Chapter-4 GR Log (Gamma Ray Log)

By

Dr. Jorge Salgado Gomes

3/4/2013 1 Chap -4 Duration of this chapter: 3 classes (135’) Core Gamma

Educational Outcomes

• Review the concepts of formation radioactivity

• Review the most radioactive lithologies/minerals

• Use of GR log for correlations

• Use of GR to match logs with cores

• The use of GR log in sequence stratigraphy

• The use of GR to detect water entries in cased holes across perforation

– NORM – Natural Occurring Radioactive Minerals

3/4/2013 Chap -4 2

Electromagnetic Radiation Spectrum

Use of GR Log

• Detect pay from non-pay zones – Pay = reservoir; non-pay= shales (non reservoir)

• Correlations between wells (sequence stratigraphy) • Indication of lithology and source rock type (marine,

continental) • Picking coring points • Useful for geosteering with MWD-Gamma • Determine the net to gross ratio for volumetric calculations • If faults are present, the fault throw detection is easier than

other conventional logs. • Detect water breakthroughs in cased holes • Detect casing leaks (cross-flow behind pipe)

3/4/2013 Chap -4 4

How the tool works ?

3/4/2013 Chap -4 5

detector

K U Th

1,3 ... 1,6 ... 2,4 ... 2,8 MeV

E (MeV)

n spectral

selective

integral

all impulses above a treashold of energy

channels

Condition:

open or cased

hole

water/mud or

dry hole

Unit: MeV (Million electron Volts) Recording speed: 1800 ft/hr – satisfactory definition of a 4 ft bed

Schematic Scintillation Detector

API GR Calibration Pit UNIVERSITY of HOUSTON

Integral Gamma Measurement

3/4/2013 Chap -4 8

by Lecturer

Integral activity is effect of 3 contributions

I = (K + U + Th)

Unit: API-unit

API facility is constructed of concrete with an admixture of radium to

provide 238U decay series, monazite ore as a source of thorium, and

mica as a source of potassium.

The complex GR response

• Determine different minerals

• For complex mineral identification

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TYPICAL GAMMA-RAY RESPONSE TO

SEDIMENTARY ROCKS

Mineral - ray Ref

(API)

Quartz, Dolomite, Calcite (clean) 0 H; S

Plagioclase (Albite, Anorthite) 0 S

Alcali feldspar 220 S

(Orthoclase, Anorthoclase, Microcline)

Micas (Muscovite, Biotite) 270 S

Shale 80 …150 H

Kaolinite 80 …130 S

Chlorite 180 … 250 S

Illite 250 … 300 S

Montmorillonite 150 … 200 S

Sylvite 500 H; S

Carnallite 200 H; S

(S - data from Schlumberger 1989; H - data from a Table of Hearst, Nelson, 1985, made on the basis of values from Brom and Driedonks 1981, Edmundson, Raymer, 1979, Fertl , Frost 1980, Patchett 1975, Reeves 1981, Tixier, Alger 1970)

Gamma-ray API Values for Minerals

3/4/2013 Chap -4 12

by Lecturer

Application 1:

1. Plot sand line (minimum)

2. Plot shale line (maximum)

2. Design lithologic profile

shale – sand separation

Note: Consider the caliper

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• Basis: correlation between shale content and gamma activity

• Assumption: only shale and clay are radioactive components in rock,

no other radioactive minerals

First step: Calculation of “gamma ray shale index”

log response in

zone of interest

log response in a

zone considered

clean (shale free)

log response in a

shale zone sh

cn

GR

GR

GR

Application 2: Shale content calculation

cnsh

cnGR

GRGR

GRGRI

GR

3/4/2013 Chap -4 14

by Lecturer

Second step: Select & apply a relationship IGR vs. Vsh

)12(33.0

)12(083.0

2

7.3

GR

GR

I

I

GR

Vsh

Vsh

IVsh Linear relationship (upper limit)

Tertiary clastics (Larionov, 1969)

Mesozoic & older rocks (Larionov, 1969)

Application 2: Shale content calculation

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Second step: Select & apply a relationship IGR vs. shale content Vsh

Application 2: Shale content calculation

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Note: Sand with Mica or Feldspar - "radioactive sandstone"

Baker Atlas, 2002

3/4/2013 Chap -4 17

Application 3: Clay Mineral Identification

• Clay minerals show different Th/K ratios for different mineral composition

• Used for clay mineral identification

• Combination with other properties (Pe, neutron) recommended

3/4/2013 Chap -4 18

Mineral Identification from Spectral gammalog

0 5 15 10 25 20 30

2

10

12

6

8

4

K (

%)

Th (ppm)

