CPG Exploration Techniques & Seismic Interpretation 3 — Questions and Answers
Question 1: What is the primary advantage of 3D seismic surveys over 2D surveys for prospect evaluation?
- Full spatial sampling eliminates out-of-plane reflections and enables accurate structural mapping (Correct answer)
- 3D surveys are always cheaper per line-kilometer acquired
- 3D surveys automatically convert time to depth without velocity models
- 3D surveys eliminate the need for well control
Correct answer: Full spatial sampling eliminates out-of-plane reflections and enables accurate structural mapping
3D seismic provides full spatial sampling, removing sideswipe and out-of-plane energy that distorts 2D profiles, and enables accurate volumetric structural and stratigraphic mapping.
Question 2: Which exploration method measures variations in the Earth's gravitational field to detect subsurface density contrasts?
- Gravity surveying (Correct answer)
- Magnetotellurics
- Seismic refraction
- Induced polarization
Correct answer: Gravity surveying
Gravity surveys detect lateral changes in rock density; salt bodies (low density) produce negative Bouguer anomalies, while dense basement rocks produce positive anomalies.
Question 3: In reflection seismology, what is a 'multiple' reflection?
- A seismic wave that has reflected more than once before being recorded at the surface (Correct answer)
- The primary reflection from the deepest target of interest
- A reflection produced by two separate source points
- An artifact introduced during deconvolution processing
Correct answer: A seismic wave that has reflected more than once before being recorded at the surface
Multiples are seismic events that have undergone more than one reflection, often between the sea floor and the water surface or between two strong reflectors, and can mimic primary reflections.
Question 4: What does the 'dominant frequency' of a seismic wavelet control in exploration?
- The vertical resolution of the seismic data, with higher frequency giving finer resolution (Correct answer)
- The depth penetration of the seismic signal, with higher frequency going deeper
- The horizontal (lateral) fold of the seismic acquisition
- The polarity convention used in seismic interpretation
Correct answer: The vertical resolution of the seismic data, with higher frequency giving finer resolution
Vertical resolution is approximately λ/4, and since λ = velocity/frequency, higher dominant frequency yields shorter wavelength and finer resolution of thin beds.
Question 5: A CPG candidate is evaluating a prospect mapped as a four-way dip closure on seismic. What additional risk must be assessed that this structural map alone cannot fully address?
- Seal integrity and the presence of a top seal capable of retaining hydrocarbons (Correct answer)
- Whether the seismic data was acquired with dynamite or vibroseis sources
- The exact UTM coordinate system used for the survey
- The age of the seismic processing software
Correct answer: Seal integrity and the presence of a top seal capable of retaining hydrocarbons
Structural closure confirms trap geometry but does not prove seal; without an effective top seal, hydrocarbons will migrate through even a perfect four-way closure.
Question 6: What is a 'time-depth conversion' in seismic interpretation, and why is it critical?
- Converting seismic two-way travel time to true subsurface depth using velocity information for accurate well placement (Correct answer)
- Converting the seismic amplitude scale from decibels to linear units
- Translating seismic frequencies from Hz to milliseconds for wavelet analysis
- Converting a 2D seismic line to a 3D volume by interpolation
Correct answer: Converting seismic two-way travel time to true subsurface depth using velocity information for accurate well placement
Time-depth conversion uses velocity models to transform seismic time horizons into true vertical depths, which is essential for accurate structural maps and drilling targets.
Question 7: Which of the following best describes the concept of 'seismic anisotropy' relevant to shale reservoir characterization?
- Variation in seismic wave velocity depending on the direction of propagation through aligned fractures or minerals (Correct answer)
- The change in amplitude with offset due to changes in lithology
- The lateral variation in reflection coefficient along a single horizon
- The difference in velocity between P-waves and S-waves in isotropic media
Correct answer: Variation in seismic wave velocity depending on the direction of propagation through aligned fractures or minerals
Seismic anisotropy in shales arises from aligned clay minerals and fractures, causing velocity to vary with propagation direction, which affects fracture characterization in unconventional plays.
CPG Exploration Techniques & Seismic Interpretation 3
What is the primary advantage of 3D seismic surveys over 2D surveys for prospect evaluation?