lec 4-1 well log correlation
DESCRIPTION
Detail explanation of how to easily correlate logs using the combination technique of a quick look process and in depth analysis. six technique of correlation are discussed and how relate to one another.TRANSCRIPT
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Well Logging and Formation Evaluation
PAB2084 & PCB 2044
Well Log Correlation
Dr Ahmed Salim
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At the end of this lecture you should
be able to correlate differentreservoir units using various dataset with an emphasis on well Logs.
Learning Outcomes:
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CONTENTS
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1. Introduction
2. Correlation Data
3. Correlation types
4. Correlation plan and tips
5. Conclusion
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Log correlation
Correlation of logs involves the identification ofcharacteristic patterns orsignatureswithin the logs, and
then matchingthese to the same orsimilar patterns in
neighboring boreholes (pattern recognition).
Correlation or stratigraphic correlation is a geological
term referring to theprocessby which twoor more
geological intervalsare equatedeven through they are
spatially separated.Stratigraphic Cross Section is the best demonstration of a
correlation results.
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Why Well Log Correlation
Well-to-well correlation studies permit accurate subsurface
mapping and the determination of:
The elevations (Tops) of formations present in the well
Whether well depth has reached a known productivehorizon, and, if not, approximately how much remains to be
drilled.
The presence or absence of faults. dips, folds, or
unconformities;
The thickening and thinning of lithologic sections; or lateral
changes of sedimentation or lithology.5
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Correlation Data
Cuttings, cores and well logs constitute the main
data used for correlation, with well logs playing a
special role because they are long, continuous
recordings.
Additionally, they are useful because they are
unbiased physical measurements and oftenavailable in a large number of wells in a field or a
basin.
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The best combination of logs for the correlation process
will depend upon the nature of the rocksbeing correlated.
Most logs used are Neutron, Gamma ray, Density, Sonic,
Resistivity and SP.Many geologistsprefer to use just the GR log or a
combination of GR logs with SP logs, because they are
commonly available and they carry a relatively strong
lithological signal.
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Correlation Logs
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Correlation Logs
Nuclear Spectroscopy Logs (NGS) reflect lithologybetter than most of logs and practically insensitive to
fluids.
Conventional resistivitycurve (commonly base lateral
electrode array with 2.5 meter electrode spacing R2.5) is
sensitive to the fluids, so it can be used to correct
affection of fluidsand for reference shale tracing.
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Correlation Logs
Neutron logs
Their sensitivity to clay mineral content makes them
particularly suited to correlating coalbearing strata.
Gamma ray
Since it the most frequently run measurement, recorded in
all types of borehole conditions, and is a good lithology
indicator (its response in marine bands covering large areas)it has an important role in correlation.
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Correlation Logs
Density
Some formations, notably coal, anhydrite and halite have
density values that are usually diagnostic, giving excellentfingerprinting potential.
Resistivity
Guard logs have good vertical resolution and are relatively
insensitive to borehole conditions, so they are potentially
useful correlation logs.
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Types of Correlation1.trace lateral continuity2.Lithology - lithologic correlation (lithostratigraphy)
3.correspondence in ageand in position
(chronostratigraphy)4.stratigraphicsuccessions
5.fossilcontent (correlation of two fossil - bearing beds )
Biostratigraphy
6.chemical, petrophysical, magnetic, or geophysical
properties
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Physical Criter ia for Correlation
1.Physical continuity:
In this photograph, we
can be certain about
the relationships ofdifferent beds, because
we can visually trace
each bed over longdistances
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Examples: Lateral and li thology
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Lateral and lithology
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2.Lithology:
Similarity of rock types in associated outcrops can beassessed through a comparison of lithological
properties including rock type, texture (roundness,
sorting, orientation), and color
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Physical Criter ia for Correlation
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Rock properties
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3.Mineralogy:Similarity of the mineral composition of rocks in
associated outcrops allows for their correlation,
especially if distinctive or uncommon mineralsare present.
4.Position in sequence:
Rockpositionin relation to distinctive rock unitsabove and below may aid in correlation.
