gener­ally west-southwest. This well is located in the thicker part of the sand

• body. These data suggest that this sand unit was deposited by paleocurrents flowing in a south-southwesterly direction at this ·lu~tion. I .' .

Figure 2-19

-'

\....--)

CROSS-BEDDING IN UNIT "B" INDICATES NORTHWARD TIIICKENING AND AN EAST-WEST TRENDING LINEAR SAND

OIP MAGNITUOE DIP AZIMUTH wELL CODE 1241

SOUTH LOUISIANA ~~_ww~~ _ ~ ~ __

SUMMARY OF O,P

DIRECTIONS

1-1;---

----1-1 ~ 1-+---'---

1 i I \ I \ I I I I I I ~ .. - . ~-. ~- .. , --I' I· ,I.<'\-""T m·\ ::!-,: ::

• GOOD • FAIR II POOR

•' , , .­, , ,

The four dips in the basal part (Unit "B") of the thick sand in this example illustrate a special type of cross-bedding. These dips are consistently toward the north, but become progressively steeper near the base of the sand. Notice that each of the four dips is measured near the base of a separate sand lobe, as shown by the S.P. curve. These dips define a series of ssnd wedges or lenses that become thicker to the north in the direction of dip, or, conversely, that are thinning to the south. The southward thinning is inferred to be stratigraphic convergence within the basal part of the sand body.

Subsurface maps show that the northeasterly structural dip is less than 2°. The dips in the shale, Unit "A". approximate structural dip.

The sand is part of a deltaic sequence in the Miocene of South Louisiana. The dip pattern in the basal part of the sand suggests that this well is located on the south side of a lenticular sand body. The trend of the sand body is interpreted to be nearly east-west, perpendicular to the direction of maximum thickening. Subsurface ma,ps support the interpretation made from the dipmeter log.

Figure 2-20

CROSS-BEDDING IN UNITS "B", "CO AND "0" INDICATES A MAJOR CIIANGE IN SAND TRANSPORT DIRECTION BETWEEN UNITS "B" AND "CO

WELL CODf .,1'

SOUTH LOUiSIANA

I-~----

DIP hfAllNITUO£

20" roo 40" eo- &0-j i I I I.

"

I I I I (1)10*20"30''

+ HOl£ DEVIATION

DIP AZINUTH

• GOOD ... FAIR

e POOR

SUMMARY OF DIP

DIRECTIONS

When dipmeter logs show vsrying dip patterns in both magnitude and direction in areas of low structural dip, as in this example, it is generally diffi­cult to separate structural from stratigraphic dips. In many cases accurate interpretation is possible only if the interpreter is extremely familiar with the area or has recourse to other subsurface information.

In this example, a fault has been picked in the well between the depths of 9530 and 9600 feet, based on E-log correlations. The change in dip between units "D" and "E", therefore, is the result of a fault which can be localized at about 9570 feet by means of the dipmeter log. No faults can be picked above this zone in an area of closely-spaced well control and good correla­tions. Therefore, the shallower anomalies are probably of stratigraphic origin.

The structural dip on the top of the sand in Unit "B" is locally about SO_7° south, based on closely-spaced well control. The dips in Unit "A" may approximate structural dip, but the rate of dip is only 2°_3° in this inter­val. The dips in Unit liE" are assumed to represent structural dip in the lower fault block, although this cannot be completely verified. The origin of the dips in Unit "F" is not known.

In order to evaluate accurately the dip patterns in Units "B", "e" and "DIt, the effects of structural tilting must be removed by rotation. The dips have been rotated, and the results have been plotted on Figure 2-211..

Figure 2-21

"

DIPS IN FIGURE 4 AFTER ELIMINATING STRUCTURAL TILTING BY ROTATION. TIlESE DJ PS JNDJ CATE SOUTHWARD TRANSPORT

IN UNITS 'c' AND 'D' AND NORTIlWARD TRANSPORT IN UNIT 'B",

WELL COOE lSI.

SOUTH LOUISIANA.

OIP ,. AGN ITUOf

'-r----1ffiflll

+ HOLf DEVIATION

01,. AZINUTH su ........ ., '" OJ,.

DIRfCTION'

The dips in Units "A" through liD" on Figure 2-21 have been replotted sfter removal of the effects of structural tilting. The structural dip in Unit "A" was assumed to be 3° S 15° W. The dip in Units liB", "e" and "D" was assumed to be 70 south.

The northward rotation has increased the north dip in Unit "B" and decreased the south dip in Units "e" and liD". Also notice that the rotation has reduced the,width of the aZimuth pattern in Unit "B" and broadened it in Units Ife" and linn.

If it is assumed that all structural tilting has been eliminated, the remain­ing dip would be only cross-bedding. The original dips in Units "e" and "D" suggest that the sediment transport direction was southward for these units. The sand transport direction in Unit "B" waS northward. This indicates a 1800 shift in the direction of paleocurrents, or a change in the agent of sediment transport.

Figure 2-21A

. ) ~.

WELL CODr 12Z6

SOUTH LOUISIANA

TWO SEPARATE AND DISTINCT SAND BODIES, UNITS "B" AND "CO, IIECOGNIZED BY TilE

CIIANGE IN DIP PATTERN WITIIIN TilE SAND

DIP MAGNITUDE DIP AZIMUTH

0' 10' 0' 100' I I I i

----~H~~-------

I I I I 0" 10" ro- Xl"

+ HOLE DEVIATION

• GOOD • FAIR QI POOR

SUMMARY Of DIP

DIRECTIONS

It \

The massive sand below 11,700 feet on the log appears to be a single thick sand unit on casual inspection of the E-log. The dipmeter log, however, shows a distinct chsnge in the dip pattern between Units "B" and "C". This change in dip pattern is evidence thst Sand Units "B" and "e" are two sepa­rate and distinct sand bodies. A re-evaluation of the E-log shows slight differences in log character between the upper and lower parts of the sand.

The dips in Sand Unit "B" are interpreted as cross-bedding, which in turn can be used to determine the sand transport direction, after the effects of structural tilting have been removed. Sand Unit "e" appears to be more massive, and since there is no distinct cross-bedding pattern, the dips in Units "A" and "D" represent structural dip. Notice that nearly all of the dips in the sand below 11,900 feet reflect structure and not cross-bedding.

Other wells have been drilled in the vicinity of the well in this example. All have penetrated a sand that correlates with Sand Unit "B". No sand has been found that correlates with Sand Unit "e". Apparently Unit "e" is a separate sand body of small areal extent.

Figure 2-22

DIPS IN FIGURE 5 AFTER ELIMINATING STRUCTURAL TILTING BY ROTATION. TlIESE DIPS INDICATE AN EASTWARD TRANSPORT DIRECTION FOR UNIT "B".

WELL CODE 1226

SOUTH LOUISIANA

DIP MAONITUDE

I I I I CI' 10" 20" 30"

+ HOLE DEVIATION

DIP AZIMUTH

• GOOD • fAIR o POOR

IUMMAfty 0'01'

DIHECTIDNS

The dips from the well on Figure 2-22 were replotted after removing the effects of structural tilting. The structural dip assumed was 7° S 30° E, from interpretation of the dips in Units "A" and "0" on the dipmeter log on Figure 2-22. This dip corresponds closely to that determined by aubsurface mapping in this area.

The results plotted in Unit "0" after rotation have a small northwesterly component, indicsting that the dip probably does not exceed 6°. After remov­ing structural dip, the dip directions should be random.

The original dips in Sand Unit "B" (determined by the process of rotation) range from 3° to 20°, and they are interpreted as cross-bedding. They indi­cate an easterly sand trsnsport direction. This direction differs st lesst 20° from the direction indicated prior to rotstion.

Figure 2-22A

t-'!

.. _. -.. -... -....... __ .. -_._. _._ .. ---~·-···---·--I

PART 3

LITHOLOGY FROM LOGS

I

l

f I i

CONGLOMERATES

Conglomerates present some of the most difficult problems in quantitative interpretation of fluid content from electric logs. Even when fully wet, they usually show a similar character to that of good oil and gas sands. The S.P. curve usually shows values ranging from those shown by clayey sands to those shown by clean, highly porous sands; and the resistivity values are always higher thsn those shown by porous wet sands containing water of comparable salinity.

Two factors are responsible for this characteristic appearance of conglom­erates on electric logs. First, because of the irregular porosity caused by extreme variations in particle size, invasion of mud filtrate and sometimes whole mud is usually deep. Secondly, the formation factor is higher thsn in sands of comparable porosity because the tortuosity factor is higher. In a conglomerate, the current paths must bend around pebbles and sometimes boulders composed largely of highly resistive material. The result is an increase in the bulk resistivity analogous to that resulting from an increase in the length of conductors in a metallic circuit.

Conglomerates can often be distinguished from oil or gas sands by their vertical patterns. In a conglomerate the resistivity usually increases toward the base of the bed due to increase in the size and number of pebbles, whereas in an oil or gas sand the resistivity may decrease in the downward direction if the bed is penetrated near the oil-water contact.

If there is any doubt about the interpretation, the electric log should be compared with the mud and cuttings log, the drilling rate should be checked, and core and cuttings samples examined.

1

CONGLOMERATE

NEWHALL AREA

CALIFORNIA

31GG

S3GO

noo

,aoo

5100

1400

.SOO

.00.

"'00

i

I

I , , ,

, I

;

I

""'"

-

t I

( ~

1 fC1

....... r-.

~

<=

"'1=

fC7='

"'7"

"'- ......

lOONV +

I I I 4~& ,.

e- . I-e:: ..

.

..

.~ ~ -I-

c.. ~ , "" ::: J~ '" -

r ~ ,< ..,

.D l.- t , l-

61 K ~ ~ ~f: :: ~~ ~

)}-~ .... = - .• -

:~ L;r

~ .

'~ I'. ~ " -~ -I!; :..

? < ~ ::

~,

" <, ~.

-~ J' ;-- --

1 , ~ -<

"" -! ... - ~

l( ~ ~ .. , ,>l

,... ,

. """ < ., , -

~ ~ .~ - .,. .;,

( A ~ yj

, - ..

... - $I ~

" Figure 3-1

~-

SILICEOUS SHALES

DIATOMITES

The characteristic electric log pattern through a diatomite bed is as shown on Figure 3-2. The resistivity curves show low values and approximately coincide, as they do in porous argillaceous shales, but the S.P. curve looks similar to the curve obtained opposite a uniform sand.

The reasons for this behavior become readily apparent when we consider the factors controlling resistivity and self-potential. Diatomites are very porous but have little or no permeability because of their fine grain size. Consequently they contain a large amount of format~on water which cannot be readily displaced by oil or gas or by mud filtrate from the bore hole. This accounts for the low values shown by all three resistivity curves. The uniform negative departure shown by the S.P. curve results from the uni­formly high porosity and siliceous composition of diatomites. The S.P. current circulation at a diatomite - argillaceous shale contact is, therefore, similar to that at a sand-shale contact.

An electric log pattern of this type shol.ld always be checked by cuttings, sidewall samples, or cores, however, because a uniform, non-argillaceous siltstone shows a similar pattern, but siltstones usually do not maintain the same uniformity through thick beds. Another check, which should be made if cuttings or cores are not available, is the _time interval between pene­tration of the bed and the logging run. A permeable salt water sand will quite often show little or no invasion effects on an electric log run a few hours after penetration, and will, therefore, show an electric log pattern similar to that of a diatomite.

Figure 3-3 shows an interesting example of diatomite interbedded with clayey oil sand. The thicker diatomite beds are easily identified by their dis­tinctive electric log patterns similar to that shown on Figure 3-2. The thinner beds would be difficult to distinguish, however, were it not for the density log. The unusually low bulk density of the diatomite is ascribed to the low grain density of the diatom shells and the high porosity. Diatom shells are composed- of hydrous silica which has a grain density of 2.0-2.2 gm./cc. compared with an average sand grain density of 2.65 gm./cc.

2

DIATOMITE BED

RM =2.4 OHM-M

AT 84°F

SOUTH BELRIDGE

CALI FORNIA

~ c-, , ~

... l -

8

0

§

~

I

A

+- § c-

\-,~

i

SP I~ol tiN +

-" ,." AM -10" '

• - --

- . .. .. , ~ , -

I

I I, I I

,

0 H S A~ •

AO = 10' --

-

..

-

AO = 20' 1= -

- - - .. . c- .

o 0 H S ~ ~ ~

Figure 3-2

l'lo -- f'I':)5r- .-.71

~ t-<

/

- - 1 - F" -t- - J-

T , =t\

,~ -' ,

SAND t--, I- -

, ,

" -:--! , ,

--

~ I

R::

-~

i= -.:. -- --j .

T -, t---l3i - -j - -

- e- -. j J _ [3

" --- -

fo ~

--+- - i g ~--l--- - -.1

I" r __ ! !r-r --l~~-I n..1 '- I : 1

1---', __ j " . ,

! ! ' f'I

I. 7J J.!O

Figure 3-3

OPALINE SHALE

Upper Miocene siliceous shales in California developed in large part from diatomite which is still preserved in some localities near the basin edges where subsequent deposition was minimal. Deeper burial and solution pro­gressively changed the diatomite to opal and finally chert as the end product.· Recent grain density determinations from cores (1) and denaity logs from wells(2,3) have shown the opaline stage to be more widespread than was previously recognized. The reservoir quality of these shales in some areas was undoubtedly enhanced by the microfracturing in the opaline stage of alteration.

Figure 3-4 shows how these opaline shales can be distinguished from sands or chert by comparing the density log with the S.P. curve. Normal sand response is apparent in the top 120'. The caliper shows mud cake, the S.p. indicates sand, and the bulk density averages about 2.2 gm./cc. Assuming an average sand grain density of 2.65, the average porosity would be 27%, which is a reasonable figure for thst sand. The next 200' looks like normal Silty shale on the IES log and the bulk density averages about 2.15, which is a reasonable figure for silty shale at that depth. Below 4650', the top of the siliceous shale section, comparison of the S.P. curve with the bulk density shows that the negative deflections on the S.P. correlate with abnormally low bulk densities except for a few high resistivity - high density streaks which are either hard sandstone or dense chert. The abnor­mally low bulk densities are now known to result from opal in the shale. If we assume an average grain density of 2.2 for opal, the total porosity of the more porous intervals would be in the range of 25% to 33% which is con­firmed by core analysis. The effective porosity, however, is considerably lower because the matrix porosity of the shale probably accounts for more than half of the total.

