introduction to seismic interpretation el amal

8/12/2019 Introduction to Seismic Interpretation El Amal

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Shell Exploration & Production

Copyright:ShellExploration

&P

roduction

Ltd.

Introduction to Seismic Interpretation

By:

Hosny Diab

Explorationist Seismic Interpreter / Onshore Exploration Team

Shell Egypt N. V.


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How oil trapped Technology used video


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Seismic Acquisition operations

Seismic acquisition offshore Seismic acquisition onsho


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Long PeriodMultiples

Short PeriodMultiples

UpcomingWavelet

Sca

Recordin

Ground Receiver CouplingReceiver Frequency Response

Array Effects

Refractions

Ambient andCultural Noise

Refractions

Q-Factor

Reflection

Coefficient

Interface Losses

SphericalSpreadingDowngoingWavelet

ShotHole Free

SurfaceGhost?

Source Effects

LowVelocity

Layer


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3D seismic Video


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Zoeppritz equations simplify to:

Acoustic Impedance Z:

RC =Z2- Z1

Z1+ Z2for (near) vertical incidence

Z = V where: is densityV is velocity

What can be seen on seismic data?

RC: Acoustic impedance co

between 2 different materia


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Shell Exploration & ProductionConvolutional Model for Synthetic Seismic Trace

Rockcolumn

ReflectivityAcousticImpedance

from sonic & density logs

Reflectorresponses

Syntheticseismogra

Sourcewavelet

0

Minimumphase


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3D seismic cube configuration Video


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VariableDensity VariableWiggleDifferent Seismic Dis

Color Schemes

Seismic section display


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Seismic-to-Well Tie

Process of

correlatingtheseismicsignalclose

to a wellbore to well

information(synthetic

seismogram, lithology

log, deep-reading

resistivity log, tops)

To identify seismic

reflections for horizon

interpretation; in

calibration for

quantitative


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synthetic deep-reading resistivity


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Seismic terms

Wavelet: a seismic pulse usually consisting of only a few

which represents the reflection shape from a single positiv

reflectorat normal incidence

Event: general feature in seismic data

Explicitevents are features depicted by amplitude extrema

peak)

Implicitevents are features depicted by terminations of expevents (faults, unconformities)

Trace: a vertical recordof seismicamplitudesat a given

or 3D grid coordinate (time or depth),

Fault shadow: zone of reducedimagingqualityin the fo


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Seismic terms (Cont.)

Grid: a 2-dimensional array to store horizon, attribute and f

with a regular x/y sampling

Horizon Slice: a horizontaldisplay of seismic amplitude

extracted at a constant distance from a seismic horizon, p

for viewing stratigraphic information (Coherence data)

Attribute: a measurement executed on seismic data, with v

base geometries

Traceattribute: along a trace, e.g. Phase

Horizonattribute: along a horizon, e.g. Amplitude

Windowattribute: between horizons or within a fixed gate, e

energy


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Structural (Slip) Vector / Volume dip & azimuth:

A volume attribute that represents lateral change of phase, ecaused by tectonic deformation of subsurface strata; commo

highlighting of faults and flexures in timeslices and horizon sl

Inversion: a method of restoringbroad-band acousticim

signal of the subsurface from the ordinary band-limited refl

signalof seismic data. Techniques used: Sparse-spike Inversion: deconvolution / whitening plus addin

frequencies from well data

Model-based Inversion: both low and high frequencies are ad

interpreted borehole measurements, extrapolating away from

along horizons


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Flattening: datuming of vertical and horizontal seismic disp

parallelto a seismic horizon .

A flattened timslice is also called horizon slice.

Useful for interpretation of stratigraphic geometries

Mis-tie: inconsistencybetween 2 interpretation of the sam

features on different seismic displays, e.g. Crossing 2D line

inlines-crossline displays of 3D seismic. Also in seismic-to-

Jump correlation: identification of a seismic event on eithe

a fault for regional horizon interpretation.


