CPG Petroleum Geology & Reservoir Characterization 2 — Questions and Answers
Question 1: Which diagenetic process most commonly reduces reservoir porosity in deeply buried sandstones?
- Dolomitization
- Quartz cementation (Correct answer)
- Feldspar dissolution
- Chlorite coating
Correct answer: Quartz cementation
Quartz cementation is the dominant porosity-reducing diagenetic process in deeply buried sandstones, as silica precipitates in pore spaces at elevated temperatures.
Question 2: A reservoir has a porosity of 22% and a water saturation of 35%. What is the hydrocarbon pore volume fraction?
- 0.077
- 0.143 (Correct answer)
- 0.057
- 0.220
Correct answer: 0.143
Hydrocarbon pore volume = porosity × (1 - Sw) = 0.22 × (1 - 0.35) = 0.22 × 0.65 = 0.143.
Question 3: In sequence stratigraphy, which systems tract is deposited during the early phase of sea-level rise following lowstand?
- Highstand systems tract (HST)
- Falling stage systems tract (FSST)
- Transgressive systems tract (TST) (Correct answer)
- Lowstand systems tract (LST)
Correct answer: Transgressive systems tract (TST)
The transgressive systems tract (TST) is deposited during the early phase of sea-level rise, characterized by retrogradational stacking patterns.
Question 4: Which well log is BEST suited for identifying gas-bearing reservoirs through the 'gas effect'?
- Gamma ray log
- Neutron porosity log (Correct answer)
- Spontaneous potential log
- Caliper log
Correct answer: Neutron porosity log
The neutron porosity log shows anomalously low apparent porosity in gas zones due to the low hydrogen index of gas, creating the characteristic 'gas effect' crossover with density logs.
Question 5: What is the primary control on the distribution of carbonate facies in a rimmed shelf platform?
- Siliciclastic sediment input
- Distance from organic reef/mound margin (Correct answer)
- Fluvial discharge patterns
- Deep marine currents
Correct answer: Distance from organic reef/mound margin
In rimmed shelf platforms, carbonate facies distribution is primarily controlled by the reef/mound margin, which creates a high-energy barrier separating fore-reef, reef crest, back-reef, and lagoon environments.
Question 6: A fault with a normal sense of movement that exhibits an antithetic orientation relative to the main fault is called a:
- Synthetic fault
- Antithetic fault (Correct answer)
- Transfer fault
- Listric fault
Correct answer: Antithetic fault
An antithetic fault dips in the opposite direction to the main (synthetic) fault and has a reverse sense of displacement relative to the main fault block movement.
Question 7: Which type of unconformity occurs when sedimentary strata overlie igneous or metamorphic basement rocks?
- Disconformity
- Angular unconformity
- Nonconformity (Correct answer)
- Paraconformity
Correct answer: Nonconformity
A nonconformity exists where sedimentary rocks overlie crystalline (igneous or metamorphic) basement rocks, representing a significant gap in the geologic record.
CPG Petroleum Geology & Reservoir Characterization 2
Which diagenetic process most commonly reduces reservoir porosity in deeply buried sandstones?