(1) Chevron Research Project 14,706, Dec. 31, 1962.

(2) Geological Item of Interest No. 94, Producing Item of Interest No. 38, "Formation Evaluation of Frsctured Shales - San Joaquin Valley, California," R. R. Johnston, W.O.!., 1964.

(3) "Application of the Density Log to Fractured Shale Evaluation," J. C. Wells, Formation Evaluation Committee Report, April, 1964.

3

OI'ALI NE SIIALE

CAIIPfR """'''''' ...... _1 I US Ii 1 ".1.1 r

0/0.""."" ... BULl( DENSITY

-, .

.~ ,.,,' " ·H· . . . ...... "~~, "":';.:'~;

-+

Figure 3-4

FRACTURED SIIALE RESERVOIRS

The characteristic electric log pattern of fractured shale reservoirs in California is as shown on Figure 3-5. The S.P. curve resembles that pro­duced by a finely interbedded sequence of argillaceous shales and sands of varying porosity or clay content. and for the same reasons. Given similar relationships between mud resistivity. formation water resistivity. and streaming potential. the resultant S.P. curve will primarily reflect differ­ences in porosity snd concentration of clay minerals. In the case of the fractured shale. the more siliceous or cherty beds will also be more brittle resulting in greater frscture porosity.

Resistivity curves in fractured shales are usually very erratic showing extremes of contrast opposite what appear to be similar beds from the S.P. curve and lithology determinations. Unfortunately. for quantitative evalu­ation. these varistions more often reflect erratic changes in formation factor over short intervals. because of the irregular nature of fracture porosity. rather than changes in type and concentration of formation fluids.

Wet sands interbedded with siliceous shales can sometimes be distinguished by comparing the resistivity curves for invasion if the sand beds are more than three or four feet thick. For thinner sands and oil sands. a wall resistivity log is useful if the hole is to gauge because mud cake does not form on fractured shale unless the shale is pulverized sufficiently to approximate intergranular porosity.

Figure 3-6 shows a contact between siliceous shale and dolomitic shale. The dolomitic shale has a lower matrix porosity and is not as brittle as the siliceous shale. Consequently it shows an electric log pattern intermediate between that of siliceous and argillaceous shale. The density log. however. shows the contact quite sharply because of the high grain density of dolomite compared to that of silica or opal.

4

r

'-

FRACTURED SHALE

ELK HILLS

CALIFORNIA

-. >- r--

~ ·f· ~t:. · . ~ · . ~

f=~~ - 0 -~I --. - f- .

- ~I . f-t--I-

-= ., :Ii - ~t=

..

= t-o

- ~ I-

-

~ f-f-

-~ = · . ~, - - -j-H-

" '" :r 60'.

=i ; =r~ ~

-j ,

I +-=!.

-

J-

I:J g 1- I- ! .

-. ~ 8 ~ PECI N PruHAr.11

" t 8

., '

" .• 1."

'r;f<:k' "J .. - ,', f- ·,I~

~ g I~-'

.. o g --

,,; .

<,

'"

~.

.. '

a'I • 1 o o

'. "" -'. ," "

" , ..

: ......... :.. .. ........

~ ...... : .. \',

Figure 3-5

I~'-' ,---1---

---,' ,

I --,-'

L- '-.---' .

-' --.

, _. ---_-·0.- __

SILICEOUS ANO DOLOMITIC SHALE

'.=~- ~I ----,- _ ..

~ -.,..,.--~ ....... "'_:~. __ _ .0':"_

:..:..:;,.:;;:.::::=+~~-!.- .. ":--=::-

.. ---- _ .. -::::::::::j:=:::>- .' . .......

t- •• ~.

~ntCt4 D£fOITY 'lOlCf.U.J.T !l2f9'. UlINCIOU Wrnt" ClJCtO p IJ.O

I CCIIWClIVlfY," AI> CiOIlIIALLY OoUiu ! ~~.~ SIP-~ CVTTI.JQ ;tn~ ~I

......

" . __ .".

,.,. I I I I

"'" ,

-__ L.

- f-t--+-;-f-.

~r - .. .L~ . , - "~ - -;- ~.'~

- -i.-t--t' - rl--~ , ,

.t.... ~ , . . . .. - c....: .c. , ':? :

i -;-f-- -r-...,... , '- f-- .. ~ ,

'- j , " . "'" I-'~ f-. f- -'- ':: ~ - I-~-t ~ l- f--- .;

f-. f-- .. ~ c-I-~-" R=-~

~:'"

i-l---'<'!C": ~: , H-'1~ C+' -,--.. j[

; , I 1

+" .-+-t.-,

'T -,

+ -i.- I

~- .. - -4 .•

~ , ~ .. ~ , , -, -; , -"" I

. . -'--~ .. .... ::r ... _. - -....; -+ '"1' , .. .. -I-~ Y :

....J: .. _ - .. .. , =-I

1+ - ... - --, - -.-- . :.-

c- ,·1-: . ~ .. T c..

t:-,::-=t= ~t±; --;

f-' , ....

i- 8- '1'J = __ {lL.K O£:sax. , Z5 ,2,~ .. ] ,: . i2.~,-i--'r.s:

Figure 3-6

SILTSTONE

In some areas, siltstones may be sufficiently clean to show electric log patterns similar to thick sand sections or even diatomites if the log is run soon after penetration. (Figure 3-7). Cuttings, sidewall samples, or cores are necessary to distinguish this siltstone from sand. If the log is not checked against samples, serious errors in sand counts will result.

Note that the resistivity recorded by the 16' Lateral curve is lower through the siltstone than through the shale below. This indicates either a much higher porosity in the clean siltstone or saltier formation water.

5

I

SILTSTONE FILLMORE AREA

CALIFORNIA

CLEAN SILTSTONE

ARGILLACEOUS

SILTSTONE

AND SILTY

SHALE

......... _--- ~.~ . --

-,-'" -

j-----­

. -=~---~-:-~:.~ ~- .. ~ ... , ..

( .. - ",. -.. . -

c ______ ," .. , -~--

~::.-: : ~. ~~::.~ .. ~~~·-~~M~~M~~·,~~.~--

~

r--... ' .' .... _ .... ;1=:..:"-. :.: . "'--~. '.:-: - '~~.:'=

f-I-f--....... " ...... -

.- t· - .. -.-. -.

.. -- . ..... .. ...

~ ...... ~ ..... JLj .. . ~.::::. :~' .. !.: :.': :

4'i!---" ~ .... -- ~.-.

§~-'~:::E~~:' -][ .. l _ _ ..• _ .

. . . - .' T~ ti' Pk"tJ ..

"'" ~_(()~$.f" r-(f/-'!'4/ ~_/~ IV/. ...

Hc--' .••• _ I

.: i § .

I- '--- .... : ::j ,

! . _ ....

·1 I

.' ----- - - - 8 f- -.-.. 1 -~-- .. -.,. f--_. __ " ____ 4 ._ ••

--"--- ......

:-'-"-'--.

~-L-"~~ Hi-- -' '.

i--'-" I ~.-­

. --'

~ . f=-::: I-~' r-- .

I-f--'.' I-r- . 1-1-"

;:: I- f--_ ..

! .

, .'

Figure 3-7

BENTONITES

A pure bentonite always shows a positive departure on the S.P. curve (i.e., to the right of the shale line on the hour glass type log), and a resistiv­ity value lower than the average shale value on all three curves. (Figure 3-8). .

The reason for this behavior is the super clay effect of the montmorillonite group of clay minerals. The minerals of this group absorb more water than the kaolinites and illites and are even. more selective to the passage of ions.

The modern induction-electric log combination now enables us to identify bentonite beds in sandy intervals as shown on Figure 3-9. The high contrast in conductive (low resistivity) formations on the conductivity curve causes bentonites to stand out from ordinary shales even when interbedded with sand, provided that the sands do not contain super saline waters. Maximum water salinities in California are about 3000 GIG or 50,000 P.P.M. Conse­quently, sand conductivities do not approach the conductivity of bentonite.

6

BENTONIT ES

THORNTON

SACRAMENTO

CALIFOR

AREA

VALLEY

NIA

t3

,

-

= -

= : --

= = ---

1 -... 4 ~

Q;;

I '" § .~

.. - il

i

i

i

-

;-

--\

1- -

1- f-

Figure 3-8

I :J

... .. 8 f-. - - ~ -- f-- .. - - f- - f- f- . .. -

) - . 1--- ~. I- - -- - ... - -- -

.-. . - " .,- - -- -. f- f-- BENTONITE --"'-

. - - . ~ ;;.--- .. . - - '-

~,~ . - .. - ..

... --- . .... -- f- - -- -- .", - I --- . - .. - ~.' . - -- f-- · - f- --

.. - --r' - - - I--~ I- 1--- -- f-~~ -"

-- -~ .I ~

,-- . -' .- 0 _. f-- .. ... - - --: l- I- -- - - l-.. - , .. f-- ---c - f-- -- .- J. . - - - .- - 1- .

If. - - - c-. -~ .- _. - I- . .. -- ... - ." ... -

.. -- ... ... i 1--- - ~ .. . r-- _ .. ;;::> .. - . - . - · ..

- . c- _. ~ r.-.- .. . . . . j" _.

" _ . - - . -. .. - - ..

... -- . -~

....- ... . - -i:'" - - -

J .. ( §

- - f--. - . _. " . .- < • - f

... f--..

J. . . '.~

- ... :.;,; .,/ -- ._- -- - c · - ...

..... ~ .- - • -- ..

~ i--. -;=; - - - --k ~ .. " '-- ... . . +. _.L~ ' . b: -.... -_. -- -f- - - - ~'. - lj. ~ v" -. .- - .. ~' C- .- - · -

-- -- -- -f- t -..:' '. • b . - - ~

"'i .. 8 •

~ .-0 .,r -. = .- - <~ 1 _. -. - . .

.. .-- - - BEN 'rONITE

l. ." .' 1 ,/ .. - -- -.

'.\'" .. ..... _.- .- ~

.~

lj' .. _ . r' .... - - - -- y - - _. - .-. -. > - .-. I--- .' -f-- - . - -- f-. - .- ..

~ - .

.- c - .. -, . . - .-. .. --.. -8 . . .

'7 - -- - _ . .; k

- - - .. · . ~. " - - .. .-

.' y

.- - ......, C .. . -~

.. ._. - ..

- -- f-- ;.:.:.: . - - .. - f--- .-

_. -- .. <\: iL- l- -.--

c::: ..... . - C- .. -- ? ",- .. --~- · - - . _. .. ~-. .. - f-. --. .. II ~

- _. _. --c-- - . ..- -- -, • 8

-.- -- .... -. -- .•..

16' NORMAL INDUCTION IV v ,U ~JU 0

-1-1 , 0 1O0

INDUCTION ~--........................... -.............. ?~. 0 ..l92...

SPONTANEOUS· POTENTIAL 0 RESISTIVITY CONDUCTIVITY m ~

100 millivolts ...

ohms. m'jm millimhos/ m 1000 ili -Oh";i.~

Figure 3-9

)