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Guidelines for 3D seismic interpretation

Faults interpretation


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Guidelines for the Interpretation of Faults

Interpret all visible faults - in order to maximise the unde

of deformational history and the controls on trapping and flo The definition of appropriate selection criteria for faultsto

interpreted as 3D planes is essential to be used

along the entire Subsurface Interpretation workflow (struc

reservoir model building, upscaling, reservoir simulation).

Sequencingfaults for interpretation should consider struc

settingand kinematics.

As a minimum, all faultsthat directly affect volumetricsm

fully interpreted, i.e. those faults that are (potentially) sea

occur in (potential) trap geometries. Generally these faults


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Common orientations and shapes of faults

Most hydrocarbon accumulations occur in

Structuraltraps involving extensional to moderately transpredeformation,

Their faults tend to be rather steep(ranging from about 60w

displacement for extensional faults through nearly vertical str

faults to reverse faults of about 60dip in mildly transpressio

regimes).

Fault shape is controlled by the magnitude of differential s

between the horizontal stress axes,

Bends and kinks can occur if the stress field is laterally variab

All faults are either straightor at least have constant curv


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Choosing the most suitable digitisation direction

Fortunately many 3Dsurveysare orientedsuch that tseismic grid is aligned with the predominant dipdirect

(azimuth) in the subsurface, and are thereby also align

most faults,

it will be sufficient to generate two sets of arbitrary lines,45with the seismic grid

It is important that the corner coordinatesof used arb

lines are stored, as otherwise the interpretation on suc

cannot be revisited or corrected.


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Interpretation strategy

The seismic evidence for faults is

implicit(reflection terminations), ambiguous(not all reflec

terminations are caused by faults)

incomplete(intervals without reflective interfaces also lack e

faults).

may have many different geometries including (self-)branch

Good interpretation practice means taking into account

kinematicconsiderations, The specific geophysical respon

rock competence of each interval when making choices with

ambiguous evidence.

Generation of fault planes by linear interpolation or triang


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Fault (discontinuity) highlighting volume in support of structural interpretation:

Structural Vector (lat

Small scale faultsCoherence (lateral amplitude change)

(vertical displacement > 0.25 wave length)


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Where and how to pick

Pick preferably at the hanging-wall terminations (above the fa

as the seismic image below the fault plane is often of poorer qua

shadow) and does not provide a good contrast between continunfaulted reflections and clear terminations towards a fault plan

If fault plane ref lect ions are present but do not coincide with th

wall termination, better ignorethem because, as very steep fea

are much more sensitive to inaccuraciesin migration velocit

Interpret fault segments consistently from upperto lowertip.

Split-the-distance method. In this workflow one would start int

with a very large increment that can be divided by 2 for a numbe

ideally the power-2 system 1-2-4-8-16-32-64, but the system 5-1

80is often easier to manage.

Fault junctions and amalgamated faults: shape complexity in


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NigeriaData

raw seismic


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NigeriaData

withHorizon&

Fault

Interpretation


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Horizon unconformity interpretation


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Guidelines for 3D horizon interpretation

Horizoninterpretationshould be executed afterinitial fau

interpretation

The minimum set of horizons:

all unconformitiesand sequence boundaries

major lap surface and maximumfloodingsurfaces

Other levels may also be needed: timeto depthconversiostructural modelling & kitchen/maturitymodelling

Startwith shallowhorizonson obvious events and to inte

by-step from topto bottom, as structural complexity increa

imagingbreaksdown.

Correlate a articular horizon on a coarse rid of lines awa


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Guidelines for 3D horizon interpretation

Ensurethat there is nomistiesof horizons and faults

It is then safernotto interpretcloserto a faultplanetha

traces.

Jumpcorrelationsacross faults:

Get an idea about the throwdistributionalong the interface

two blocks by tentative horizon interpretation

Work topdown, starting from levels with confident correla

the fault.

Baseyour choiceon sequencecorrelationrather than eve

correlation

Take discrete sedimentaryfeaturessuch as unconformitie


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Unconformity: as significant breaks in vertical velocity trends.

Its interpretationdepends on the recognition of characteristic reflectiongeometriesrather than

information


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Exercises


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introduction to seismic interpretation el amal

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