Lec 4-1 Well Log Correlation

5/28/2018 Lec 4-1 Well Log Correlation

1/60

Well Logging and Formation Evaluation

PAB2084 & PCB 2044

Well Log Correlation

Dr Ahmed Salim


2/60

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:


3/60

CONTENTS

3

1. Introduction

2. Correlation Data

3. Correlation types

4. Correlation plan and tips

5. Conclusion


4/60

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.

4


5/60

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


6/60

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.

6


7/60

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.

7

Correlation Logs


8/60

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.

8


9/60

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.

9


10/60

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.

10


11/60

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

11


12/60

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

12


13/60

Examples: Lateral and li thology

13


14/60

Lateral and lithology

14


15/60

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

15



16/60

Rock properties

16


17/60

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.

17



18/60

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.

18



19/60

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.

19



20/60

Fossi l content

20


21/60

Chronostratigraphic Vs Lithostratigraphic

21


22/60

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.

22

Done by methods of absolute age

dating and/or analysis of fossilassemblage. They are not used

commonly for petroleum geologists.


23/60

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.

23


24/60

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.

24


25/60

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


26/60

The reference horizon with the feature of high resistivity mudstones

Delta front depositional subfacies


27/60

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 ))

27


28/60

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).

28


29/60

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


30/60

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.

30


31/60

Walther 's Law

31

(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.


32/60

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.


33/60

2. The sequence stratigraphic frameworkis constructed by

correlatingthe keysequence stratigraphic surfaces(chronostratigraphic markers)


34/60

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.


35/60

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 .


36/60

Well Correlation Plan

36


37/60

37

Example ofa Well

CorrelationPlan

The skill of stratigraphic correlation


38/60

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.


39/60

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


40/60

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


41/60

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.

41

Tips for Correlation


42/60

42



43/60


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.

43

Ti f C l ti


44/60

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.

44


Ti f C l i


45/60

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.

45


Ti f C l ti


46/60


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.

46


47/60

47

Repeated

Sections


48/60

48


49/60

49


50/60

50


51/60

51

LATIHAN


52/60

52

WELL #456

C

OIL

OIL

LATIHAN


53/60

53

OWC

D

C

WELL

#123

OILOIL

LATIHAN


54/60

54

OWC

D

C

WELL

#123

OIL

OIL

LATIHAN


55/60

55


56/60

Extension to seismic sections

56


57/60

57

CORRELATION


58/60

CORRELATIONPROBABI L ISTIC to DETERM INI STIC Modeling

58

Conclusion


59/60

59

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


60/60

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.

Documents

Lec 4-1 Well Log Correlation