J

n

BENTONITE

Microcaliper MicroLog Microcaliper MicroLog

-

F~

~

~ li -

~

~ E

Ef= ~ F 1= ~ ~§

1= PIERRE SH.J I

~~~

•

DENVER BASIN

COLORADO

(r"O~'" t.ly /-.. 0 .... Y,-ehl:>

Fre{ A A." e",,)

Bentonites can often be recognized by observing hole enlargement with corresponding low resistivities. The Microcaliper-MicroLog or MicroLaterolog surveys make good logs for this use.

Figure 3-10

SANDWICII BEDS {Finely Interbedded Sand and Shale}

Finely interbedded clean sands and shale, where the individual beds are less than two feet thick, show an electric log pattern similar to that of silt­stone regardless of the fluid content of the sand. Figure 3-11 shows an electric log example where, in the cored interval, the individual oil sands are less than six inches thick. The interval was completed for an initial production of 80 BID oil. Figure 3-12 shows an induction-electric log example where the individual gas sands are probably one to two feet thick. This interval made a good gas producer.

Quantitative evaluation is not possible where bed thickness is less than three feet because the true resistivity of the sands csnnot be determined with our present tools. However, we can detect thin, permeable sands with wall resistivity devices and short spacing velocity logs. Then, if mud , logging or sidewall sampling indicates hydrocarbon saturation, and the sand count appears to be sufficient, the interval is further evaluated by a drill stem test or a production test.

7

I

... t;;:.

- I~ .. .... --,' '3 i~ ..., . c::.

A .

Q..: 1

L'd

'.

l.!!I'

SANDI!'J ell BEDS WIIEELER RIDGE, CALIFOIINIA

.1>

•

F.... ~ I" ~ li r-:.

"' < ~ L

I:s: ~ 1'1 i

1< ~

4

- .--

~

'" ID o o

g o

... ?

:' K. " I l

~-'

.~ . " .. ~ -,

'y' -. -.~--

,r.. ;'7" , .... -'" -.- . __ ""': -

.i .-' .. t'

I ." 0"

"~

~ .-.... -l-~~~-t.;~r+H--t--fl~rl-1 S __ I---P!Id-+-t--t-li-t--t--t--t ~ ~.~ ~ -- ~

&~i-t~;/~,'4--r.I_~~r.~.1-~-I~--::~~~~'~~-t-l-r-t-Ir-t-;~

~ P( NT~N~O, S: ~OEt n L - .tlm Iii, of:s +

R~SIS~I~IT' • ~l>hl\l1s. I~I m

b : , , : , t ; ;

21 t'

Figure 3-11

j

SANDWICII BEDS SOUTH TEXAS

16" normal (0-10 ohm scale \-

1111.1111111 Induction (0 - 10 ohm scale)

I -'--"-~ -- , "

-+- -r- -

I

','

r::-Perfs @ 11,262' - 320' , Flow gas ARO 4650 to 4750 MCFPD

on 1/4" ck, GOR 5-1/2 to 6 OO/MWJF. Sonic porosi_ty 40%; SW 54% Rwa anomaly .4-.2

" , ... -' , , , I I

Figure 3-12

~J

7

GLAUCONITIC SANDS

On electric logs, glauconitic sands cannot be distinguished from other sands with like permeability and porosity, but can be easily recognized on gamma ray logs because of their high natural radioactivity. The high radioactiv­ity value results from the presence of radioactive potassium, K-40. in the mineral glauconite. Figure 3-13 shows a typical example from the Lower Tertiary in California.

8

PRESSION RADIOACTIVE EX

OF GLAUCON

GREEN SA

ITIC NOS

RIO VISTA A REA

CALIFORN IA

§ 0 ~ t.J t.J Q:

r • ~

GAMMA RAV~ rf~ NWTRON ~

RADIATION INfl"mYIHtft"'" ~fl RADIATION INHN"TY INC."",

~ ~~11fffi:J'! . I. " .. - -~ --r-~" i" ... j.,.rJ---;-, + - , ... : i'1-1" . ~ -,-'fH··· +W~ , . Tr ,. · ., _. .. . ._. _ .. . t-· . , ., . I ;~<r- I--r-I·-1·-~·f .j). -, -+"- H=t::-.~~c.::1-~±- -j-. "i~ t ' -', -, ~-H~r -.I,

_@r'-'~d · ." __ -:--1:.::-.\:_, r: . .'-:- ! .. · ~~ ~'l B-~-1 :.;'~ .• 1 ._--f--, ; -i . ~t=4 ..

. - '- T" -.~ .. 1S ;'-i .. 1-- :- ~- --1.-I<l 1:1'-' '-1 "'IS + t'r ' '-I-- j_. . .

f1-t::r '--'- , . '--r- ,c'[-' r-'~ ~. 'r- -f.- , I"~ , -! .. -,-+.;~ }, '::r~i!'

-T j _c_, --r-I" 1- - .. ·~-t,~l~

,=.~ or"~ , L 1--- I- -1-.- I

i' '-, 1-. ,. - r--c-- ,

, -1- ' .. 1

' . "

l- I- -+ .,

-e- -ti .. r -J - - . -\ .-

=1--' ,=t _, ':--,+- "1- --I"

r- ,-I- - -I· . '

~ ~ -- l'l~ .. , I

$.J:i~ - ,- 1-- >" -+-1---I- -- ... '~ -1 '

--- "'I-.i-· -- '. "J I>: -.. I-

L .. r. h 1-1- ... kf'

--f- 1-' - -- 1'-.. , ~-- I-I-

- ,

, -- r- -- --""1--

· ! -" 1"-1 , - "1 I

.:..j.:.' .- t-- Iy'c -r

~ .. 17 .--t .

-~ l-I- ... j- ..

-- r;. l.- 1-'" -__ I ••• ~

- I-I-t-. "'1-

·-t+ I--.. - t~ " : 1-1-1

r ~

l_f- .-:. ,., .1 I 1 • I . I I: , •. L

r!--} .. I':':;!

--.....; .

,

~....(.! .,-, ..

, I I , I

I , ,

I

~ ;.;-_~_~~~ -4

"~i '"

. , , ~ I I

, 1 -,

, ! I

Figure 3-13

'----)

\.....,)

ORGANIC SIIALES

NODULAR SHALE

The Nodular Shale is a subsurface term for an organic, phosphatic shale 'that ~rlies an unconformity at the top of the middle Miocene in many parts of the Los Angeles Basin in Southern California. Where the middle and lower Miocene are absent, the Nodular Shale rests directly on schist basement or is separated from the schist by a thin basal sand or conglomerate. (Figure 3-14).

The Nodular Shale is a black, hard, fissile shale. It is highly organic (up to 10% organic carbon by weight) and contains bands or nodules of brown phosphatic material. The phosphate mineral is predominantly apatite. Car­bonate, usually dolomitiC, is associated with the apatite and is also present in the non-phosphatic portions of the Shale. Core analyses yield a range of 1 to 18 per cent carbonate by weight.

The high resistivities of the Nodular Shale (20 to 1000 ohmmeters) are attributed to its high organic carbon content as are the low densities,and velocities. The S.P. expression varies from that of a normal shale to a siliceous shale with irregular negative departures. Comparative properties of intervals with differing S.P. are as follows:

Resistivity Organic carbon con~ent Density Velocity Drilling rate Clay content-

Negative S.P.

Very high Very high (12+%) Very low (2.1 gm./cc.) Low (7300-11,000 ft./sec.) Fast Low

Shale-type S.P.

Moderate to high High (5-9%) Low (2.2-2.4 gm./cc.) High (13,000-15,400) Slow Moderate

For additional detail on the Nodular Shale, please refer to W.O.I. geological report, "Sentous Study - Inglewood Field, California," by W. J. Plumley and T. L. Wright, dated August 1964. The above description was taken from this report.

9

j NODULAR Sf/ALE

LOS ANGELES BASIN, CALIFORNIA

lI'OHflJlfOOS I'OffHflU millivolh

RfSISflYlIY COHDUCflYlIY ohm.-m'/m millimhos/m .. ~

A'''~'M ".".."

" ~ HOI.IoUol I' •• a ItKMJC110H •

'H' -"OIJetlOH

~ __ .... _ ....... _ ... .e.

,~1 ;1-,':-{ ;(,

'I \" , II I ; t

~ -I .: ; I I 1 , '. ..,-.

.. ,/. .-r.,o' " f :~> ' ". " " , i '-<.!; , 0 ~ : •• :,.

, '. !; ""'-J ~"

1 i!' ~ ~."~"-'~--'~~'+---'-'~f'-'-'~' ': ::: ~ 'I' "i. ~,'~I" '~ : :'<-:,~ . L I, ,I, , -c' >-: !; : i ~ 'Jf--:---' .c:+---:~ ~.-.:--l

I, ,j ~ J ' .3-_.;- j " c::s ,'/ ~--+~~-, '. ,. ')

0, 'I ~ ~: ~ :~" ' Ii: : ,¥, I ,~,_: .. ---£ ,_ , 1 ' ~ , i' "'" ~c <;-"' '.~:."--' 'I :!: ~ itb : , ~ : , . :.~"::' , I ! <..3 (. <- .... __ .

'~:+ll,+-Ii f-+--ri -j ~ 1-,' 1---+----'-:-;; :?: r<:-, :=:-1,

" ,I ~.. . j ~f=-I----r~r_j ; -~--~--~~r--l

• --L : : ',roc";£t..==- I =~ I j i 1 !" ~ --;~'-:il"~"""~=C'-1---' --r--: -:'-o'::"~f~'

itl ",I ~ ,I{':'~~ ~' " " i':::'~=-= , II .' j ,- '.. . ~ I-:-ri I' +i-"I~-rTt-tI,,-.-j, ~ ::, :'~;'~ 1 :~~ 1_' '-j..l"'+-f=-I-l-+++-l ~ I 0 ','~,===5

, 1'3 j I <::5 [:,:5 ' ,~. , ..>C, , . p , , " :;e; {i' r-:- ,..--,- .. c ' -~"~~.i' ,::: '---'- f~~,""" -:--'-,' ' . 1 C~_. o u, _I P'"_ ;., I .. ~

-.,1 1: ~ : ,I~' .1 .~ ,-1- '~:t -:;..;-;- , 'I ~-i't-mm~ ~ II ,'L: -" II IIIIII~I";

: I

CALIPER HOI' Di.t.M.. IN ....o.r.

_~ I •

m

-{! ______ e. -- ___ 'Jl,

CORRECTION I,

GA.UlJ,/C(

BULK DENSITY

J,

--~t---5I-""--~""r---l '.~~ .;~ , ,-",~ .. , . ..l ~' , ...

Figure 3-14

<:. / Yt. t;./ Ir\l~ I D - T t'r8 N R 'U W J L,,- (,; fo .. 11.1 "-Co INA fQ,.I1.Y Jl.p v-<y" ,

CARBONACEOUS SHALE C!'<It! K #- ~ I ;\'I<..K,I\ztei (,,,,,-1-'1 I

SLIGHTLY RADIOACTIVE ARGILLACEOUS LIMESTONE

IIIGlR.Y v, . •. '0

CARBONACEOUS SHALE

RADIOACTIVE SHALR ~

Low radioactivity from limestone end dolomite ~' or sands.

I

Cerbonaceous ahales can be recognized by the abnormally high gamma ray activity. Laterolog to right a180 shows higher resistiVity in carbonaceous shale interval than in the other shale sections.

Garrana-radiation 8

I

I

Laterolog Resistivity

LODGEPOLE LS; '::'1::

BAKKEN FM.'

THREE FORKS FM.

1'-"

",y "111 I FM •.

II SII.

~ DUPEROW FM.

1-1-1--1--1--1-­I-I--

-

WILLISTON BASIN

NORTH DAKOTA

Figure 3-15

)

COAL BEDS

Coal beds are normally highly resistive with little or no S.P. development on electric logs. The electrical expression is, therefore, similar to other nonporous rocks such as dense basalt, cemented sandstone, dense carbonates, and evaporites. They can be easily recognized, howover, if a velocity log or density log is available for comparison.

Unlike other nonporous sediments, coal has a low velocity, about per second or a travel time of about 140 microseconds per foot. of coal is 1.0-1.8 gm./cc.

7,100 feet The density

The accompanying example, Figure 3-16, shows the comparison of the electric log and Sonic log opposite a relatively thick coal bed. The density log can also be used to distinguish coal from other high resistivity rocks if the hole is not bsdly eroded.

Fractured cosl may look like a good oil sand on an electric log, and if fractured sufficiently to form a mud cake, it will look like a sand on the microlog. (Figure 3-17). A velocity or densHy log is therefore essential for proper identification.

10

n

COAL BED ELECTRIC LOG AND VELOCITY LOG

POWDER RIVER BASIN, WYOMING

Sonic S P

-- - " Resistivity 140 microseconds /Ioot 40

0 f--- _ --i-- -_ -0

~- F"'i - _ r: - -_J--' ~

-~ - -- --f--- c- - - 1- -- -~ -

==1:::.-ft -- '= ::..j= -t::- f--- f---,

+ - - -I--- ::- - --- -

= = - - .:;- :- - - ,F- -- 1= 1= :-:.:

- - -:- : - --

- - -- J'.. - - -- c._ r.:: ---

I-!= =-1= -

~ f--- --

1-- -1- -- - -- I;::: == 1= -- H'

~~ - .. _. -

I-f--- = /'-' -- 1- -- --=: ~ - '-J--:- r:::. I=-1= -

1= 1= --~ ~ ~- - - 0 -1-- --

- --

f---- f-:o I:-H \- --- --- \-- :- I- --- - -- c:-- ~ ---- ---- - -, 1= 1-- ~ I- -- --

i, 1- 1= f--- l-'-~ • -7 - 1-1-~

- -- 1- 0

0 --Le:

0 - -

1- -..J I- I-

!-- -<t: 1-0 1-(.) I-Ll F- j-- -

1= 1= I- I~~ ::{ -f--I- I-- IS ---I~ =- i= 1-- - .. -

00

~ 0_ - --00 - 1- i--' -'; 1-' t:---

~o

- :.-:.: l-f--- 1- - - - - -

1= ::':: --t=- =::: :- 1-- _0

-o-

f:- I- I-- j~ \--

1--1- - 00

-0-

1-- S-

- 0 c-~ .. l::::::=

.- .. - 0

- - -

Figure 3-16

COAL TERTIARY FORT UNION FM OF THE POWDER RIVER BASIN

BIG HORN CO., MONTANA

This is an example of a fractured cosl bed displaying SP development, filter cake buildup, and positive ML Separation, all of which are similar to overlying and underlying sandstones. Lack of a velocity log to differentiate thi. coal from sandstonea. and lack of good aample quality. could lead to the aieiuterpretat!on as sa oil bearing sand in view of the very high accoapanyina re8iativity. A g~ methane gas kick accompanied the drlllina ot thb coal. /1\ e If: 2. Z ~ -'f-r.

16" Nonaal. Induction

(

Hicrolog Res is ti vi ty

10 fool ~v o _n

HicrOCal1per

R fl 0)

- - -- - -. --=:- .= =- :-1= =-

.~ .. -.

I J'>.~ =c _: c:c __ :: .:::

1""-':: -:: -=-.:::

-1< 1:-- . _. i: P: i ,--i--! -

•

I

•

:=:

~i?, - 1=:: -.:-::

_.t<='.:_

: ~F ______ .1:::

r=­i=

,. F

Figure 3-17

SANDSTONE REEFS

A high resistivity intervsl in a sand-shale section with low resistivities above and below msy indicste hydrocarbons, a fresh water aquifer, a coal bed, or unuaually low poroaity. Figure 3-18 is a good example of the latter case, and shows why porosity logs are now run in all exploratory wells.

Aside from quantitative evaluation there may be geological significance to a porosity change such as that shown on Figure 3-18. In this case we are looking at a calcareous sandstone member of the upper Miocene Santa Margarita formation which is probably the cliff-forming Ostrea titan-Pecten estrellanus reef exposed in canyons north of Coalinga, Californis.

11

-',~;tJf;~j~~:, :~ " SANDSTONE REEF

SP.()NTANEOUS POTENTIAL 0 RESISTIVITY g INTERVAL TRANSIT TIME millivolts ~ ohm.- m'/m 8 MICROSECONDS PU fOOT

• 016"·M T_'.-2.' • 10 0 SHORT NORMAL ,n 11<0 11l'L--_. __ - .5.Q

-H' -_.

0 200 2'" 200 150 -,

INDUCTION P- •• -•••..•...•••.••.•.•.••..•..••.• ~P 0 200 tr···········~1'l1'· ;;'. it:··········· ~

I IJ III IC "1 , , -, I ,y ~.-~,:?

.. , , , 1-' ~

, I .. ---, , , I . .J~. , , '" u u - , ; ,

! § I'- § , ~ -I 1/ h

, : ;- , h ,

In 7 ='1 15-f-'-I , , ; ! I , I : --ri' ,

I , , : , r T ~ ; I " , ,

I i ! ~ . ; I I : : ~

: ! u I? ~ I- u • , , ..;.? § §

, , ., I '1 1 : i : 1«· ,':;:> h- , I , , I <'"'

:7; I) : , I , , ~ :>; I I .J

/ rr I c: '< ,

; .. <:, J , . ,

.-"' . -'--. -..:;:: ; , '. -"' , I ;

. / I i i ..

-S ~., , , . ..., ; ; , -, . .. .. - -

! , I _. -: ; ) 1 ~I , - , : ~

"" l:\- I i , 7. I: .! . ~

--; I i> , ,

__ .3 I- --, ! § § I- .~ , : l- I

i , , I .. " .. -

