CPG Exploration Techniques & Seismic Interpretation 2 — Questions and Answers
Question 1: What does a flat spot on a seismic section typically indicate?
- A horizontal fluid contact such as a gas-water or oil-water interface (Correct answer)
- A uniform lithology with no impedance contrast
- The base of the salt body
- A processing artifact from stacking
Correct answer: A horizontal fluid contact such as a gas-water or oil-water interface
Flat spots are horizontal reflections that cross stratigraphic dip and represent fluid contacts where acoustic impedance changes at the phase boundary.
Question 2: In seismic stratigraphy, what is an 'onlap' configuration?
- Reflectors terminate against an underlying surface in an up-dip direction (Correct answer)
- Reflectors terminate against an overlying surface
- Reflectors converge and thin toward a structural high
- Reflectors are truncated by erosion at the top
Correct answer: Reflectors terminate against an underlying surface in an up-dip direction
Onlap occurs when initially horizontal or inclined strata terminate progressively against an inclined surface in an updip direction, indicating transgression or basin filling.
Question 3: Which seismic attribute is most directly related to pore fluid type?
- Poisson's ratio derived from Vp/Vs ratio (Correct answer)
- Acoustic impedance from P-wave velocity alone
- Dominant frequency of the wavelet
- NMO velocity from stacking
Correct answer: Poisson's ratio derived from Vp/Vs ratio
Poisson's ratio (derived from the Vp/Vs ratio) is highly sensitive to pore fluid type because shear waves are less affected by fluid content than P-waves.
Question 4: What is the purpose of applying a Normal Moveout (NMO) correction in seismic processing?
- To align reflections from the same subsurface point recorded at different offsets (Correct answer)
- To remove multiples caused by water-bottom reflections
- To correct for variations in surface elevation along the seismic line
- To convert time-domain data to depth-domain data
Correct answer: To align reflections from the same subsurface point recorded at different offsets
NMO correction removes the hyperbolic moveout effect so that reflections from the same subsurface point at different source-receiver offsets can be stacked coherently.
Question 5: A geologist observes high-amplitude seismic anomalies (bright spots) in a gas-prone basin. Which scenario would cause a FALSE bright spot?
- A tight carbonate with high acoustic impedance overlying a low-impedance shale (Correct answer)
- Gas-saturated sandstone with low impedance beneath a shale
- Coal seams within a clastic sequence
- Volcanic sills intruded into sedimentary rock
Correct answer: A tight carbonate with high acoustic impedance overlying a low-impedance shale
A hard kick from high-impedance carbonate over low-impedance shale produces a strong reflection that mimics a bright spot but contains no hydrocarbons.
Question 6: What does the term 'velocity pull-up' describe in seismic interpretation?
- An apparent structural high beneath a high-velocity body like a carbonate or salt (Correct answer)
- A reduction in seismic velocity due to overpressure
- The increase in velocity with depth in normally compacted shales
- Correction of velocity errors introduced during NMO
Correct answer: An apparent structural high beneath a high-velocity body like a carbonate or salt
Velocity pull-up is an artifact where reflectors beneath a high-velocity body appear structurally higher than they actually are because travel times are shortened.
Question 7: In the context of seismic facies analysis, what does 'chaotic' reflection pattern suggest?
- Mass transport deposits, slumps, or highly deformed sediments (Correct answer)
- Well-bedded, laterally continuous deep-water turbidites
- Fluvial channel fill with high lateral continuity
- Carbonate platform interior with uniform deposition
Correct answer: Mass transport deposits, slumps, or highly deformed sediments
Chaotic seismic facies lack internal organization and continuity, which is characteristic of mass transport complexes, slumps, or deformed sedimentary packages.
CPG Exploration Techniques & Seismic Interpretation 2
What does a flat spot on a seismic section typically indicate?