introduction to seismic interpretation el amal[1]

Shell Exploration & Production

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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 onshore


Long PeriodMultiples

Short PeriodMultiples

UpcomingWavelet

Scatterers

Record ing Instruments

Ground Receiver CouplingReceiver Frequency Response

Array Effects

Refractions

Ambient andCultural Noise

Refractions

Q-Factor

ReflectionCoefficien t

Interface Losses

SphericalSpreadingDowngoing

Wavelet

ShotHole Free

SurfaceGhost?

Source Effects

LowVelocity

Layer


3D seismic Video


for (near) vertical incidence

• Zoeppritz equations simplify to:

• Acoustic Impedance Z:

RC = Z2 - Z1

Z1 + Z2

Z = V where: is densityV is velocity

What can be seen on seismic data?

RC: Acoustic impedance contrast between 2 different materials

Shell Exploration & ProductionConvolutional Model for Synthetic Seismic

TraceRockcolumn

ReflectivityAcousticImpedance

from sonic & density logs

Reflectorresponses

Syntheticseismogram

Sourcewavelet

0

Min

imum

phase


3D seismic cube configuration Video


Variable Density Variable Wiggle

Different Seismic Displays & Color Schemes

Seismic section display

Shell Exploration & ProductionSeismic-to-Well Tie• Process of correlating the seismic 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 interpretation

• Match relative amplitudes between seismic signal and synthetic.

density /

sonic

GR / ca

liper

deep-re

adin

g resis

tivity

markers

TVD / tim

e

imped

ance

reflectivity

synth

etic


synthetic deep-reading resistivity


Seismic terms• Wavelet: a seismic pulse usually consisting of only a few cycles which

represents the reflection shape from a single positive reflector at normal incidence

• Event: general feature in seismic data

– Explicit events are features depicted by amplitude extrema (trough – peak)

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

• Trace: a vertical record of seismic amplitudes at a given shot point or 3D grid coordinate (time or depth),

• Fault shadow: zone of reduced imaging quality in the footwall (below) major faults with a distinct velocity contrast to the hanging wall (above), can also be caused by wider fault damage zones with anomalous velocity

– Effect is usually aggravated by strike acquisition


Seismic terms (Cont.)• Grid: a 2-dimensional array to store horizon, attribute and fault data with a

regular x/y sampling

• Horizon Slice: a horizontal display of seismic amplitude data, extracted at a constant distance from a seismic horizon, powerful for viewing stratigraphic information (Coherence data)

• Attribute: a measurement executed on seismic data, with varying 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.g. RMS energy

– Volume attribute: multi-trace (change) measurement, e.g. Coherency; represents lateral amplitude change, e.g. At reflection terminations; commonly used for highlighting of faults and abrupt stratigraphic variations in timeslices and horizon slices.


Seismic terms (Cont.)• Structural (Slip) Vector / Volume dip & azimuth:

– A volume attribute that represents lateral change of phase, e.g. As caused by tectonic deformation of subsurface strata; commonly used for highlighting of faults and flexures in timeslices and horizon slices.

• Inversion: a method of restoring broad-band acoustic impedance signal of the subsurface from the ordinary band-limited reflectivity signal of seismic data. Techniques used:

– Sparse-spike Inversion: deconvolution / whitening plus adding low frequencies from well data

– Model-based Inversion: both low and high frequencies are added from interpreted borehole measurements, extrapolating away from boreholes along horizons

• Isochron: TWT isoline, either from seismic datum to a horizon or as isochrone thickness, measured between 2 horizons, with wave travelling vertically assumption


Seismic terms (Cont.)• Flattening: datuming of vertical and horizontal seismic displays

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 same features on different seismic displays, e.g. Crossing 2D lines or inlines-crossline displays of 3D seismic. Also in seismic-to-well tie.