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Physical Criter ia for Correlation
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In the figure shown here,
the limestoneunit at
localityAcan be
correlated with the lowestof thefourlimestone units
at localityBbecause of its
position between the gray
shale and the sandstone
units.
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Physical Criter ia for Correlation
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5.Foosi l content:
fossil organisms succeed one another in a definite
and determinable order, and therefore any time
period can be recognized by its fossil content.
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Physical Criter ia for Correlation
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Fossi l content
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Chronostratigraphic Vs Lithostratigraphic
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Stratigraphic correlation
The stratigraphic correlation can be dividedinto the
four scale as follow:
The worldwide scale;the regional scale;
the basin (or depression) scale; and
The oilfield scale.
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Done by methods of absolute age
dating and/or analysis of fossilassemblage. They are not used
commonly for petroleum geologists.
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Methods of stratigraphic correlation
(1) The concept of similarity
Reflected on log curves with the similarity of amplitude,
shape ,thicknessand association etc.
Four aspects:
type well;reference horizon (or key horizon/bed);
sedimentary cycle; andthe rule of facies change.
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Type well
It is the well, which is the most completeinstratigraphic
successionexposed,strataof the most thick, without
strata loss and abnormal thinning or thickeningcaused
by fault and erosion, with good quality and complete
logging series, with system analysis data of core and test,
clear of chronostratigraphy.
The type well is relative, it is chosen in the process of
stratigraphic correlation.
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Reference horizon (or key bed)
It is the horizon, which lithologicalfeature is obvious,
steady on lithology, the thinner in thickness, the farther
horizontal extension and easy to identify on the log curves.
Generally, the Reference horizon is isochronal.
Reference horizon can be divided into main and secondary
according to difficulty to identify and distributionrange.
The more the reference horizon is the more reliable the
stratigraphic correlation is.25
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The reference horizon with the feature of high resistivity mudstones
Delta front depositional subfacies
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Sedimentary cycle
It is geologic record left in the strata with periodicvariationdue toperiodic variation of geologic agent.
Periodic variation in the strata features (such as lithology,
thickness of bed, color, fossil, and lithology associationetc.) can be expressed on the log curves exceptfor rock
color.
Therefore, cycle can be studied comprehensively by log
curve combined with lithology, rock texture, thickness andfossil. These features can be analyzed and plotted cyclic
curve.(TransgressiveRegressive (T - R ))
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Sedimentary cycleAccording to the change of
features (bed scale),second
order cycles ( I, II and III) can
be determined on thebackground of the first order
cycle (formation scale)
exhibited as regressive cycle
(reverse cycle).
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The rule of facies change
The connection between the vertical and lateral changes offaciesobserved in outcrop and subsurface is made by
Walthers Law.
As discussed byMiall (1997), vertical changes in litho- and
biofacies have been used to reconstruct paleogeography
and temporal changes in depositional environments and,
with the aid of Walthers Law, to interpret lateral shifts of
these environments. As a note of caution, however,such interpretations are
only valid within relatively conformable successions of
genetically related strata.29
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Walther ' s Law
(M iddleton, 1973):in a conformable succession theonly facies that can occur together in vertical
succession are those that can occur side by side in
nature.
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Walther 's Law
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(Posamentier and Allen, 1999):the same
succession that is present vertically also is present
horizontal unless there is a break in
sedimentation.
In other words, a vertical change of facies implies
a corresponding lateral shift of facies within a
relatively conformable succession of genetically
related strata.
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A prograding delta is a good illustration of the Walther's Law concept.
1. The reconstruction of paleodepositional environments via
facies analysis is an importantpre-requisite for sequence
stratigraphic interpretations.
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2. The sequence stratigraphic frameworkis constructed by
correlatingthe keysequence stratigraphic surfaces(chronostratigraphic markers)
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3. Sequence stratigraphic cross section, showing key surfaces, within-
trend facies contacts, and paleodepositional environments. Within-trend
facies contacts, marking lateral changes of facies, are placed on the
cross-section after the sequence stratigraphic framework is constructed.Facies codes: A--meandering system; B--braided system; C--estuary-
mouth complex; D--central estuary; E--delta plain; F--upper delta
front; G--lower delta front--prodelta.