I ;- ~

-, I ; ----- .. : Ie - , I

: , I i I T , , :-..:! Ee-' ... I- 4 r ,

,\, 1-0" ; I , ~ . ''Y DI.J .-

I I .. ,. . . J • ( • , -, 8 I ~-I- 8 . 1 . -- ~-.---i .t;.; I ; I •

~':::.:' ~:-== -,; I I '" II i~ __ , ; , ~

I " ; h~- ~.f-.I-.I- ...... !---. . .-- -. F::. ...... (. , I I ' I i"T Ie- ~_:.J. -1-.,-·H-,··..,-· 'J

k:':: _ ~ . .:.:. . _ I ...... I I I ... ~ . 5L..."7· :.-.-!-...... -.~ j:-. ---- .. _.1-'-, I

I t-..' I I I -1$- -4_" _~ .. .. l-; .. ~ ..... 1:.. ........ - ·-----1 I .. -. - ,

Figure 3-18

4' ., ! ~'

~: ~ f . "

r

CARBONATES AND EVAPORITES

LIMESTONE VS. DOLOMITE

With the advent of the continuous velocity log and the density log.we can now distinguish clean limestone from clean dolomite by comparison with the neutron log. In a carbonate section the velocity log .and the density log are affected by both lithology and fluid in the pore spaces whereas the neutron log responds primarily to the amount of hydrogen in the rock. In pure carbonates all of the hydrogen will be in the pore spaces. Conse­quently, the lithology do~s not affect the neutron log.

Figure 3-19 shows a comparison between the neutron log and the Sonic log in the "B" zone of the Aneth Field. Two intervals, 5491'-95' and 5537'-44', appear to have the same maximum porosity on the neutron log but show much different values on the Sonic log. The upper interval is limestone with a matrix veloCity of about 21,000 feet per second, whereas the lower interval is dolomite with a matrix velocity of about 24,000 feet per second.

12

•

c./ (W /f\.W I

i

i

I

\

'1 - -r 1/-1 5 - I< 2 It E ~+I<".t... .. d.. 0 i " # 12.-~ fJA..ltfl.J'o k

_.

LIMESTONE

DOLOMITE

:

ii I I I I

SU.NOAlO (O\JHT, MCOHO

. ~. E

.-

--

. .... ~~

I I I

SONIC LOG I H U lV !,,,,~, .. !..~.A ~!! tTl", I

_1 .. ·.­• _1. ,'- ,.,

. ··cy'" _ ~ .. r=

"",,»<Y . ~.QlL ~~ Of '~lfIWHJ.. SwlC ,. SU' n.u~ .. .. .. WIll ''V' to ,utI,. J t . .u·, SW$C TO HiS .. ~))l

'''''"'' i.tN JUJ!f ... _IIAH._. UL<IL. •. Dill 10 562, 0<. " ... >I' '" ..

Figure 3-19 .,

r 1

1 ,-.'

j

I \ .. J

:

1-:.

tt

''I

I -

DOI.OMITE CAVERN ANADARKO BASIN, OKI.AHOMA

. :::-:...

HUNTON 1l0L(I!IITE

I I iii

, : SYLVAN LIMESTONE

•

,-' GAMMA RAY 1 10 100 1000 2.0

OHM-METERS

I:i r=

2.2:1 2.11 gm./cc.

Figure 3-20

VERTICAL FnACTVRES

Vertical fracturing in carbonates may be detected by comparing different types of resistivity curves with acoustic velocity and amplitude logs as on Figure 3-21.

The formation is Ellenburger dolomite in the Texss Panhandle. The Sonic log shows the consistent low porosity of the matrix throughout this interval. However, from 12,210-12,324 feet the ILd is reading higher than the LL8 whereas above and below this interval the relationships are normal. This is believed to be due to near vertical fracturing in this interval where the LL8 current is somewhat short-circuited by the mud in the fractures whereas the LLd current cannot,be short-circuited in this manner.

It will be noted that the ILm also reads higher thsn the LL8 in the upper part of this interval but in the lower part, the two curves sverage about the same. This might be interpreted as an inconsistency in the thesis. However, there is considerably more difference in the ILd and LL8 resistivi­ties in the upper part of the interval than the lower. The failure of the ILm to conform to the theGIY is believed due to the poorer resolution at higher resistivities of the ILm.

The fact that the Proximity log sometimes reads higher and sometimes reads lower than the other logs is believed due to the directionality and sampling volume of the device. If a fracture(s) is within the sampling volume (Le., more or less parallel to it), the "short-circuiting" occurs and the reading is low. If the sampling volume consists of matrix only, the reading is high.

This thesis is confirmed by the shear wave amplitude log and by DST. Sub­stantially more permeability was indicated by the test of this zone than would normally be inferred from the low porosity shown by the Sonic log.

13

, ' i I •• _. . -_.--- ............... --+--+-+, ........ . • I I . I:' , I .. I' .. , il" I

___ " .~_~. :1066 ;:,.., .!-~

--1:-__ , .. ! .... ' , ! ~

--'

. -,

, .,

0>' •

I ,

1

VERTICAL FRACTURES

12200

T

c z o ~ :!

.. , I CQmpr .... Waw4 St I. (m."J

_ ...... '=_,....; 7 S<iol< Scali (p m/ fJ 1 ~ __ ..7--~~-'-- -'"'

5/leQr WaY' t.mpHluch

'Compu'lioooi Wove Ampllh,dt

f I-~--~~--~--r.:~~<=:d=-~~r-~--~~--:--i--~~~~--~-i r.;

~

12300

., .,

, ,

Figure 3-21

· .,

ANHYDRITE

Anhydrite is a dense rock which cannot be distinguished from other nonporous rocks on electric logs. Its velocity (20.000 feet per second) is similar to that of low porosity limestone. The gamma ray log shows it to be nonargil­laceou8 with an expression similar to that of pure carbonates.

The density log. however. distinguishes anhydrite from all other common sed­iments. As shown on Figure 3-22. the bulk density is the same as the grain density. 2.95 gm./cc. The !lotted line on the density log shows bulk density from core anslysis. Although the anhydrites were partially cored. no analyses were run.

The example is from a well in the Paradox Basin near Blanding. Utah. The formation is upper Paradox of Pennsylvanian age.

14

Figure 3-22

';-' -' ,

SALT AND ANHYDRITE

Distinguishing between salt and anhydrite in an evaporite sequence can be a difficult problem with nothing but regular electric logs, radioactivity logs and cuttings. Salt and anhydrite are both dense rocks which show similar' characteristics on electric and radioactivity logs, i.e., high resistivity, no S.P., low gamma, and high or low neutron depending upon hole effect and hydration. Anhydrite can be recognized in cuttings, but salt (halite) is seldom recovered because of its solubility. If salt muds are used, as is customary in evaporite section, the effect of salt beds on the mud is negligible.

Until the development of velocity logs, the caliper log was the most defini­tive tool for locating salt beds. Figure 3-23 shows how salt and anhydrite can be distinguished on the Sonic log by their velocities. Rock salt has a velocity of 15,000 feet per second compared with 20,000 for anhydrite or a transit time of 66.7 micro-seconds per foot compared with 50 for anhydrite.,

15

.'

, )

~

, , ., '-~ -. y ,

".,C ",

~ '" ,

WELL A

TRANSIT TIME

.;M. SEC I FT.

WELL B

8 .. p_7 ..... 9 __ 60 ...... _ ..... 5P 81...'O_7 ..... P __ 6_P_,_5,O

I ! · I I . i --I

I I '

• I

I I I I

100' I I I

I I I I

I I '\ I • • I I

--------- SALT

,-'-' - ANHYDRITE

WINKLER CO. t TEXAS

Figure 3-23

1 : ,

r

SALT (NaCI) IIIENTIFICATION

Microcaliper NicroLaterolog

..

~ HOLE ~~ . ENLARGEMENT

.

~~

E~ l:i.

~.

PARADOX BASIN COLORADO

~= .

CHARLES FM.

WILLISTON BASIN

NORTH DAKOTA

C. /I\.W/J,tw J 2.. 2-HN - l4lP, TtLi! (AII-l'OI'fI1,'t\.. (o ..... pM.IA'I)

tt: I V;-e T,.,.' btj.1 I .,ttA.,..1.-te2v .... ~ I (OVIA-ty

H

~ '"

J

Sodium chloride has the logging properties of low gamma radia­tion, high neutron and high resistivity. However, all of these properties may not be apparent on many logs. The solubility of salt in drilling fluid may result in considerable hole enlargement. If the hole enlargement is greater than the depth of investigation of the survey, the drilling fluid becomes the investigated medium. The MicroLog, MicroLaterolog, Neutron and E.S. normal surveys may be affected by hol~ en­largement.

The upper example is a typical case where hole enlargement results in the log reading mud resistivity. The lower example shows very little hole enlargement by the caliper measurements and the high salt resistivity is recorded. The well with small hole enlargement was drilled with mud saturated with salt prior to drilling the interval shOlffl.

Figure 3-24

SALT BED IDENTIFICATION

The following is sunnnarized from "Salt Bed Identification from Unfocused Resistivity Logs," by J. R. Lishman, 1960, and for more detailed information the reader is referred to the original work.

ABSTRACT

Salt beds are almost infinitely resistive. They differ from other infinitely resistive beds in that they are usually soluble in the drilling fluid, and give rise to enlarged bore­holes. An infinitely resistive bed, lying between shales, may be recognized from the characteristic shape of the electric log resistivity curves; and the ratios of the apparent resis­tivities which they show. Anyone of the curves may then be used to compute the hole diameter, and hence decide whether the bed is salt.

When a washed out salt bed lies adjacent to another infinitely resistive bed in which the hole is to gauge, the configuration of the curves is characteris~ic. Apparent resistivity ratios again help to identify the salt.

.. .. .. * * Unless the hole diameter is known, the reading of anyone curve csnnot be used to determine whether a bed is of infinite resistivity. Ratios between curve resdings, however, can be used. In beds of low to moderate resistiv­ity, the reading of the 64" normal curve is never more than!£!:!!. times the reading of the 16" normal. However, in thick beds of high resistivity, the ratio is generally much higher than four. The R64"/R16" ratio is approxi­mately 5 for an infinitely resistive bed 80' thick and ratio increases with increasing bed thickness. For beds thinner than 80', the R64"/R16" ratio is less than 5, but for these beds the ratio of the 18' 8" lateral curve to the 16" normal curve ranges between 12 and 15.2.

16

----------T I I

1

'" 1

1 1 I I

-------------- -r -' I 1 I I

I I I L

I

''-.- -) I L

... L -' I

I 1 I ;

I L

I 1

I .L 1

-

BEIIAVIOR OF ELECTRIC LOG RESISTIVITY CUIIVES IN SINGLE INFINITELY RESISTIVE BEDS

• 1'* "*0111014 '000

I· ..' .0'.... ~ • ____ .. ________ !2;.~

I --- I _flL!.~ I I -- 1

II , - -

1 1. - "5 [1.[C11100£ 0),1 8(0 . -+-H--t-+-i

i I , --

" 'i-'

---- I - - --- -

_lItl'~ ! - ----

II 1 ' -1 1 " ,

" I , / 1 I " I II 1 1

J II .1 /

1\ .... ,,--++ - -, (\.((rllOO( ,,. no

- 411NOA Ihi 1[0 III' co J.

I 1 1 1 ,

1 ,

1 _I / ,

I 1\ ,

J ,

1 V/ -1 ',Ii"

,'.).

Figure 3-25

)

----=--, '.

~ • • o • · •

§, ·1

" , . , i I . , .~

o

----- ... .......... - ..

\ /' /"'----... /

'-- -t-

I-JI-- l-I- I- ~L

I-- l-

I-I-- l- I- I- ' '

- f-~ H H

h I----< f- f.- I--

I ~IIII~IIIII~

/'--:: -I I

....... J

W z 2 5 o o W :r: (/)

~ >­etl--

co~ I-­

W(/) ...J­et(/) :r:W (/)0::

et w :r:!:: ~z :>­o It. O::Z x­I-

It. (/)0 W >(/) 0::0 :>w om ~z OW ...JW ~

01-_w 0:10 I­o~ Wz ...J_ w>-

l:> z o (/)

0:: ..::( n. ::E

8

...J

Figure 3-26

COMPARISON OF TlIEORETICAL ELECTRIC LOG CURVES TIIHOUGII A SALT liED 1111'11 AN ACTUAl, LOG

lUI FOH TIIEOHETICAL CUHVES = ,05

flO

SPOHUHIOUHOlfHllAl a .. 1111",,11> I RE51S'TlVITY

ohm •. mt/m RUI$TIVITY ohm~ m'/m

R'O - . \I " \.

_1 ", '

I

»

I

THEQRETICAL CURVE S

......... " ....... . . . • ; I ...... .

o 11'.- LAY"."'- &0

000

' I ' •• I • I 1 , I • '/ Figure 3-27

POTASH

Potash (sylvite) can easily be distinguished from salt (halite) on gamma ray logs and used as msrker beds in saline basins (Figure 3-28). These logs are from two wells in the northern psrt of the Paradox Basin, Grand County, Utah.

As in glauconite, the radioactive isotope of potassium, K-40, accounts for the high gamma response. The average K-40 content in naturally occurring potassium is 0.0119~ as compared with 93.08% K-39 and 6.9% K-41, the stable isotopes. .

Sylvite, being a dense rock like halite, shows a similsr response to other dense rocks on the neutron log. Potash beds, therefore, cannot be confused with radioactive shales or porous glauconitic sands when gamma ray and neutron logs are used together.

17

, '-!

C.,

\

f\.~/;e H -'2.61 -70 ~Cf M,tJ--'+\~ r"+1'01 #(

Gunu.

. I~. • . . .... - -

-~

: : t ~'. ......

. . , : ! : : 'r .

.:._:_tll. ~ " Figure 3-28

VOLCANICS

Figure 3-29 shows the typical appearance of dense nonfractured basalt on an electric log. The resistivity is very high and the S.P. departure low. The S.P. curve also has the rounded, sluggish appearance which is a characteris­tic of dense rocks.

Unfortunately, however, dense, nonfractured basalt represents but a small portion of the total section of volcanics in many areas, and it is difficult, if not impossible, to recognize most volcanic rocks by the electric log alone. Pyroclastic deposits may run the whole gamut from dense rocks to sands and shales depending upon size of particles, alteration and cementa­tion, and flows may be sufficiently fractured or vesicular as to approximate the appearance of fractured cherty shales, fractured or vugular limestone, or conglomerates.