• Jump correlation: identification of a seismic event on either side of 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 understanding

of deformational history and the controls on trapping and flow

• The definition of appropriate selection criteria for faults to be interpreted as 3D planes is essential to be used

– along the entire Subsurface Interpretation workflow (structural and reservoir model building, upscaling, reservoir simulation).

• Sequencing faults for interpretation should consider structural setting and kinematics.

• As a minimum, all faults that directly affect volumetrics must be fully interpreted, i.e. those faults that are (potentially) sealing and occur in (potential) trap geometries. Generally these faults are also the ones that are to be included in the static reservoir model.


Common orientations and shapes of faults• Most hydrocarbon accumulations occur in

– Structural traps involving extensional to moderately transpressional deformation,

– Their faults tend to be rather steep (ranging from about 60° with normal displacement for extensional faults through nearly vertical strike-slip faults to reverse faults of about 60° dip in mildly transpressional regimes).

• Fault shape is controlled by the magnitude of differential stress between the horizontal stress axes,

– Bends and kinks can occur if the stress field is laterally variable

• All faults are either straight or at least have constant curvature in the direction of their displacement,

– At larger faults, this rule may appear to be broken if the fault position is offset at incompetent intervals with plastic rather than brittle deformation.


Choosing the most suitable digitisation direction

• Fortunately many 3D surveys are oriented such that the seismic grid is aligned with the predominant dip direction (azimuth) in the subsurface, and are thereby also aligned with most faults,

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

• It is important that the corner coordinates of used arbitrary lines are stored, as otherwise the interpretation on such lines cannot be revisited or corrected.


Interpretation strategy• The seismic evidence for faults is

– implicit (reflection terminations), ambiguous (not all reflection terminations are caused by faults)

– incomplete (intervals without reflective interfaces also lack evidence for faults).

– may have many different geometries including (self-)branching,

• Good interpretation practice means taking into account

– kinematic considerations, The specific geophysical response and rock competence of each interval when making choices with ambiguous evidence.

• Generation of fault planes by linear interpolation or triangulation between manually interpreted ‘segments’ may be easier if the manual ‘seed’ interpretation is oriented in the direction of highest irregularity of fault shape, i.e. normal to the slip vector.


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

Structural Vector (lateral phase change)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 fault plane) as

the seismic image below the fault plane is often of poorer quality (‘fault shadow’) and does not provide a good contrast between continuous unfaulted reflections and clear terminations towards a fault plane.

• If fault plane reflections are present but do not coincide with the hanging-wall termination, better ignore them because, as very steep features, they are much more sensitive to inaccuracies in migration velocities.

• Interpret fault segments consistently from upper to lower tip.

• ‘Split-the-distance’ method. In this workflow one would start interpretation with a very large increment that can be divided by 2 for a number of times: ideally the power-2 system 1-2-4-8-16-32-64, but the system 5-10-20-40-80 is often easier to manage.

• Fault junctions and amalgamated faults: shape complexity increases towards the lateral tips of fault planes, where the local stress fields start interfering. This implies that interpretation density should usually increase towards fault tips.


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 fault

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 conversion, structural modelling & kitchen/maturity modelling

• Start with shallow horizons on obvious events and to interpret step-by-step from top to bottom, as structural complexity increases and imaging breaks down.

• Correlate a particular horizon on a coarse grid of lines away from wells, and make sure you always close a loop back to your starting point to verify that the horizon of interest is consistently picked.


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 than 1-3 traces.

• Jump correlations across faults:

– Get an idea about the throw distribution along the interface between two blocks by tentative horizon interpretation

– Work top down, starting from levels with confident correlation across the fault.

– Base your choice on sequence correlation rather than event correlation

– Take discrete sedimentary features such as unconformities, incised valley fills and channels as anchor points for jump correlation


Unconformity: as significant breaks in vertical velocity trends.Its interpretation depends on the recognition of characteristic reflection geometries rather than on amplitude information

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“Exercises”

introduction to seismic interpretation el amal[1]

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