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4. Lithostratigraphic cross-section. Three main lithostratigraphic units
A--a sandstone-dominated unit;
B and C - mudstone-dominated units, with silty and sandy interbeds. Formations B and C are separated by Formation A .
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Well Correlation Plan
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Example ofa Well
CorrelationPlan
The skill of stratigraphic correlation
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The skill of stratigraphic correlation
1. The hierarchy of sedimentary cycle
and reference horizon can be identified
by use of logcurves of SP / GR /
COND / ML / R0.5
etc.
The reference horizon used commonly
is : thick mudstone interval, oil shale
like comb on conductivity curves, thincoal, thin volcanic tuff interval with
high resistivity and high GR, thin
limestone interval.
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limestone
coal
C81
C82
Thin volcanic tuff interval
with high AC , high GR and low DEN
Oil shale with high AC , high GR and low DEN
Skills for stratigraphic correlation
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Skills for stratigraphic correlation
2. The logsplot will be shifted to
correlate between wellsfollowing the
procedure as :
upper before downThe easier beforethe more difficult;
The near before far;
large interval beforesmaller one
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As a starting point, align the depth scale of logs
and look for correlation as shown below or if no
begin to slide one of the logs.
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Tips for Correlation
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Tips for Correlation
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Tips for Correlation
The section should show reservoir lateral and verticalfacies changes, markers continuity, missing & repetition
sections, completion & prod. testing notes, etc.
Good markers can be organic shale, coal/lignite,
limestonebeds, glauconite, sideriteetc. which has good
continuityand correspond to the geologic eventssuch as
maximum flooding.
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Ti f C l ti
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Startthe correlation with the whole log section of
individual well, make zonationbased on electro facies thendefine all markers and zones of interest. Indicates any
missing and repetition section.
Carry out a detail correlation of objective reservoirs.
For reservoir connectivity indication use also fluid contents
and contacts,pressure data and productionperformance
data
Prepare a good tabulation (database) of geologic data such
as depth of top & bottom of reservoir, net & gross
thickness,faults depth etc.
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Tips for Correlation
Ti f C l i
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1.For initial quick-look correlation, review major
sandstones using the SP or GR curves.
2.For detailed correlation work, first correlateshale
sections. shale deposition commonly cover large
geographic areas.
Sands vary in thickness and character from well to
well and contain fluids
3.Initially, use the amplifiedshort normal resistivitycurve, which usually provides the most reliable
shale correlations.
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Tips for Correlation
Ti f C l ti
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Tips for Correlation
4.Use colored pencils to identify specific correlation
points.
5.Always begin correlation at the top of the log, not
the middle. Because structures like faults tend to die
upward towards the surface
6.Never force a correlation.
7.In highly faulted areas, first correlate down the log
and then correlate up the log.
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Repeated
Sections
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LATIHAN
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WELL #456
C
OIL
OIL
LATIHAN
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OWC
D
C
WELL
#123
OILOIL
LATIHAN
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OWC
D
C
WELL
#123
OIL
OIL
LATIHAN
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Extension to seismic sections
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CORRELATION
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CORRELATIONPROBABI L ISTIC to DETERM INI STIC Modeling
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Conclusion
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Conclusion Wells correlation studies permit accuratesubsurface
mapping and the determinationof different reservoirparameters.
Cuttings, cores and well logs constitute the main data used
for correlation, but the latter is the most useful because its
consistency and availability.
Reservoir units can be correlatedaccording to their
lithology, age and in position,stratigraphic successions,
fossil content and other chemical or petrophysicalproperties.
Conclusion
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Conclusion
Correlation is a tedious process planning and steps to befollowed are important with precautions of repeated and
missed sections, which are due to dipping and different
structures.
Modern correlation is done using Artificial NeuralNetwork (ANN),Dynamic Programming (DP), etc.
Sequences predicted from well logs have to be linked to
seismic data; with VSP,synthetic seismogram; to make the
correlation basin wide.