5. Task Geoscience - Structural Interpretation Methodology

7/29/2019 5. Task Geoscience - Structural Interpretation Methodology

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Structure 1/1

Image log & dipmeteranalysis course

Structural interpretation

methodology


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Structure 1/2

Structural interpretation from borehole images

Dip analysis

Structural zonation Structural boundary interpretation

Curvature analysis

Integration with logs and seismic data

Fracture characterisation Fracture description

Fracture distribution and sampling bias

Influence upon flow

Structural issues in deviated wells In-situ stress analysis

Interpreting in-situ stress indicators

Geomechanical applications


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Structure 1/3

Primer


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Structure 1/4

Plane orientation elements & notation

Dip: maximum inclination of plane from horizontal.

Azimuth: direction of maximum inclination as compass

bearing from 0-360.

Strike: trend of any horizontal line on plane, 90 from azimuth.

Reported as dip/azimuth, e.g. 45/045.

Azimuth

North

Horizontal plane


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Structure 1/5

Upper hemisphere stereoplot

Planes plotted as poles. Centre of circle horizontal.

Rim of circle vertical (unless

otherwise labelled).

Dip denoted by distance

from centre of stereoplot.

Azimuth denoted by angle

clockwise from top ofstereonet N).

N

45/045


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Structure 1/6

Dip azimuth and strike histograms

Dip azimuth rose histogram

Petals denote dip directions

Small petal: few dips in this direction

Large petal: many dips in this direction

Used commonly for visualising beddirections.

Strike histogram

Petals denote strike of plane

Symmetrical about centre

Used commonly for fracture work

N

N


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Dip tadpole plot:Glyphs representing feature attitude.

Dip azimuth plot:

Symbols represent feature dipazimuth, scaled from 0 (North) to

360 (North)

Depth (ft)

881

882

883

884

885

886

887

888

889

10 30 50

Tadpole Plot

120 240

Dip Azimuth Plot

0 360

Increasing

inclination

N

39115

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N E S W N

115

Tadpole plot emphasises dip domains,

Azimuth plot emphasises azimuth domains.

Dip data representation


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Each bed is drawn as an arrow pointing to

its dip azimuth. The plot is built from the base of the study

interval up-section, with the tail of eachfeature arrow placed at the head of theprevious arrow.

Sections of consistent dip azimuth becomeapparent, boundaries may be distinguishedas sharp or gradational.

Arrow length varied with pick confidence toemphasise good data over poor.

Used to identify subtle structural changes.

Base of interval

Top of interval

Dip azimuth vector walkouts


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Each bed is drawn as an arrow oriented to its dip magnitude(where right is 0 and down is 90).

The plot is built from the interval base up-section.

Sections of consistent dip magnitude become apparent,

boundaries may be distinguished as sharp or gradational.

Arrow length varied with pick confidence to emphasise good data

over poor.

Used to identify bulk structural zonation.

Base of interval

Top of interval Dip angle

Cumulative dip magnitude plots


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Dip analysis

Analysis of bedding fabrics and

key structural surfaces to produce

a bulk structural zonation


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Objective setting Initial visual analysis of succession

Review of image data

Dip picking

Structural zonation

Structural dip determination

Structural boundary interpretation

Analysis of folded successions Integration with seismic data

Outline


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Objective setting

Objective

Verification of seismic

structure

Bulk structure model in

area of poor seismic data

due to e.g. shallow gas

Fault location and

orientation to plan

sidetracks

Structural input to a

deterministic reservoir

model

Reorientation of fabrics in

core

Input

Regional information.

Zones of interest.

Budget?

Advice on acquisition practice.

Output

Specification of information

required.

Objectives met?


