Introduction to Seismic Interpretationl

8/13/2019 Introduction to Seismic Interpretationl

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Shell Exploration & Production

C o p y r i g h t : S h e l l E x p l o r a t i o n

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r o d u c t i o n

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Introduction to Seismic Interpretation

By:

Hosny Diab

Explorationist Seismic Interpreter / Onshore Exploration Team

Shell Egypt N. V.




How oil trapped Technology used video




Seismic Acquisition operations

Seismic acquisition offshore Seismic acquisition onsho




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




3D seismic Video




• Zoeppritz equations simplify to:

• Acoustic Impedance Z:

RC =Z2 - Z1

Z1 + Z2 for (near) vertical incidence

Z = V where: is density

V is velocity

What can be seen on seismic data?

RC: Acoustic impedance co

between 2 different materia



Shell Exploration & Production Convolutional Model for Synthetic Seismic Trace

Rockcolumn

Reflectivity AcousticImpedance

from sonic & density logs

Reflectorresponses

Syntheticseismogra

Sourcewavelet

0

Mi ni m um ph a s e







3D seismic cube configuration Video






Seismic-to-Well Tie

• Process of

correlating theseismic signal close

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




synthetic deep-reading resistivity




Seismic terms

• Wavelet: a seismic pulse usually consisting of only a few

which represents the reflection shape from a single positiv

reflector at normal incidence

• Event: general feature in seismic data

– Explicit events are features depicted by amplitude extrema

peak)

– Implicit events are features depicted by terminations of expevents (faults, unconformities)

• Trace: a vertical record of seismic amplitudes at a given

or 3D grid coordinate (time or depth),

• Fault shadow: zone of reduced imaging quality in the fo




Seismic terms (Cont.)

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

with a regular x/y sampling

• Horizon Slice: a horizontal display 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

– Trace attribute: along a trace, e.g. Phase

– Horizon attribute: along a horizon, e.g. Amplitude

– Window attribute: between horizons or within a fixed gate, e

energy





• 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 restoring broad-band acoustic im

signal of the subsurface from the ordinary band-limited refl

signal of 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





• Flattening: datuming of vertical and horizontal seismic disp

parallel to a seismic horizon .

– A flattened timslice is also called horizon slice.

– Useful for interpretation of stratigraphic geometries

• Mis-tie: inconsistency between 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”




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 faults to

interpreted as 3D planes is essential to be used

– along the entire Subsurface Interpretation workflow (struc

reservoir model building, upscaling, reservoir simulation).

• Sequencing faults for interpretation should consider struc

setting and kinematics.

• As a minimum, all faults that directly affect volumetrics m

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

occur in (potential) trap geometries. Generally these faults




Common orientations and shapes of faults

• Most hydrocarbon accumulations occur in

– Structural traps involving extensional to moderately transpredeformation,

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

displacement for extensional faults through nearly vertical str

faults to reverse faults of about 60° dip 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 straight or at least have constant curv




Choosing the most suitable digitisation direction

• Fortunately many 3D surveys are oriented such that tseismic grid is aligned with the predominant dip direct

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

most faults,

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

• It is important that the corner coordinates of used arb

lines are stored, as otherwise the interpretation on suc

cannot be revisited or corrected.




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

– kinematic considerations, The specific geophysical respon

rock competence of each interval when making choices with

ambiguous evidence.

• Generation of fault planes by linear interpolation or triang‘ ’




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

Structural Vector (lat

Small scale faultsCoherence (lateral amplitude change)

(vertical displacement > 0.25 wave length)




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 ignore them because, as very steep fea

are much more sensitive to inaccuracies in migration velocit

• Interpret fault segments consistently from upper to lower tip.

• ‘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

80 is often easier to manage.

• Fault junctions and amalgamated faults: shape complexity in




Nigeria Data

raw seismic




Nigeria Data

with Horizon &

Fault

Interpretation




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“Horizon unconformity interpretation”




Guidelines for 3D horizon interpretation

• Horizon interpretation should be executed after initial fau

interpretation

• The minimum set of horizons:

– all unconformities and sequence boundaries

– major lap surface and maximum flooding surfaces

• Other levels may also be needed: time to depth conversiostructural modelling & kitchen/maturity modelling

• Start with shallow horizons on obvious events and to inte

by-step from top to bottom, as structural complexity increa

imaging breaks down.

• Correlate a articular horizon on a coarse rid of lines awa




Guidelines for 3D horizon interpretation

• Ensure that there is no misties of horizons and faults

• It is then safer not to interpret closer to a fault plane tha

traces.

• Jump correlations across faults:

– Get an idea about the throw distribution along the interface

two blocks by tentative horizon interpretation

– Work top down, starting from levels with confident correla

the fault.

– Base your choice on sequence correlation rather than eve

correlation

– Take discrete sedimentary features such as unconformitie




Unconformity: as significant breaks in vertical velocity trends.

Its interpretation depends on the recognition of characteristic reflection geometries rather than

information




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“Exercises”









Documents

Introduction to Seismic Interpretationl