Consequently, unless the volcanics in a particular area are known to b~ solid flows, cuttings and core samples are essential for electric log inter­pretation. A gamma ray log, if available, may also help considerably because volcanic sediments normally show higher natural radioactivity than other sediments. Hydrous silica or opal in vitrir. tuff may be identified from the density log. (Figure 3-30).

Figure 3-31 shows an electric log very similar in character to Figure 3-29. In this case, however, two of the highly resistive intervals are tar sands. As explained on Page 19 and illustrated on Figure 3-32, a velocity log will distingUish tar aands from dense basalt.

18

l'O~lnl~~ 'OIl~JtU I ... ,tt..-oIto

VITRIC TUFF AND VOLCANIC SANDS WEST SI DE COOK INLET! ALASKA

._.l,"---..... '!...._--..!,-L_

'f$I$HVUY ..... "'.-""1'"

l' 1,' _ _ t..l ...... , •.••••..•• J..l.~~~

BULK DENSITY

7

.J ~ ....

INTfRVAl TRANSfl TIME _.,...c..,. ... 1OOf

\ Figure 3-30

r )

TOP OF VOLCANICS

I} i)' .. ., ~f>

E EOOEN

VOLCAN lOS

WESTE

WASHIN

RN

GTON

-

..

, .f-•

,. I'

, .

'1- f-

- f-f-

7 •

I''': :: I" - -f-- .. . 'I"

\- f-

i

• §

1/

f-

)

j .. fi I-

I- H· 1 - ~ - . 1- 1- I~ f> I~ H- ">

• 8 ..

- -

100 MV +

Figure 3-29

,>'

SIIALLOW., UNCONSOLIDATED TAR AND GAS SANDS

As shown on Figure 3-32, continuous velocity logs are excellent for identi­fying shallow tar sands and gas sands because of their abnormally low velocities. In the tar sands the velocity approaches that of tar itself as the sand grains in these loose sands are literally floating in tar. In the unconsolidated gas sands, the low velocity is attributed to a combination of compaction and the attenuation effect of gas bubbles in the pore sp~ces.

On the example, which is from Huntington Beach, California, A-C are shown by the mud log to be tar sands and D-K are probably low productivity gas sands.

Similar low velocities have been noted in partially aerated sands at shallow depths.

19

. J 1 ! '--~.

TAR SAND

TA R SANDS AND

VOLCANICS

OXNARD AREA CALIFORNIA

TAR SAND

(/)

u

z

'"' U

-I

o >

- ---.-.--+---- .. -. --. ~

~.-.

-, -. --­". r' .........

~.' r': -l-l---L--J..,!L.j-LT '~

t-- ••

," ,

1--

c'" ' , ' , I ___ •• J

, .

E

.. ~ ." ....... .

( I (I :

/" 1 -r'-~--.'

:'-: 'r i ' , , , . - <.. I ! I ,., ,,' , .. I i J-- ' --'-j : l ( ~,.~/ ' '-:- I · , I • I I

I, __

I. - , , . i) ~ ,f.j, ~--c'~..c.

I) ':::!===-i

, ,

,-=

--:- _.

-•

; -\' - -~:;oi -· ., . ! • . . •

: '.' I · , , " ' , § . __ ~.",JOOI~_ i " '): ;. .. I

I, .... - " , . . . i" _____ _

--- .

t ' --~

Figure 3-11

~fSISTIVITY ohmL ml/m

RESISTIVITY ohnu, m"m

o •• J;:l .0 '.. •• -. ,'~ '" '

o· •• r;.' .' ;+:~ '~.r! t:::P1,.,j --, ,

.

:0 Ie: ~

p ; l- I

... • '. ~: I

" • '-F ~ ... _. 1

'il! - . ~

I I

l- e-. j .. k' 0

~ i I • ;

:l1 !> 1 l-I--

0 l.- .. Po I

0

I .~

1-1"; .' 3·<· ... ' '.' I'

. ,:\, _ ... L~

S'OHI AHIOUS·POIIHIIAt mlillvoll.

r

I " "".~~O"!!.A':!.:' HM, --­...... ~~ .. ,.---,

.-j----. f-I- . - ~-

. '- H-,.-. L I. •

I I- . 1- .•. .. ..

I-'~ I- _. H - . ~

1- ! :. 'I"'~ . . v- . I~' .f-f- .' . - H - . '- -

Figure 3-32

10

,

o ,

7 •

MINERALIZED ZONE

Mineralized zones, which are commonly associated with volcanic dikes, can sometimes be recognized on electric logs by unusually low resistivity and erratic (usually negative) S.P. departure. The unusually low resistivity ia caused by metallic conductivity of mineral veins and the S.P. is affected by oxidation potentials.

On the example I-ES log, Figure 3-33, the mineralized zones are immediately above and below a volcanic dike or sill at 9315'-9470', and at 9940'-9970'. The lower zone may have been connected to the dike by a high angle fracture or fissure •. The vein mineral primarily responsible for the metallic con­ductivity in this well is pyrite, which was recovered in large amountS in cuttings samples from the three zones.

The geologie significance is obvious because any oil accumulation predating the dike or sill would be dissipated from the volume of sediments affected by the intrusive.

20

i I '-')

SPOHI AHf OUs-rO I f HIIAI millivolt,

{-

I ., , J

1 I . I i, L

I I '1

I +1', ··t··

[I .

. 11 I i

MINERALIZED ZONES

RESIS11VITY ohms, mt/m

o --- " .. NOIM4l--20 I

o

r

CONDUCTIVITY milJimhos/m • ~;.;.1,09!.,.

tff40 "'OUClION

·1

.~----~.- _ ..... _--.. r . . . ~ ...

. -. -". -

.L.~.l ~ ++-1#4. -_ .. ! .;.. ..

I i l--l~-4----~----~----

j. .. ;--=-.

I ;

=

Figure 3-33

BASEMENT ROCKS

The term "basement" usually denotes any igneous or metamorphic complex which lies beneath the sedimentary section. Igneous basement rocks are usually referred to as "granitic basement" and the metamorphics are identified by rock type, geologic age, or formation name like "Franciscan basement" in California.

Solid granitic basement is easily recognized on electric logs, as shown on Figure 3-34, but if the rock is deeply weathered or covered with a basal conglomerate, or granite wash, containing large granitic boulders. a solid core 20 feet or more in length may be required to distinguish true basement' from overlying sediments. (Figure 3-35).

Metamorphic basement is usually not as well defined on electric logs because these rocks are normally fractured and/or deeply weathered, resulting in a secondary porosity which may equal or exceed the porosity of overlying sedi­ments. For example, fractured schist normally resembles fractured, cherty shale in electrical character and weathered serpentine may be indistinguish­a~le from' argillaceous shale. Consequently, good lithologie information may be necessary for electric log interpretation. If density and/or velocity logs are available, however. top of basement is usually well defined because of the higher grain densities and matrix velocities of most metamorphiC minerals compared with overlying sediments. (See Figures 3-14 and 3-36).

21

1 1 1

--I 1 1 1 1 1 1 1 1 1 1 1

II

1 1 1 1 1 1 1 1 1 1 I

.J

'~

)

GRANIT BASEME

Ie NT

TEJON A

CALIFOR

REA

NIA

TOP OF BASEMENT

1100

1101

1400

1100

1600

noo

1&00

'''0

2000

~ c>

, il'

f ~,..

= -f-

'" ! I' f-

f-

--.

SPONTANIOU3 POTlNTlAL 50 ",nlivolt. + o

:J

~ *F :::::

f1

~

f= ~

~.,><

RESISTIVITY

ohms 1Il~ ~40

Figure 3-34

)

--..;

----

J ...-l_L.

I --

-" --

=t =t = -::J ~

--+

o RESISTIVITY o CONDUGTIV .q 1 1 ~I i- 100 .. ITY

11 _., 1 ~ .. ,.' -+ +..... -0 I - - - : :._tt.f!:H---f.-I-

T - -. . Ii'-I-

f-

I-<

-

-'WQ ,. -. I~t.: :. :l£bo' r. I - -' : .', J;;: t-l00t-

__ -+ . II"'?J --r;. ,.' . \--,=. ,. -,. \-

0

-n -

.---

- ..

f- .-.

I-1-- ...

• I

i

- l-. I~ -. E' :

"

r • b " •. f-.f'" ~ -

r.-- .. L..i(...:J.,;1

. ! t;. f- ~ .

r"'-•

l-~ .. ·t-tl-l-

1< .... - H,'

~ .. __ I-

_ 1_ : .. ~~~I-l-~

1-" '. - - 1-1- r t

• -b" . 10;.' • -".: - ,.. -

I t-1i=tr+"':t.-j;J.,hl;' ;r- -1--:1-1- I< I::~::. _ tr: f~' _I- -1--- ::, -. ~+-I- '-r-lC-'L

r

~ . "' _ +r:+;:=Fil-+tll (VV _1.1. r:;

.·t~_l--· I ..... I J.: ~._.+.:.L --\- ... 1.1 .... -. - .. -I- -+- •

. >'- -,- .-[-f.<.. .. -~ .:-~ -.~ - i - j;;; ~ l~ L- -

.. -.... f--__ j_:~I~

·1 . I.~t: I- _. l+-

-1-11-' .: I:tl-t~5' ~~~-,t tJ'·~t.:-1'1"~-tfl-~·~fi~: 'r t

I __ }- _ ... " .. r,.' b. l-

- - ~ +1-I- " .. ~

I. .

. l- I-­- ·1-

_ ._ .. f-......... -' I

.. '~~ '~k 1_" -f-I I J .. 1 1- I j . - . t,: ,".. .

0- UI.lln,··-t-J-±-tiii .1 _J .. 1.

- .... 1000 Figure 3-35

-

-

>

......

. --'--< i

~ ~6

: "

~ ~'

i ~ J~

f'. ..

1-

, --- ,I-- '

r":::; , , ~J=I'\;< "

II

if

"

'. "

.. ~~ ,I;'. ~ ~

Figure. 3-36

'. -. :~

-'.i

.,

\.

" . ,-

PART 4 :-, \-. " .-.,"

'~ 1; \-

STRUCTURAL INTERPRETATION

FROM lOGS .'

. -,'.11 f '7,.':. : _.'t.: .

. ,

. "

. '-1 '- ~~;

•. ,1 •••. _ . ~, .

-~-,

" >., ,',

, ,

STEEP DIPS

The effect of moderately steep dips on an electric log is an apparent thickening of equivalent intervals when compared with a log showing a normal section with low dips. If the dips are very steep, 50° or more, other effects become apparent as shown on Figures 4-1 to 4-4. In a thinly bedded section with contrasting resistivity (bedding anisotropy), the resistivity curves become rounded and flatten out because the depth of current penetra­tion may include several beds, causing horizontal bed thickness effects in addition to the vertical effects which are dependent upon the ratio of bed thickness to electrode, or coil, spacing. In thick beds with relatively uniform resistivity and little or no permeability (no invasion effect), intervals of steep inhole dips may be detectep by the effects of micro­anisotropy. A microanisotropic formation is one which is more conductive (less resistive) parallel to the bedding than normal to the bedding. Any formation with a preferred grain orientation parallel to the bedding will be electrically anisotropic, but shales and shaly siltstones will normally exhibit this property to the highest degree.

The effect of steep dips in an anisotropic shale section is most apparent on the induction-electrical (IES) log as shown on Figure 4-3. The radial, 3-dimensional current pattern for a short, 2-electrode spacing like that of the 16" normal results in a resistivity value greater than that parallel to the bedding but less than that normal to the bedding, and this resistivity value does not ch&nge appreciably with radical dip changes because of the predominance of the hole effect on a non-focused short spacing system. The current circulation which produces the induction log is in horizontal loops around the hole, and the hole effect is negligible because of the focusing coils in the system. Therefore, ~.a shale section, the 16" normal will usually show a higher resistivity than ths induction curve where the inhole dip is less than 40°. Depending upon the anisotropy ratio (RN/RP), the curves will cross when the inhole dip is between 40° and 60°, and above 60° the induction curve will show a higher resistivity because the current loops must now travel obliquely across the bedding.

The response of the IES log opposite thin resistive beds may also provide a clue to the amount of dip. If the dips are low, the induction and short nor­mal curves will indicate approximately the same apparent bed thickness, and if the bed is uniform, the peak values of the two curves will occur at essentially the same depth. However, with steep dips the bed thickness is exaggerated, particularly by the induction device because it has a much greater lateral investigation. Also, the peaks on the two curves usually occur at different depths with the normal curve characteristically deeper. The average dip of the intervals shown· on Figure 4-4 is between 65° and 70°. In beds A, B, C, and D, the induction curve indicates greater apparent thickness than the short normal, and at Band E, the normal curve reaches a peak value about two feet deeper than the induction.

Anisotropic effects on the conventional electric log, which is a combination of 2 and 3-electrode systems with different spacings, are not as apparent in low dip section·s as they are on the. IES log. Although the longer spacings are affected more by the horizon,tal component, the difference is usually too

1

r •

small to be detected readily in long shale intervals. However, when the dip exceeda 500 in thick shale or shaly siltstone intervals, the longer spacing will show higher resistivities than the short normal with the separation increasing with the dip as shown on Figure 4-2. High dip effects on the conventional electric log are described in detail with an excellent example of a long interval in Geological Item of Interest 096 by Mr. H. E. Nagle,· W.O.I., which was distributed in July, 1964.

Conditions other than steep dips which may cause similar relationships between the resistivity curves are mud saltier than the formation water, particularly in large holes, and hydrocarbon saturation in oil shales like the Green River. in Utah and Wyoming.

2

/

'-,-r-

i-~ - ,- , __ I==> ~ f'.'f-:- __ -,-- "

i ,-

i

i

i -

I

~'"

1--/-,

i H- ,

.. ..

- 0;,

, '

'" . ..

"

"

"

"

.. .-

I -

INTERVAL THICKENING FROM

HOLE DEVIATION AND DIPPING BEDS

-

\I"

I " ,~Ja '" I - , I -\-

I

i I--

. : .

.. "

I "

Figure 4-1

)

)

STEEP DIPS

ELK HI LLS

CALIFORNIA

100 MV +

S.P. Curve Of Equivalent Section

With Low Dip ••

-

: 5100

."

sMa

ltoo

6.,.

1- '\

I,,,, ' \

X

'-r t -I-f-i--

I- I-~t-o 01+

1-1-

I-

1-1-1-­

/--1-

I-f- C'­I­

I ' 11-'

, 1+-

I -

I-

I- 1-·

H- \ I- ,--l-4

l-t­t-.~

Figure 4-2

j

16100

14000

i {

f , ' .. •

'p

INDUCTION LOG SIIIFT

CAUSED BY STEEP DIPS

IES

IES

IES

/

" <',

'"~ "~ ," "

< .'

DIP

DIP

T ,

80

90

.. ;1- ., 'H/f"", >f

STEEP DIP BED TIIICKNESS AND BED DISPLACEMENT EFFECTS

A

B

C

D

E

c .. ~~