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Scales of measurements

15m

1cm

Borehole

images

and

dipmeters

3D seismic

Core

Fault throw (m)

Cumulativefau

ltdensity

(faultspe

rkm)

0.0

1 1

100

10000

10000

100

1

0.01

Real geology and

limitless resolution

but limited coverage

& hard to see largescale structure

VSP

Fabrics resolved down

to fractions of an inch

using microresistivity

and acoustic tools, an

inch using dipmeters

and inches to feet using

LWD devices

Seismic finds large-

scale reflective

packages but littleinternal detail; VSP

adds more detail to

under 10m

resolution

Open hole WBM resistivity tools

Open hole OBM resistivity tools

Open hole acoustic tools

LWD tools: density, resistivity,

gamma-ray, photo-electriceffect


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Faultdamage

zone

Look at all scales!

Core to seismic


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Initial visual assessment

Objectives:

Identify major structural zones and

bulk structure

To identify areas which require further

detailed examination

Data required: Automatic dip results

Open hole logs to identify lithology

changes, etc.

Known stratigraphy

Use overview scale (1:500 or 1:1000) to

identify major zones in the context of the

open hole log suite.


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Close examination of images (often 1:5 to 1:20 scale)

Assessment of automatic dip data Are computed dips representative of primary bedding fabric?

Do events in computed dips represent structural boundaries?

Are dip artefacts present?

Do outlier dips represent fractures or over-steep beds?

Recognition and description of artefacts Evaluation of types of features that are visible

Core comparison where possible

Construction of dip classification types

Picking of a small number of manual dips to assistinterpretation of automatic dips

Is data representative and reliable can analysis of theautomatic dip data satisfy objectives?

Image review


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If automatic dips are representative fabric orientations then

analysis can continue: Classification of automatic dips using log cut-offs and

inclinations (e.g. shale beds, sandstone beds from

gamma-ray log, over-steep beds where inclination is over

15 above the background dip) Infill picking of bedding fabric where automatic dips have

low correlation coefficients or dips are absent

Detailed manual picking over intervals where structure is

thought to change

Dip analysis to produce a structural zonation and

representative dips

Reclassify and analyse?


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Pros and cons of this approach

Pros

Rapid turnaround Provides a bulk

structural

interpretation

Adds value to a

traditional dipmeteranalysis

Cost-effective

Cons

Using modern image logs as dipmetersmisses a significant amount of information:

Data is low resolution

Detailed description and classification of

textures is not done

Automatic dips are placed at the centreof each correlation window; the position

of a features within this is not known

Fractures and faults are unlikely to be

imaged

Fine-scale fabric variations are omittedbut these are important in understanding

the sedimentology

Manual dip picking allows much greater

interpretation resolution and confidence


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Systematic pass at appropriatescale (often 1:5 to 1:20)

Fit sine curves to features

Assign categories based on

static image character, open

hole log response and context

Adjust scales and reverse

colour-scaling to get the most

from the images and reclassify

dips if necessary Flag important features that

require further analysis

Manual dip picking


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1:10 scale, section 1.5 metres

Manual dip picking example


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Where possible use static image to define major

changes in lithology and nature of fractures (hereresistive is bright, conductive dark).

The dynamic image captures more detail within lowcontrast intervals and allows dense dip picking.

Dip categories may change after structural dip isremoved, as anomalous orientations become moreevident; dips are finalised after initial structuralanalysis, derotation and facies picking.

Dips are assigned a quality based on the confidenceof category type assigned, fit of sine curve andfeature continuity in image.

Manual dip picking


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Structure 1/25

Dip analysis and structural dip determinationVector walkout plots, cumulative dip magnitude plots,

Interactive stereographs, derotated tadpole tracks

Initial dip data assessment

Dip tadpole and dip azimuth plotsInterval stereoplots and rose histograms

PRELIMINARY STRUCTURAL

ZONATION

STRUCTURAL ZONATION,STRUCTURAL DIPS

Zone boundary interpretationCurvature analysis plots, integration of evidence

from images, well tops and other available data-sets

STRUCTURAL INTERPRETATION

OF DIP DATA

Fracture and fault analysis,curvature analysis

ITERATIVE!

