View
824
Download
33
Embed Size (px)
Citation preview
Shell Exploration & Production
Cop
yri
gh
t: S
hell
Exp
lora
tion
& P
rod
uct
ion
Ltd
.
Introduction to Seismic Interpretation
By:
Hosny Diab
Explorationist Seismic Interpreter / Onshore Exploration Team
Shell Egypt N. V.
Shell Exploration & Production
How oil trapped & Technology used video
Shell Exploration & Production
Seismic Acquisition operations
Seismic acquisition offshore Seismic acquisition onshore
Shell Exploration & Production
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
Shell Exploration & Production
3D seismic Video
Shell Exploration & Production
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
Shell Exploration & Production
Shell Exploration & Production
3D seismic cube configuration Video
Shell Exploration & Production
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
Shell Exploration & Production
synthetic deep-reading resistivity
Shell Exploration & Production
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
Shell Exploration & Production
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.
Shell Exploration & Production
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
Shell Exploration & Production
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.
Cop
yri
gh
t: S
hell
Exp
lora
tion
& P
rod
uct
ion
Ltd
.
Shell Exploration & Production
Guidelines for 3D seismic interpretation
“Faults interpretation”
Shell Exploration & Production
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.
Shell Exploration & Production
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.
Shell Exploration & Production
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.
Shell Exploration & Production
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.
Shell Exploration & Production
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)
Shell Exploration & Production
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.
Shell Exploration & Production
Nigeria Data raw seismic
Shell Exploration & Production
Nigeria Data with Horizon & Fault Interpretation
Cop
yri
gh
t: S
hell
Exp
lora
tion
& P
rod
uct
ion
Ltd
.
Shell Exploration & Production
Guidelines for 3D seismic interpretation
“Horizon & unconformity interpretation”
Shell Exploration & Production
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.
Shell Exploration & Production
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
Shell Exploration & Production
Unconformity: as significant breaks in vertical velocity trends.Its interpretation depends on the recognition of characteristic reflection geometries rather than on amplitude information
Cop
yri
gh
t: S
hell
Exp
lora
tion
& P
rod
uct
ion
Ltd
.
Shell Exploration & Production
Guidelines for 3D seismic interpretation
“Exercises”
Shell Exploration & Production
Shell Exploration & Production