~~~

:::::;:::;~;~ ~ --~ ---.. -"

......... 6FF40

16" Normal

-----,---,

-~ c. _____ _

Figure 4-4

I ""

:j 'J <~l ,

)

'c' ,

UNCONFORMITIES

·Recognition of unconformities usually requires careful correlation of two or more logs, good age dating by paleontology or palynology to estabish a hiatus, and dips from cores or dipmeter computation to identify the feature as a dis conformity or an angular unconformity.

Figure 4-S shows an excellent example of truncation beneath an angular unconformity and an onlap sand above the unconformity. Many of the better stratigraphic traps were formed in this way. Consequently, recognition of unconformities, end angular unconformities in particular, is one of the first objectives in regional correlation •

. .

3

1;lillT"~' g I· . I ,j !" I I ONLAP AND UNCONFORMITY 1 1.-,', i . .J. .. f-

rrij-R~-IH" -j; /' .... - - ..

I : t . /, f -j l L~' _ / ":'1 .. '

! ... . •. ::~'-I", .. -·/-.1-I: i 1::..1- ~ \ .. :. -. ./-[- i " -I -0 ••• -1-- I--I 4 •• I . i .- - -

: :ENTRADA SS

:':l~l-' ,:". . ~

• : . _ CARMEL SH _

."

.. -

'.:...:1 :.. LJ:,Un- • -' ......... ~ .•.. ,i1l::{:: .'x'. ••••••••• , •••• i ............... • •••• , •••••••• 0"1 NLAP'_t. \ •••••••••••• 1 •••• ••••••• ttL' • t ••• t ••••• t. 0 '-.' .• •••••••• ' ••• It ..... tf .t ... "T' .......... -0. ••• ..-~. ~'''::I······''· .. '.,', .... :: ... :,t\', °0 to' ': .of:. ': .................... fl.J.~ .•••• NUGGET SSt ••••••• 10 I •••••• ••••••.••• '-~' ::::.1 ~f , •••••• ," ':. '''i.' 'r ,", 'o' t: ;' .. I J..l. ./, •••• 4 •••• i •• /." ••• "i! •••• ,. , •••• (;.. • .!\ -,- . ~O" " ·····r····· ····t·~· ...... '1' .•••• , " m\C'" /" •••••••••••••••••••••••••••• tt'0 ~L

':.·::.· .. :,i/p.R ~~ -" j.~" r- / ~:-'r"Vl-' .

~ :l'+ ~ . !-"t .. ·

• ,

+ 1·-~ = H-- l- i

~: LYKINS FM. 1.-.. --L ..

:·c· .. R-~.~:~-/ .... ~

; .. I

l :; ~ "'-t-" -~--

1- -- + -

- -.-~ - -.~ - -" --

oL ' ,,- -+_

' .. ,,' :- . -

'. ' . , • "t- - ."'

-, ---, 1

f-1----

I· ~ -- ~ ..

v,. DENVER BASIN

COLORADO ~ WYOMING

-!--

-_.

- - .. " --

, ~'L I·

- .

I--

I-~ -1-

.. , " • .:.l.

.~.­f-. -: --- •. ,

Figure 4-5

(64" normal not shown)

.

"J

..

NORMAL FAULTS

Missing section on an electric log when compared with logs from nearby wells may be indicative of normal faulting. Because of the continuous record, provided by electric logs, many more such faults can now be recognized and defined than was possible when drillers' logs and core records and cuttings were the only material available for detailed correlation. However, as was pointed out in the section o~ correlation, electric logs by themselves may be misleading. Many workers have become "fault happy" by disregarding the possibilities of rapid facies changes and/or unconformities. Erroneous fault interpretations can usually be avoided if the regional and local lithologic relationships are studied in conjunction with the correlation of electric logs. To this end, all faunal evidence and core and cuttings records should be assembled and plotted on the logs. '

Figure 4-6 shows a good example of correlations based on recognition of lithologic units and log character to define a missing interval. Note the absence of the Timpas limestone on the log to the left.

Disturbed and unusual fluid relationships associated with faulting may also be recognized on electric logs, particularly if the fault has a strike-slip component, S8 shown on Figure 4-7.

4

,

NORMAL FAULTING , -- -- 8 , -

,- ..

.

IWtl s .

I· i 'f-I?

§ NIOBRARA

~~t"J.r~I· ~~i~ .D ().

__ "_'-1.."; ~

TIMPAS '" CaDELL i':-'. ,:.!) CARLILE_I-

1-· j._- .-.-

\­GREENHORN

GRANEROS-

I-f· . NOTE THICKENING ON DOWNTHROWN SIDE

CaRR

CaRR

" •

TIMPAS ~ ~ CaDELL --II:'ift~, _

CARLILE

.

PIERCE FIELD DENVER BASIN

COLORADO

§

I~

._­-

-

NIOBRARA

CaRR

CaRR

TIMPAS CaDELL CARLILE

GREENIlOHll

1-1-· GRANEROS

Figure- 4-6

.1

,

:l ~/ UNUSUAL FLUID

RELATIONSHIP

RESULTING FROM STRIKE-SLIP FAULT

SALT CREEK AREA

CYMRIC FIELD

CALIFORNIA

WATER OIL

1100

1600

noD

IUO

tlOO

noD

.~.- .

f

-r- . - - ....... +-+--t-'rt- -_. ,-t-t--

I.- -I -.j- ... ..' +-t-F ..., ,,' .... --1-+ +--I- ..... 'Ol_-;-...!.--'+-.

.1.. ,. _ .:,...-+-. 1-"+ . , ..... ,--r- 't- t- ... '. t-.,...,... ,

1. 1"·· .•.•. • -;. --1 -_ - ...... ,."

I· t·, .... -~-.. , . -.- " ."--.. ~ , •. . .•• i . t-. t-

, ,..... ...,-· . t-.~~, ....... , ~-!.­

I··t· ."lj/f.- ., ... -_ i I.~-t- .•• ,-. -I~!, J.::. .: L;':::;:" .~-l ... : ... &--

'. !~ii~$L -::t ""'.j- -4- . · . ..~ __ ~ . .l-.--y_

'Ok\-- -I- - -~.-+ .. - ... -,-.. -:~ _-:-'--+ ..... -

t· r- . - f-- .-

~ ....... _. 1

- ~.., . ,,-• • . t • , • • .. ..

-io.· ... ·I·· .. •· I .. -I ....... -

100 MV + 0 ohm. mV.. 'to

Figure 4-7

REVERSE FAULTING AND OVERTURNING

Repetition of section by reverse faulting and/or overturning may sometimes be recognizable on electric logs, particularly if good dip information and faunal records are available. Approximate similarity of character, though indicative of repetition, is not by itself sufficient criteria for reverse faulting or overturning. However, when abrupt changes in amount or direc­tion of dip and/or unusual faunal relationships are noted, the electric log may be used to define the structural feature. Recommended practice for recognizing repeated section is to obtain two copies of the electric log and compare detail by superimposing one interval over the other on a light table, making allo~18nce for differences in amount of dip. If overturning is sus­pected, one of the logs should be turned upside down. Character similarity which might otherwise be missed sometimes becomes readily apparent when duplicate logs are used in this manner.

Figure 4-8 shows the effects on an rES log in a deviated hole which crossed the axis of a tight anticlinal fold. The inhole dip changed from 260

right-side-up to '760 overturned in the interval 4160'-4260'. The normal anisotropic effect which causes separation between the 16" normal and the induction curves disappears at about 4200' as.the dip. approaches 900 where the well crossed the anticlinal axis. Below that depth the beds are all overturned with respect to the hole and the inhole thickness is increased about ~ times by the steep dips as shown by the interval between the con­ductive beds A and B.

Figure 4-9 shows an interval repeated by a thrust fault with a 150' interval of drag folding in the fault zone. The steep dips in the fault zona are indicated by the.cross~over of the resistivity curves as explained in the discussion on steep dips.

Figure 4-10 shows a sand twice repeated by a possible combination of over­turning and high angle reverse faulting. The vertical line on the right represents a well 330' away in which only the lower occurence of this sand was found. Faunal evidence and dip information, though meager, agree with the picture as drawn. A similar break thrust structure is shown on Figure 4-11 except that, in this case, the overturned beds are below the fault.

Figures 4-12 and 4-13 illustrate the effect of a high angle reverse fault on an overlying thick salt section. The structural movement was absorbed by the salt in diapir type folds draping over. the fault scarp. Repetition of thin shale markers by overturning is clearly shown on the Gamms-Sonic log of part of the salt interval by comparing a right-side-up copy with one turned upside down.

A unique application of a velocity log used for the location of a fault ·zone is illustrated on Figure 4-14. The well crossed a rev'erse fault where Cambrian carbonates were thrust over Cretaceous sandstone. The abrupt change in velocity plus the abnormslly low velocity at 3625'-40', which probably reflects a gouge or crushed zone, definitely locate the fault. The velocity log of this example is a single receiver log which is now largely replaced by two receiver logs.

5

, 1

•

Application of a focused wall resistivity log and'caliper in locating a fault ia shown on Figure 4-15. The fault zone around 7700' was brecciated causing the hole to erode beyond the range of the caliper. That a fault was responsible for the brecciation was, of course, proven by other meana.

6

, "

,",'. , "

, ) " ',--, ., , ,

,J

,-I

'--/

,

.. ..--,- ,

•

23' "

: 19' "

, 2~'

, -/s,

79'

, ,

, , ,

,

, -J' r

,

52'

, ,

, ,

, I , i

;

! ,

I 1 I'Ll ;

.'

~ - r"1 +

SPONTANfOUS-POTfNTlAl millivolts

'---'-'

• -~-., ~.:-~~

I

J

.1 1 t t- 1- it =t-jj~: ,. :' r,t '1""" 'j') "t..i.' !- ,.-."" '-t r' r . '1 +'t . " -e-r

~. t ;·;L ~1l:.HT' Ii·: ~'" ~ till' f ""j'

• ,-~ "t,.l,-I-'--1 , I I

o

o

I

.-1 j I , ,

.• ~. I " ,

' j t ' .I I , : / : ,

, I

. I- r' 'i- '! ; .• -' -I' . +-- ,,-., .. r-. " . I I I : , I

: , jO 1000 -~ o

'00 1000 1000

RESISTIVITY CONDUCTIVITY ohm •• m'/m mllllmho./m. ~

STEEP DIPS AND OVERTURNED BEDS

. '

Figure 4-8

, ~) I \..... ...

REVERSE FAULTING - r-. _.,- -,

; ,

-,

..-

i

R~ N

H

~ ,r.. ,

o

f-

LARAMIE BASIN

WYOMING

I

a

-b -

,

- - c -- -d ~ ---

--, - < -, -- -,

-• pO: , -

a .. - --- I

- b

,

-- -

, c ---d I - --I I

Note uSe of short normal for correlation. Correlations a, b, c, and d show bed repe­tition from reverse faulting. Correlation is based entirely on electrical resistivity character.

Figure 4-9

REPETITION OF SECTION BY

OVERTURNING AND

REVERSE FAULTING

SALT CREEK AREA

CYMRIC FIELD

CALIFORNIA

, :..: - ll- -. - ~ ., -

600

A - ~-- ..

100

= - .

· -~

-. B 800

-SOD

·

= -1000 t---. ::: - -

- -.-

- -liDO

-

- .

1200

1500 B I

= .. · •. 140D

.

- . . :/_r~· .. I~OO

. - -··1

1'00 . ~ f-I

.-j: - -

.A

· ~ noD . .-,. 1

- .:; - j -, ,-~ ,

.. -I. · · B

1800 .,.

