CPG CPG Basin Analysis & Structural Geology 2 — Questions and Answers
Question 1: Strike-slip (wrench) faults most commonly create hydrocarbon traps through which structural mechanism?
- Extensional rollover anticlines
- Flower structures and en-echelon folds (Correct answer)
- Salt diapirism
- Compactional drape over basement highs
Correct answer: Flower structures and en-echelon folds
Strike-slip faults generate positive and negative flower structures and en-echelon folds along fault bends and stepovers that can form effective traps.
Question 2: Subsidence analysis (backstripping) in basin analysis is primarily used to:
- Calculate present-day porosity
- Separate tectonic subsidence from sediment loading effects (Correct answer)
- Determine current pore pressure
- Estimate hydrocarbon column height
Correct answer: Separate tectonic subsidence from sediment loading effects
Backstripping removes the effect of sediment loading to isolate tectonic subsidence, which reveals the basin's driving mechanism and thermal history.
Question 3: Which kerogen type is most prone to generating oil rather than gas?
- Type III (vitrinite-rich, humic)
- Type II (algal/marine, liptinite-rich) (Correct answer)
- Type IV (inertinite, recycled)
- Type I and IV mixtures
Correct answer: Type II (algal/marine, liptinite-rich)
Type II kerogen, derived from marine algae and plankton, has a high hydrogen index and generates primarily oil at peak maturity.
Question 4: Structural closure in a four-way dip closure trap is defined as:
- The highest spill point minus the structural crest elevation
- The area enclosed by the lowest closing contour around a structural high (Correct answer)
- The net pay thickness times the areal extent
- The distance from the crest to the nearest fault
Correct answer: The area enclosed by the lowest closing contour around a structural high
Structural closure is the area enclosed by the lowest structural contour that completely surrounds the closure, limiting the trap's capacity to hold hydrocarbons.
Question 5: In the context of secondary migration, hydrocarbons move from source rock to trap primarily via:
- Diffusion through tight shale
- Buoyancy-driven flow along carrier beds and faults (Correct answer)
- Compaction-driven lateral flow only
- Osmotic pressure gradients
Correct answer: Buoyancy-driven flow along carrier beds and faults
Secondary migration occurs as hydrocarbons, being less dense than water, rise buoyantly through permeable carrier beds and along faults until they are trapped.
Question 6: Unconformity traps rely on which geological relationship to form a seal?
- Lateral facies change in a reservoir
- Truncation of reservoir beds below an impermeable erosional surface (Correct answer)
- Salt dissolution collapse
- Diagenetic cementation along a fault plane
Correct answer: Truncation of reservoir beds below an impermeable erosional surface
Unconformity traps form where porous reservoir beds are truncated by erosion and then overlain by impermeable rocks, sealing the upslope edge of the reservoir.
CPG CPG Basin Analysis & Structural Geology 2
Strike-slip (wrench) faults most commonly create hydrocarbon traps through which structural mechanism?