100% Illite Ave. Feldspar Line

Ave. 100% Clay Line

Potassium Evaporites

Micas

Glauconite

Feldspars

Smectites and Mixed Layer Clays

Illite Clays

Kaolinite

Heavy Thorium Minerals

Chlorite

Baker Atlas, 2002

Baker Atlas, 2002

3/4/2013 Chap -4 19

4.65 ppm Uranium in acid igneous rocks

• forms soluble salts (uranyle), transported in water (sea water 3ppb dissolved

Uranium)

• three ways passes into sediments (Serra, 1979):

- chemical precipitation in acid (pH 2.5 - 4.0) reducing environment

- adsorption by organic matter, or living plants and animals

- chemical reaction in phosphorites

Stagnant, anoxic waters, low rate of sediment deposition,

which typically produce black shales

(North Sea Jurassic „hot shales‘)

Note: Uranium Content of Source Rocks

NGS Response to Trace Uranium in Clean Sand

3/4/2013 Chap -4 21

Th/U indications Th/U indicator for environment:

• Th/U > 7 continental, oxydizing

• Th/U < 7 marine, grey ... green shales

• Th/U < 2 marine, black shales, phosphates.

Source rock

indication from

spectral gammalog,

Baker Atlas

document

3/4/2013 Chap -4 22

Influences and corrections

• absorption of radiation

– influence of caliper, mud density, casing

– influence of tool position (centralized or sidewall)

• bed thickness (thin beds show reduced effect)

• formation density (influences depth of investigation)

• logging speed

– influences statistics

– influences vertical resolution

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QC - Quality Control Gammalog The gamma curve should agree with other shale indicators (except in

„radioactive beds“)

Shale values should be similar to those in nearby wells

Repeatability: curves should have the same shape and character as those

from previous runs or repeated sections

Cross-check the curve character with other logs from the same logging run.

Adapted after Krygowski, 2004

3/4/2013 Chap -4 24

3.6.2.6 Natural Radioactivity - Summary

Natural Gamma-activity controlled by U-, K- and Th- content

Two techniques are applied

– integral measurement

– spectral measurement

Gammalog is a typical „lithology log“ based on the measurement of the natural gamma-radioactivity of a formation.

Summary

3/4/2013 Chap -4 25

• K, Th, and U as source of radioactivity are concentrated in shale

shale has high gamma reading.

• Shale-free („clean“) rocks (sandstones and carbonates) usually have

low gamma intensity.

• Gammalog can be applied for lithologic profile design, shale content

estimate, and well-to-well-correlation.

• Other shale indicators: Spontaneous potential, Density-Neutron-

Combination

• Attention: Feldspathic, glauconitic, or micaceous sandstone show

high gamma radiation (K); organic matter shows high radiation (U)

Running GR Log along cores

• To be able to match core-log depth mismatch

3/4/2013 Chap -4 26 Core Gamma

Baker Atlas, D. Georgi

Core inside

Core-Log Gamma Ray Correlation Top of Core

Core GR

Example from Core Labs

Open Hole GR Core Gamma

Ray is best

correlation

curve for

Clastics.

Core porosity

is best

correlation

curve for

Carbonates ?

Let’s think

about this !

Baker Atlas, D. Georgi

3/4/2013 Chap -4 28

Gamma-Gamma Log

Interaction of incident radiation (source) with

electrons

- gives information about density porosity

- gives information about lithology

source

detector

3/4/2013 Chap -4 29

3 effects of

interaction

energy loss

Photoelectric effect

Compton effect

Pair production

probability depends on

• energy of radiation and

• atomic number of target material

Gamma Ray Interactions with Rocks

3/4/2013 Chap -4 30

An incident low-energy gamma photon (< 0.2 MeV) collides with an atom

If the energy of the gamma photon equals or exceeds the "binding energy" of

an orbital electron, then

• the gamma photon gives up all of its energy

• the electron leaves its orbit,

• and has a kinetic energy

Ekin= gamma ray energy - electron binding energy

e-

Photoelectric Effect

3/4/2013 Chap -4 31

An incident intermediate-energy gamma photon (gamma ray) collides with

an atom:

• it ejects an electron (“Compton or recoil electron”) from an outer shell

and leaves with a lower energy;

• the scattered gamma energy is a function of the angle of scattering

Compton electron

Compton Effect

3/4/2013 Chap -4 32

An incident high-energy gamma photon (gamma ray energy > 1.02 MeV)1

can be converted into a electron - positron pair when it is near a nucleus.

The electron slows down

The positron interacts with an ordinary electron. They annihilate one another

and produce two gamma-rays.

11.02 MeV is exactly twice the rest mass of an electron (mc2)

nucleus

e-

e+

electron

positron

Pair Production

3/4/2013 Chap -4 33

Gamma-ray energy as result of scattering (Photoeffect and Compton effects)

Gamma

radiation

cps

Energy of

gamma

radiation

Photoelectric effect Pe Z

Compton effect electron density

mineralogy

density

increasing

Z

Pe density

measurement

r1

r2 < r1

3/4/2013 Chap -4 34

Gamma Ray Absorption Mechanisms

Cs Co

Rock

forming

elements

In the energy range between 0.5 and 5 MeV

for most abundant elements the COMPTON-effect dominates.