· --

l';--', r-- - ~I ···-1- : I : I( - 100 MV + 0 ohms .,'L/rrt

1::------. t--"I

~ 1'\

· ·

.-

fi ·

I"-

:: --

iii

. --

~ .... -. -

20 Fi u e -g r 4 10

I • j ;

• to.

i I • •

I

, '

' .. I : : I' I; , , , . , I

, , .

--..-:)

I ,:

, ,

.. r .: . II J.; -:1 t-. ., II

I • ~ .

, :: . j I::. : ! 'I

: : : :

I •••

::: ; · '" • • p.l .

: . I

I' • ,

; : :·i .~ , .• t,

i ; : I' I· . ~-. '-!!: !

· , '. j : : j ~ .....

I" •

+;:..~ I •..

-';'.; ~ I-I I' , • •• , I

REPETITION OF SECTION BY REVERSE FAULTING AND OVERTURN S.E. DURANT AREA BRVAN CO. OKLA,

Figure 4-11

) \1 "-- ...

)

) ~

. , '. ;~.i:' proJ.,f#d ':i~,' : i,(' PU'" (t/~~)·OgJ

SOC . .1. TJOS,Rll£ K8'64I4'

JUO

.,"

7~50

7411

TD~'

ZOOO

1500

_--.....,--/

I

1000 SCALE

0

PURE STATe.·A-'

S.c. I- TJOS!RI4-E Elev. 6SS4 KIJ

UPPER HERMOSA e ____ --

A,"'--_-~

... , ./--..

" c, OCTAIL C'ORRELATION SH'OWN ..., 'ON F'OLLOWING PLA TE /~ -.... ,/ " ,/---"/

\ /' /"\./ \-...,// PA.,R~OOX

".--~' SALT /

r ___ /' ~

r'-.",/

----. ';-'\----.:"-JJ._-__ - _/ ~

~ -..... --) /-,,",,-./ '---c,-"

"--lOWE :£ '0 ~

10,110 LEADVILLE , OURAY

LISBON ANTICLINE:

I I

If I

i I I

500 0 0

~ 0 8 ~ SAN JUAN COUNTY UTAH

'" Figure 4-12

· ~ H: , I ;:: i;t!

E.XAMPLE. or

I ! I I'" ,I,,·, I ',I

ii TOP rRONT ., ~;;:. 'I' d

III JI':, , ;; II !. IHlHHII'

I !tllH44+11'

1,~l#l~~ll 79

,

i

REPEATED AND OVERTURNED ON GAMMA-SONIC LOG

I , fI

l#l!>I, fHHI: ,

D{$, j" 1'1111 iJ.hIIj 1/

/..OG HI:.. _I ft AND INVENTED TO SHOW

BEDS REPETITION or 0, AND 02 SHAUS.

j ~) •

THRUST

ON

FAULT EXPRESSION

VELOCITY LOG ROCKY MOUNTAINS

~ 8 TRAVEL TIME IN SECONDS

',iO .~, T ~ T ~ ~ ~ T ~T 5: ~ I INTERVAL TIME fOR !.p~ f£ET IN IMlCROSECONOS i~I~~~~I+~~~~~~~~~~~~,~~,~~,~,~g~~~~~~,~·I'~~~,~~.~4r~~~.~+~~.~~rll

. INTERVAL VELOCITY IN THOU~ANOS or fEET PER SECOND I

o » ~ OJ ::0

» :z

o ::u ITI -f » o ITI o C (J)

Figure 4-14

,

HOLE ENLARGEMENT IN FAULT ZONE

BELDEN FM.

A reverse fault was found in this well. The fault zone around 7700 feet was brecciated and developed marl(ed hole enlargement as indicated by the Micro­caliper.

MINTURN FM.

Hicrocaliper NicroLaterolog

na

GYPSUM BASIN

COLORADO

Figure 4":15

:j~ ~ 1

STRUCTURAL INTERPRETATIONS FROM DIPMETER DATA

Since the early forties computed dips and strikes from dipmeter logs have been used with core dips to'determine structural trends and establish the position of a particular well with regard to local structure. Until recently the computations were used individually or averaged over short intervals in the same way that core dips had been used in the past.

The detail now available from modern continuous dipmeter surveys is best handled by statistical analysis techniques. Several different approaches are used with the resultsnt display determined by the type of problem. Regional structure, unconformities,and stratigraphic detail, such as cross bedding, require statistical averaging techniques like those used in the MDSTR (Mean Dip'and Strike) computer program. This program averages dip and dip azimuth data vectorially and the results are usually displsyed in a sim­ilar fashion to thst shown on Figures 2-18 to 2-22 in Psrt 2 of this manusl. The vector averaging technique is described in detail in a report entitled "MDSTR Program for Computing Vectorial Averages of Dip and Dip Azimuth," by L. R. Litsey in the report on the March, 1965, Formation Evaluation Committee Meeting.

The statistical approach for detailed structural analys1S of dipmeter data requires several types of plots which are obtained from the SCAT (Statistical Curvature Analysis Techniques) computer programs developed by C. A. Bengston of Chevron Research. The displays which can now be automatically plotted from the computer output are shown on Figure 4-16. The six kinds of geo­logical surfaces which can be interpreted from SCAT plots are shown on Figure 4-17. The type of curvature is deduced from Dip vs. Azimuth plots like those shown on Figure 4-18.

The objectives of the SCAT progrsms are:

1. To determine the structural dip as accurately as posaible at as many places as possible along the well bore.

2. To locate and identify geometric singularities, i.e., faults, uncon­formities, crestal planes, axial planes, and inflection planes.

3. To orient structural elements, i.e., find the bearing and plunge of crestal lines and dip and strike of axial planes.

4. To determine the geometric properties of the region near the well bore, i.e., to identify regions of anticlinal and synclinal curvature, dis­tinguish between prolate and oblate curvature and conformal and disharmonic structural style.

5. To account for the effect of stratigraphic convergence on structural change with depth.

7

"'l ..... 00

" ... .. .0-f ... '"

l C

A A'

AZIMUTH DIP 180' 0 lBO' 0 45' 90'

I l­ll. W o

~

•

•

•

•

•

I~\' •

.~ ••

l·

•

• j. •

DIP-COMPONENT +90' 0 -90'

~ i=::=-~,_ _ ___ (--!---

I I , J r J , J , • , J ,

I I-____ • ___ 11. W o

-----~

• • ------

• ------ •

• • ----- • • •

TRANSVERSE DIRECTION

•

'~- . ..:.-.

C-.J

.. '"

90' L T I I

90' ", CF.

I I I I

D.. -o • I I

I • ! bJ· ..... W •

180' 270' 0 90' lBO' AZIMUTH

DIP-COMPONENT +90' -90'

I Jfl. I ~

f

•

• LONGITUDINAL

DIRECTION CROSS SECTION

VECTOR AND GRAPH-TYPE DISPLAYS OF DIPMETfR DATA

SIX KINDS OF OEOL. SURFACES

NON- PLUNGI NG CYLINDI2ICAL

HOMOCL/NE (No Curyafure)

D;r~lionol No CllrytlllU'l

I=""OL D (Si,,[ZW'tI) L-L-LL

/,,-),,'--£);r«-II"'" til M4.rimllm ClIry.;tllre.

I:~) PLUNGING CYL/ND12ICA rOLD

(Sinp'v ) CUrYH

DOME (orBd5/o) (DDU6'1p Cl.lrV~d)

PLUNGING CONICAL ':-OLD

(Sinp1v Curwrl)

5ADDLE (Doublfl Curn.t/)