Structural interpretation workflow


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Structure 1/27

Top zone 1

Top zone 2

Top zone 3

Top zone 4

Dip azimuth vector walkout plot

Overall NE dip

Top zone 2

Top zone 3Top zone 4

Top zone 1

Cumulative dip magnitude plot

0 AZIM 0

0 DIP 90

Dip analysis plots


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Structure 1/29

Further division of structure

Domain 210-30 SSEUp-holeshallowingFault drag?

Domain 110 SEUniform dip

Domain 35 NWUniform dip

Fault?

Fault?

Fault?

Subdomain 1aModerately uniformorientation; localup-hole shallowing-depositional?

Subdomain 1bVariable azimuthsSerrate dip profile

-complexdepositional dipsor faulting

Subdomain 2aUp-hole shallowingdips. Ends atsteep S dips thatmay be faults.

Subdomain 2b

Up-hole shallowingdips. Ends atfault with no footwalldrag - listric?

Subdomain 3aShallow NW dips

Subdomain 3bChaotic WNW dips

Subdomain 3cUp-hole steepening - drag?


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Structure 1/30

Dip analysis exercise: part 1

15 mins


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Structure 1/31


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Structure 1/32

Part 1 discussion

Automatic dips good;

representative of beddingfabric, but rarely captures

fractures and some spurious

dips (2554 m, 2610-2620 m,

below 2755 m).

Shales and sandstones are

present; sandstones at

2497-2543 m with some shale

partings.

Automatic dips good, even insandstones

Possible structural breaks:

Azimuth?

Dip?

Lithology?

Data quality?

Lumpers versus splitters

Start at large scale,

refine later

Structural zones and

sub-zones?

Work in isolation from other

data (e.g. stratigraphy) and

incorporate later?


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Structure 1/33

Initial dip interpretation


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Structure 1/34

Importance of scale


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Structure 1/35

Structural dip in slumped sequences


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Structure 1/36

Objective:

Break the logged interval into intervals of consistent structural

dip or dip motif. Produce a table of structural dip zones

Techniques:

Visually recognise large scale structural zones from manual

dip data plotted on vector walkouts and dip magnitude plots. Use zones of originally horizontal bedding (e.g. shales or

limestone bedding) to measure post-depositional dip.

Use stereographic techniques and statistic methods to refinezone picking and representative structural dips.

Use statistical curvature analysis techniques where noparallel bedding exists.



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Structure 1/37

Upper hemisphere stereoplots of poles to

planes and bed azimuth rose histograms.

1% Schmidt contoured poles, weighted tointerpretation confidence.

Only palaeohorizontal proxies used.

Eigenvector and Fisher Analyses.

Peak count of the weighted contour plot.

Structural dip is used to de-rotate post-

depositional tilt from bedding dips and the

method which flattens bedding most

successfully is chosen.

NEqual Area

(Schmidt)

Upper Hem.

Wtd Point Density

N = 553

2%

4%

6%

8%

10%

12%

14%

16%

18%

20%

22%

24%

26%

28%

Structural dip calculation

Schmidt

Wulff


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Structure 1/38

Subsurface dip is the sum of depositional dip, compaction,

soft sediment deformation and cumulative tectonic deformation

Subsurface dip components


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Structure 1/39

Dip analysis exercise: part 2

15 mins


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Structure 1/40

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STRUCTURAL INTERPRETATION EXERCISE

Top of interval

Base of interval

Dip azimuth vector walkout plotGreen: shale bedding

Brown: sandstone bedding

Cumulative dip magnitude plotGreen: shale bedding


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N


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Structure 1/41

Part 2 discussion

Zone Top Base Orientation Comments

1 2450 2543 6/260 Includes sandstones and

slumps; W dips at base

2 2543 2650 8/290 Rotation to N dips at top

may be depositional

3 2650 2683 10/320 As above4 2683 2713 18/320 As above

5 2713 2755 15-28/320 Progressive down hole

dip increase - rotation

into fault below section?6 2755 2770 ? Unreliable


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Structure 1/42

Part 2 discussion

Sandstones at 2497-2542 m are problematic:

Internally inconsistent cross-bedded and/or slumped

West dips at base are consistent with shales above

Internal NW shale parting dips are consistent with

shales below

Sand presence may be due to a structural change(i.e. should be derotated using shale dips above).