3/4/2013 Chap -4 35

Interactions result in attenuation (absorption) of radiation, described by

absorption coefficient a

I0 I(x) IGG(x) = I0 exp (-a x)

x

The absorption coefficient is

• connected with the absorption cross section

• related to the effect of interaction:

Absorption of Radiation

ac - absorption coefficient for Compton effect

aPe - absorption coefficient for Photoelectric effect (Pe)

3/4/2013 Chap -4 36

For many elements the photoelectric cross section shows the

proportionality to atomic number Z3.6

sPe Z 3.6

on this basis a effective photoelectric index Pe (average

photoelectric cross section per electron) is defined:

Pe = (Z/10) 3.6 Pe depends on elemental composition (lithology) - see table.

Pe - unit: b/e barns per electron

Photoelectric Effect

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Mean values for density r, electron density re , ratio Z/A, and photoelectric absorption index Pe

Substance r (g/cm3) re (g/cm3) Z/A Pe (b/e)

quartz 2.654 2.650 0.499 1.806

calcite 2.710 2.708 0.500 5.084

dolomite 2.870 2.864 0.499 3.142

halite 2.165 2.074 0.479 4.65

gypsum 2.320 2.372 0.511 3.420

anhydrite 2.97 2.96 0.499 5.05

kaolinite 2.44 2.44 0.50 1.83

illite 2.64 2.63 0.499 3.45

barite 4.48 4.09 0.446 266.8

water (fresh) 1.000 1.110 0.555 0.358

oil 0.850 0.948 0.558 0.125

3/4/2013 Chap -4 38

note

• Pe can help to discriminate between Quartz, Calcite, and Dolomite,

• Pe is one component in mineralogy-porosity crossplot technique

• Pe is extremely sensitive with respect to barite (mud!)

3/4/2013 Chap -4 39

Compton effect Bulk density and electron density

+ + + +

+

-

-

-

-

-

Number of orbiting electrons e control

probability of Compton effect

But bulk density is

controlled by

A = Z + N

e = Z

Z/A 0.5

Compton effect controlled by bulk density

3/4/2013 Chap -4 40

Probability for Compton

scattering is proportional

the number of electrons

per unit volume

density -

number mass atomic - number atomic -

)10 (6.026number sAvogadro' - N

where

23

r

r

AZ

A

ZNe

For practical purposes we define an

„electron density“ rr A

Ze 2

Compton Effect

3/4/2013 Chap -4 41

Electron density is related to the number of electrons per molecule (Z), and bulk density is related to the total atomic mass per molecule (A).

For most common Earth minerals, the ratio is constant

and thus

5.0A

Z

eebA

Zrr

r 2

3/4/2013 Chap -4 42

Mean values for density r, electron density re , ratio Z/A, and photoelectric absorption index Pe

Substance r (g/cm3) re (g/cm3) SZ/SM Pe (b/e)

quartz 2.654 2.650 0.499 1.806

calcite 2.710 2.708 0.500 5.084

dolomite 2.870 2.864 0.499 3.142

halite 2.165 2.074 0.479 4.65

gypsum 2.320 2.372 0.511 3.420

anhydrite 2.97 2.96 0.499 5.05

kaolinite 2.44 2.44 0.50 1.83

illite 2.64 2.63 0.499 3.45

barite 4.48 4.09 0.446 266.8

water (fresh) 1.000 1.110 0.555 0.358

oil (med. gr.) 0.79 0.80 0.57 0.125

3/4/2013 Chap -4 43

For practical log applications two effects

are important

Compton effect

Photoelectric effect

Density determination by nuclear measurements applies Compton effect;

the correlation between density and electron density bases on a nearly

constant ratio Z/A.

Determination of Pe applies Photoelectric effect and gives an information

about mineral composition by the strong correlation to atomic number Z

Gamma Ray Interactions - Summary

3/4/2013 Chap -4 44

Gamma-Gamma-Measurement: Tool and Calibration

source

Detector 1

Detector 2

density

Count

rate

count rate axis logarithmic

density axis linear

Short spacing

Long spacing

3/4/2013 Chap -4 45

Gamma-Gamma-Density

• Caliper and rugosity

• Mud density

• Deviation from Z/A = 0.5 (mineralogy)

• Barite

Corrections:

Primary calibration of density tools usually freshwater saturated limestones of

high purity,

Secondary calibration aluminium, sulfur, concrete blocks

SUPPORT MATERIAL

3/4/2013 Chap -4 46

Geiger-Muller Tube