Figure 4-17

'" .... 00 c ... .. ..,.. I ... ..,

~~~~~~----- ~-~---~~- ~-----

r l

90

4 "-• • • • • • • • • .. , : I· •• • • • .~ .....

180 270 0 90 180 ZERO DIP

90 ,T ,T

I .

• ;~ • • • . • • l- I' • 45

• -. t. 0

• I.- • I o

lao 270 o 90 180

RAILROAD TRACK

(NON-PLUNGING fOLD)

c

~J • • •

01 • I • I G····I.·~ I 180 270 0 90 180

LOW DIP

(DIP< 50!)

90, ,T J T

..:1 . 45

0' "- ... 0 .... "

180 270 0 90 180

HORSESHOE

(PLUNGING FOLD)

~

90

45~ • . '0' :'0·· • . .. - . • •

• • • • 0 180 210 0 90 180

POINT PATTERN

(HOMOCLiNAL DIP)

90, , T

451-

faD

•

~2'\' I· ·;..-t·11'· •

210 0 90" 180

CHRISTMAS TREE

(DOUBLE CURVATURE)

THE SIX VAliD DIP vs AZIMUTH PATTERNS

. ~ ;

,', 1:_"

.~l_

.\'

,,,~~.,

J

, -~

-,,,,:"'- ,

, .. i..!

, ,

, '

,,'

. ".' "

PART 5 :,\

" "

HYDRODYNAMICS AND ABNORMAL

PRESSURE EFFECTS

.'"

, ..

-;,c.,

, "

,. '

,,'-

I' .,'.;, .1,

, '

"

" -,,,,,._,",,,.

";,,'

" , ,

, ,.

~', I r .. l' ': ~~

.:\

: -:,

'-,"

, ".

, ,

~. ',': '. ~ . t~ "

'j. -

\f r ;

,',.

TRANSITION FROM .FRESH WATER TO SALT WATER

The normal character of an electric log water interface is shown on Figure 5-1. pIe extends from approximstely 1150' to

run through the The transition

1550' •

fresh water-salt zone on this exam-

In a sand-shale section the resistivity curves and tha S.P. curve will show opposite effects if all of the sands are wet. The resistivity curve will show high contrast in the fresh water sands and the S.P. curve will show high contrast in the salt water sands. The transition zone will always be gradational on the resistivity curves because of the changing water salinity in both sands and shales, but the S.P. curve will normally show a step-like pattern as on the example. The S.P. opposite the fresh water sands will be positive if the formation water is fresher than the mud, as in this case. and will change from positive to negative in the transition zone. Shifts in the shale base line of the S.P. curve may be caused by either a change in the shale mineralogy or changing salinity in a sand between two shale inter­vals. In a continuous shale interval as at 1450'-1550' the S.P. will not change with the change in water salinity shown by the resistivity curves.

1

1

1

,r

I

I

1

1

1

1

I I

1

1

1

1

1

1

1

1

1

1

1 :1

TRANSITION ZONE

FROM FR ESH TO

SALTY WATERS

RM "3.4 0 H M - M

AT GO°F

HUNTINGTON

BEACH

CALI fORN/A

100~V

1000

I-i-< 1100

NO REV£,

OF

rE RSAL SP

1100

1300

1100

- .

.'

. -

.

.'

-- -

+.- O-JO -

1100

- .. - -AO -10' -- -- -

::- --.-

Ali= 20' .AK/= 16" -

".

-- . ." .

OHAI-AI

Figure 5-1

I J I ,~ 1

,

IIYDRODYNAMIC EFFECTS

Partial flushing of a deep sand reservoir by meteoric waters is shown on Figure 5-2. The decreasing resistivity in the downward direction in this sand could be ascribed to increasing porosity with a resultant decrease in formation factor. decreasing hydrocarbon saturation. or increasing water salinity. The Sonic log eliminates the first cause. and the second was ruled out by the complete absence of hydrocarbon shows on the mud log and in sidewall cores·. Positive evidence to support the third cause is pro­vided by the shift in the S.P. shale base line between the top and base of the sand. Calculations of water resistivity from the S.P. curve check calculations from the resistivity and velocity curves. The salinities from those calculations are about 200 GIG in the top of the sand and 500 GIG near the base.

An unusually deep fresh water aquifer is shown on Figure 5-3. The sand­stone in the interval 12.980'-13.085' is apparently continuous to the surface outcrop on the north slope of the Brooks Range, whereas the sands with negative S.P. and low resistivity in the interval 12.700'-12,800' are not continuous. Calculations indicate a salinity of about 200 GIG in the lower sand interval compared with about 1600 GIG in the upper sands.

2

,

i 1

,.

, ,

\

i

,. ' . .

\ ".-"t

, , \.

>. , , "-:> '

~ ., .,.-r

<" ,

.\ ... , I"

! I::

-1-1+ 10

SPONT AHEOUS·POlENlIAl millivolt.

• 8

• ~

• t 0

. I L '-

.> '. ,

-' ,"

>'."

"

o 200 2000 1000

INDUCTION 1iL--_._----._._---.l.Q o 200 • .... ·--lH~T»" .. -·.,.

RESISTIVITY ohm.-m'/m -

CONDUCTIVITY millimhos/m -~

~.:.: _: .. =._.-

.: ::::- :- ~~-." '-"--··-·1 -.:~:~~ 1

,-.. La-100

'~O

·1

... -

50

ISO

INTERVAL TRANSIT TIME MlCa<»KONOI 'U fOOt

Figure 5-2

/ '- '

'--.)

,--)

FRESII WATER AQUIFER

-- -----:---

_.__ 'r I ,

~I :-------:--

.--.------,--- ·c 1-,'-,--' -'--­:-

i I I

. II i

. ! I ,

;

.

.. ,..- I

l..oc;.,.~ :

'" .., o o

-'" CD o o

-N U> o o

'" o o o

-'" o o

II

I\-~..$.lJ : ' . ~ -?j k!. I iH, Ii 1tt--+-+-++~l+Il---l iii ~-~z;:. "ti* 11Ht--+-,f-j--l--!i-l-i,-!-U-----J

I ! i \1 ~i~ , -_: . !. I , I:!' 1\ I 1 ;, .... -.:L. ,">1

j ,'" , 1=", I I ; , , TT l1l!f

1f->--l,

1 ,ill' I- i~'~r,jli ! ----t-.-: i ' -=-r.-:,...,~ m ~ ! ,n , I , I~' r ._-<

I l ", II ' i~ _~_rl r=~~j~jl.1 ~: I ~ I , ... _! , , r. 1 ' II,I ~'"-'-=>': 1- I " ! __ -'-' ' "I .--c:=-I, ' ' i -L.!] __ I :,': ~ . .,-~i-'I,··-.·j : I ,:; i- ·-:-;~fs.·I-Ttj- III --! '_., ... ;! --I-l-.J..I.!'~· I·~I Ilj 1 . . ,I ' •• I' ~ - r ., '!I- I ! ' : .. !, ..... :. -1 I ~ I ,xf-- ' I

, I, !;~-'~""I i . ~ ;-: ::: .. _~_J: : : )'\)~ .. _ i.~ I_Ii I i! "

,t. I I ' I' .' I I' 'I __ ......L_...:.---'.-L' .:.,' ~ . .. /' I I! ; ! ! I

.. j • •

Figure 5-3

r-'

!

r

ABNORMAL PRESSURE EffECTS ON WELL LOGS

During the last few years pressure effects on well logs have received con­siderable attention from log analysts. This interest has been stimulated by both geologists and drilling engineers. Geologists are primarily interested in the origin of abnormal pressures and the relationship to hydrocarbon accumulation and entrapment, whereas drilling engineers are looking at the problem of mud weight control when drilling into high pressure zones. A study group was recently selected by Mr. K. H. Crandall for a Corporation­wide survey of abnormal formation pressures, and their work to date indicates exploratory applications may be more widespread than was formerly realized.

The discussion and examples in this manual will be limited to the more obvious effects of abnormal pressures on resistivity, velocity, and density which can be readily recognized on well logs. For more complete coverage, a seli!Cted biblio.graphy is appended.

The effect of increasing depth on shale resistivities with a normal hydro­static pressure gradient is a gradual increase as porosity is decr~ased by compaction, if the shale mineralogy and formation water salinity do' not change appreciably. A sudden increase in formation pressure at the top of an abnormal pressure zone will reverse the resistivity trend because the fluid pressure supports part of the load and maintains a higher than normal porosity. Figure 5-4 shows the abrupt change from the normal Gulf Coast shale resistivity gradient down to 4940' to one of decreasing resistivity below that depth. In this area and others on the Texas and Louisiana Gulf Coast, the shale resistivity gradients are sufficiently uniform to permit quantitative calculations of formation pressure and pressure gradient and thereby provide positive mud weight control for drilling into high pressure zones.

Similar pressure effects in shale intervals are common in the Cretaceous "E" and "F" zones in the Sacramento Valley in California, as shown on Figure 5-5. The effect on the velocity log is very similar to thst on the resis­tivity because both logs are responding primarily to the changes in shale porosity. A plot of recorded pressures on tests in this well at 3890', 5397', and 6035' indicate the formation pressure gradient departs from normal hydrostatic at about 3500' and extrapolates to about 0.9 psi/ft at 7400'. The extrapolation indicates an increase of about 3 psi/ft in the interval 7300'-7400', where the most abrupt change is observed on the logs. Quantitative interpretation from logs, like that conducted on the Gulf Coast, is limited because of vertical and lateral changes in shale miner­alogy and formation water salinity.

The density log can also be used to locate abnormal pressures in shale intervals if the mineralogy is fairly uniform. However, grain densities in shales may vary from about 2.2 gm./cc. in an opaline shale to about 2.75 gm./cc. in a chlorite-mica clay shale. The mineralogy must, therefore, be checked before considering pressure effects. Figure 5-6 shows an example of a plot of shale densities from cuttings samples compared with a density

3

) '-...--.

log in a high pressure zone. This shale density plot is now part of the mud logging service offered by Baroid and has been used successfully for mud weight control on the Gulf Coast where shale mineralogy is uniform.

References

Dickinson, George, 1953, Geological Aspects of Abnormal Reservoir Pressures in Gulf Coast Louisiana, AAPG Bulletin, Vol. 37, pp. 410-432.

Hottman, C. E., and Johnson, R. K., 1965, Estimation of Formation Pressures From Log-Derived Shale Properties, Jour. Pet. Tech., June.

MacGregor, J. R., 1965, Quantitative Determination of Reservoir Pressures from Conductivity Log, AAPG Bulletin, Vol. 49, pp. 1502-1511.

Wallace, W. E., 1965, Abnormal Subsurface Pressures Measured from Conductivity or Resistivity Logs, SPWLA Transactions, Vol. 2, May, and Log Analyst, Feb.-March, 1965, pp. 26-38.

4

'-':--r"l.-

~"-, ~

'=1, i ' ;~.',' ;1' :;:t:t:t - a-

Induction (O~lO ohm sz.> _ _.J~ i'.l·~I~i~I~'i·IIIClon!diuct1Vity ABNORMAL PRESSURE EFFECT ON RESISTIVITY

TEXAS GULF COAST mmm If-16" ~orm'l (0-10 ohm 'C;11~11>'e!)11Ilri"!til;~I~!0!-[2 olua .cale)

•

~ a- Fr'~,

i ~~ .,_. Sh~i;. resistivity increaaea Vith depth __ (.72-1.0 ohms) thrC>Ullh "nomal" (.lUi5 .::) I-_jrt,"--,:--,,'~-=---=I psi/_ft.) ~rad1ent interval. I'~ , "11-;'- - , ~

,jf-

,

i

0:.. -'

I \-H-J-H ~-

--- - -

I --

I

- -

i

--

I -

-- -

I

' ..

- -.-~

Top of "Super-normal Preuure"at 494(>1-60' (max. shale r .. istiv1ty l.~ ohm.)

-

- ~ - ,I: -~ ~ -

Shale resistivity decreAaCI and rerna.lna low 1n high pres8 ure interval, gradient this example i.e 68·.92 psi/ft.

- .

. .. -- - - . -

Figure 5-4

ABNORMAL PRESSURE EFFECT ON RESISTIVITY AND VELOCITY SACRAMENTO VALLEY CALIFORNIA ,

r , SPONTANEOUS-POIlHlIAl ~ lIVITY ~ INI111VAl T.AN~ll liMo!

RESISTIVITY CON Due .. 4' ..... _4. ,.' lu, ~ ! .. ".,,,

millivolt, ohm •. mt'm milllmhos/m ...... ~ 'f " I, \' J,

~ Uo" NO.IJML -'= "D\I" .... ~ ,., ..." .

I ,~o " . -I~I+ \ ,~.., ,,~

1' ..... - . ). ......... - - ~. 'S •

1Htl'IJC1lOH '&f;' ........... . , ,-,~

- -,-1- err - - I-- - I::~ I-1-- - -- --

- 1- 1-1- _. .. ~ -- It' - . "1- ~ 1- -j..- - I- --

~ -f:i

--. I-I--.f-. - - --i . . I

1= .1-- _

-~~-· - -- I- -. -. -

!l> - .- . I-1-· I·· - -- _.- - -- -

I i

1 I- I· I- . -_I},_ b -. -- - -

- -

-l~ -- -- - ~ [}I- . i- -I- f) -

.. -l-i I--

It i Ei --~~ j£:.

,.- - .- . - .

1~~ -. 1--1._- · . -- t- -- -I-

I

i.;:;:: ~~ ~ · -·1- -· . - . -~ 1- - - ~ -1== . - · - - -.

i - · 1= _1-:- I- i -- - -,-I- I- . !- . - .. ,.. --

- - .- 1- --- -,- I- .- 1-- - - -- -- -~ --

1-- .- 1-- - - F , -- 1- _Il;' .- I -. -- -.

I=- -- --- -- -- I- .

i -~ - -- - k 1- .- j -H·+ - ... F- . -

- --- - -1-c._ - -I .. -- -f- - >.. - i- -- --

-I- 1-- .. - -.

-.- Fi-£ -- - -~ - . --- --- - - - -

i'-- - - . -. ! • Cli_ · -

1- - - i-''''' - '- -• i § -r: -- --- - -I- . - . ~~ - -- . . ._. ~ - - --

- ~ --I- . . !). - . '- i,' - 1- --·1 ~ 1-

--

-I::~ · .-- -

p~ 1- -If I· -

. ... - 1+ I-- ~-

, . . i-i i . I- I-- . :=-·1- . +- -

. 1- -I- .

=p .... 1-- -.

- E ..

1- - 1- ~ -f--I- - -

-:,l 1-1-

P .

l.-:~ 1- f---.., .

I t-- .-

f ~~~ 1 i-. - - i-

· I-I I

~ -f--

f--rf:; to< 1-1=

G .. I I ..

. flf I Figure 5--5

----,"--~- '----.-~ .-

--..... - '

SHALE DENSITY PLOT COMPARED WITH DENSITY LOG TEXAS GULF COAST

1- I

I-"f -I _

-p ______ , ____ ..tJ

C(>rtJI(CIIO" -8Uu< DINSITV

I ~~!~sth~al1e~cfutlttl~ngt-.~De'n~.~itY (eveq 30' by inud logge!')

~ C~.l1per I

I

I

I

I

I~ Figure 5-6

Geologic Interpretation From Well Logs .Eocky Mountain Log Supplement

Known Logs

Figure 1-3 -~

top; sw/se/sw, 3-T9N-R79W, Conoco Oil, #5~pollock, Jackson County, Colorado (McCallum Field).

middle; ne/se/ne, 20-TlON-R79W, Lion Oil Company, #1 Eva, Jackson County, Colorado (McCallum Field) .

bottom; sw/nw/ne, 22-TlON-R79W, Monsanto Corp., #1 Perkins, Jackson County, Colorado (McCallum Field).

Figure 1-4

left; se/se, l5-TSN-R66W, California oil Company, #1 Vernable, Weld County, Colorado (Pierce Field).

right; nw/se, 27-TSN-R66W, California Oil Company, #1 Jennings, Weld County, Colorado (Pierce Field) .

Figure 1-7

left; nw/se, 27-TSN-R66W, California Oil Company, #1 Jennings, Weld County, Colorado (Pierce Field) .

right; se/sw, 23-TSN-R66W, California Oil Company, #4 Priddy, Weld County, Colorado (pierce Field).

Figure l-S; se/se/ne, l4-T3S-RS5W, California Oil Company, #1 Benton Land and Livestock Company, Routt County, Colorado.

Figure 1-9; nw/ne, l3-T14SN-R9SW, California Oil Company, #1 Rough Creek Federal, McKenzie County, North Dakota.

Figure 2-2; sw/nw/se, l-T26N-R90W, Sinclair oil Company, #35-B Wertz, Sweetwater County, Wyoming (Wertz Field) •

Figure 2-5; sw/ne/nw, 14-T27N-R113W, California Oil Company, #1 Birch Creek, Sublette County, Wyoming (Birch Creek Field) •

Figure 2-7; c/ne/se,13-T7S-R23E, California Oil Company, #35 Redwash, Unitah County, Utah (Red Wash Field) .

Figure 2-8, ne/se/nw, 19-T29N-Rl14W, Belfer Natural Gas, 143-19G, Sublette County, Wyoming.

Figure 2-12

left; ne/nw, 7-T32N-R85W, Seaboard Oil Company, 11 Olds-Skiles, Natrona County, Wyoming (Grieves Field) •

middle; ne/se', 17-T32N-R85W, Forest Oil Comapny, 145 Govt. 17-1, Natrona County, Wyoming (Grieves Field)~

right; seIne, 18-T32N-R85W, True 9il Company and Mule Creek Oil Company, 141-18 Dumbel1, Natrona County, Wyoming (Grieves Field) •

Figure 2-13

left; ne/nw, 31-T51N-R69W, True Oil Company, I1-A Heptner, Campbell County, wy;oming (Pleasant Valley Field).

right; ne/sw, 30-T51N-R69W, Shell Oil Company, 123-30 Heptner, Campbell County, Wyoming (Pleasant Valley Field). '

Figure 2-14; se/se, 32-T7S-R44E, Standard Oil of California, ~ 41 Dry Valley, Caribou County, Idaho.

Figure 2-15; sw/sw/se, 31-T160N-Ra1W, California Company, 11 Thompson, Bottineau County, North Dakota

Figure 3-15; nw/ne 13-T148N-R98W, California Company, 11 Rough Creek Federal, McKenzie County, North Dakota.

Figure 3-17, ne/se, 22-T9S-R43E, Hose Austin Company, 11 Kendrick, Big Horn County, Montana.

Figure 3-19; c/sw/nw, 9-T41s-'R24E, Standard Oil of California, 112-9 Navajo, San Juan County, Utah (Aneth Field) •

Figure 3-24

top; c/nw/nw, 22-T33N-R19W, California Oil Company, U Ute Tribal, Montezuma County, Colorado.

bottom; nw/sw 20-T148N-R97W, California Oil Company, 11 Danielson, Dunn County, North Dakota.

Figure 3-28

left; ne/se, 36-T26S-R20E, Texas Oil Company and Gulf Oil Company, I1-X Federal, Grand County, Utah.

right; ne/se, 36-T26S-R20E, M.G.M. Petroleum, ,1 MGM, Grand County, Utah.

Figure 4-5

left; nw/ne/ne, 5-TI1N-R66W, L.B. Amer-Chicago Corp., #1 Warren-Livestock, Weld County, Colorado.

right; c/se/ne, 36-T41N-R61W, Ginther-Warren and Ginther, #1 Fritz, Laramie County, Wyoming.

Figure 4-6

left; se/se, 15-TSN-R66W, California Oil Comapny, #1 Vernable, Weld County, Colorado (Pierce Field) .

right; se/sw, 23-TSN-R66W, California Oil Company, #4 Priddy, Weld County, Colorado (Pierce Field) .

Figure 4-9, nw/sw/sw, lS-T17N-R76W, California Oil Company, #S Wilson,_ Albany County, Wyoming (Quealy Dome Field).

Figures 4-12 and 4-13, sw/se, 2-T30S-R24E, Pure Oil Company, State #l-A, San Juan County, Utah (Lisbon Field) .

Figure 4-15, se/se/ne, 14-T3S-RS5W, California oil Company, #1 Benton Land and Livestock Company, Routt County, Colorado.

Rocky Mountain Wells With Unknown Locations

Figure 3-10, Pierce Field area?, Weld County?, Colorado.

Figure 3-i6, Powder River Basin, Wyoming.

Figure 3-22, Paradox Basin, San Juan County, Utah

Figure 4-14, Rocky Mountains (United States or Canada?) .