A change in depositional slope may have led to slumping.

Use the wrong structural dip, get inaccurate orientations

from sandstone beds during analysis of deposition andpalaeoslope.


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Structure 1/43

Examine images over the boundary

Indications of fracturing, faulting, folding, erosion etc.

Changes in structural dip? Sharp or gradual?

Dip rotation trends?

Changes in lithology/stratigraphy? Biostratigraphical events?

Open hole log responses that mayindicate weathering, erosion or hiatus?

A dip pattern alone is insufficient evidence for positiveidentification; more evidence is required.

Structural zone boundary interpretation


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Structure 1/44

0 90

0 90

Parallel unconformity

Angular unconformity

Planar unconformitywith weathered zone

Buried topography

with weathered zone

Unconformities dip patterns


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Structure 1/45

Bedding fabric in lower unit truncated by overlying unit. Juxtaposition of distinctly different lithologies.

Abrupt dip change with no progressive rotation.

Presence of clasts.

Presence of drape, seen in anomalous dips. Compactiontends to make the orientation change more gradual.

Change in static image response due to weathering,

changes in cement, reworking.

Regional knowledge; unconformities are often known fromseismic data and so explain changes in dip from images

Unconformities evidence from images


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Structure 1/46

Normal fault

Fault dip exceeds maximum bedding dip in drag zone

Deformation in hanging-wall and foot-wall

Reverse fault may also display deformation in hanging-walland foot-wall but dip azimuth in drag fold opposes fault plane

azimuth.

Listric growth fault

Fault dip exceeds maximum inclination

of beds in drag zoneBut has opposing dip direction

Deformation restricted to hanging-wall

Ramp antiform above thrust fault

Only evident at ramps; flats will not

display drag.

Faults dip patterns


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Structure 1/47

Fault kinematics from displacement

FW

HW

FW

HW

Vertical well

Normal

Reverse


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Structure 1/48

Identifying faults

It is rarely possible to demonstrate

displacement unequivocally on images:

we normally describe fractures andonly inferfaults

(Parkinson et al. 1999).


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Structure 1/49

Direct observation: clear offset of bedding or fractures

Indirect observations: Change in structural dipblock rotation

Juxtaposition of differing lithofacies across adiscrete fracture

Progressive rotation into structural boundaryfault drag

Enhanced fracture densitydamage zone

Change in cement type

Fluid interface (if sealing)

Pressure change

Hole damage (commonly washout)

Change in the intensity and/or orientation ofpresent day in-situ stress features

Faults evidence from images


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Structure 1/50

Buried topography

with weathered zone

Normal fault with adjacent drag

Caution


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Structure 1/51

Upright, symmetric synform

Upright, symmetric antiform

Upright, asymmetric antiform

Recumbent similarfold (left)

Upright parallel

fold (right)

Plunging upright

antiform

Parasitic folds

Folds dip patterns


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S ti l di l


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Structure 1/53

Zone 1

5/220

Zone 2

6/140

Zone 3

8/350

1.

Cromer

Knoll

2.

Valhall3.

Pre-ValhallOriginal

Sequential dip removal

St t l di f b ddi


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Structure 1/54

Poles to cross-beds

Rotation

axis

Depositional attitude:

Cross-beds depositedon horizontal surface

Subsurface attitude:

Bedding tilted through

large-scale rotation duringregional tectonism

Tilt

If enough sets of cross-beds

are sampled, their axes of

curvature will define a girdle,the pole to which is the

structural dip.

The poles to cross-bedding

planes in a single unit

will fall on a girdle, the pole

of which (i.e. axis of curvature

lies within the plane of the

structural dip.Upper hemisphere stereoplot

Structural dip from cross-bedding

F ld


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Structure 1/55

Folds are more commonly observed in horizontal wells than

the analysis of vertical wells would suggest. Identification & classification chevrons and tadpole facing

directions, synclinal and anticlinal.

Analysis wavelength, amplitude orientation & plunge.

SCAT analysis. Dipping beds can have an effect on fluid flow, varying as a

part function of up-dip and down dip fluid transport.

Can be hard to track single bed in horizontal wells.

Problems with dip removal non-linear changes in dipsacross a fold.

Folds

Statistical Curvature Analysis Techniques


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Structure 1/56

Graphical techniques used to assess presence and attitude

of folds in a dip data-set (Bengtson 1980).

Used to identify and orientate:-

1. Large-scale fold trends in compressive regimes.

2. Growth faults.

3. Drag folds against faults to derive fault strike if dip-slip.

4. Slump fold axes; may identify palaeoslope strike.Bengtson, C.A. 1980. Statistical Curvature Analysis Techniques for

Structural Interpretation of Dipmeter Data. AAPG Bulletin 65, 312-332.

Statistical Curvature Analysis Techniques

(SCAT)


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St hi l l i f f ld


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Structure 1/58

+

N

Fold girdle

(great circle)

Pole to bedding plane

Upper hemisphere

Schmidt stereoplot

Limb 1

Centre of cluster defines

limb orientationLimb 2

Scatter defines hinge;

few dips indicates angular,

smooth spread suggestsfold is gently curved.

Inter-limb angle measured

along great circle

Fold axis is pole

to fold girdle if

fold is parallel

Fold axis is line of intersection between

mean limb orientations if fold is similar

Stereographical analysis of folds

Dip analysis exercise


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Structure 1/59

p y

Part 3

10 mins


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Structure 1/60

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STRUCTURAL INTERPRETATION EXERCISE

Top of interval

Base of interval

Dip azimuth vector walkout plotGreen: shale bedding


Cumulative dip magnitude plotGreen: shale bedding


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N

P t 3 di i


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Structure 1/61

Part 3 discussion

Fractures strike E-W and N-S; low bias as vertical well.

Fracture inclinations show normal distribution around 45; may beearly and rotated or compacted.

Faults strike E-W and N-S.

Inclinations scattered from 30-75; slightly steeper than fractures

and so may be later?

Fractures and faults are clustered into possible damage zones or

due to mechanical stratigraphy.

SCAT of whole interval shows E-W curvature axis is this the

basin axis or related to E-W striking fault population?

SCAT in sandstones is inconclusive; needs derotating?

SCAT below 2710 m shows curvature about a NE-SW trend

suggests rotation or fault drag above feature below study interval.

Part 3 discussion


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Structure 1/62

Part 3 discussion

Depth Type Comments

2543 Unconformity Sharp, no drag, few fractures, significantlithology change. Grade B.

2650 Unconformity Gradual azimuth rotation at top looksdepositional. Grade B.

2683 U/C or fault Possible drag, fracturing but also azimuth

swing as above. Grade C.2713 U/C or fault Drag? Fractures define damage zone?Faults picked striking E-W to ENE-WSW.Grade B.

2755 Data quality End of reliable inclinometry and caliper

data no geological significance. Possibledrag downwards through zone above mayindicate that a fault is present beneath.


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Integration with seismic interpretation


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Structure 1/64

Integration with seismic interpretation

X

X

B

A

A

B

Possible

faulting

well pathWNW (286) ESE (106)

crestal graben

western flank

Eastern flank


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This section has covered


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Objective setting

Initial analysis of succession

Review of image data

Manual dip picking

Structural zonation


Zone boundary description

Folded zones Integration with seismic data

This section has covered

5. Task Geoscience - Structural Interpretation Methodology

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Transcript of 5. Task Geoscience - Structural Interpretation Methodology