CPG CPG Geochemistry & Source Rock Analysis 2 — Questions and Answers
Question 1: What is the primary geochemical distinction between thermogenic and biogenic natural gas?
- Thermogenic gas has heavier carbon isotope values (less negative δ¹³C) and contains wet gas components; biogenic gas is nearly pure methane with very negative δ¹³C (Correct answer)
- Biogenic gas has higher H₂S content than thermogenic gas
- Thermogenic gas always contains more CO₂ than biogenic gas
- Biogenic gas forms only at depths below 3,000 m
Correct answer: Thermogenic gas has heavier carbon isotope values (less negative δ¹³C) and contains wet gas components; biogenic gas is nearly pure methane with very negative δ¹³C
Thermogenic methane is isotopically heavier (δ¹³C typically > −50‰) and accompanied by ethane and propane, while biogenic methane from methanogenic bacteria is isotopically very light (δ¹³C < −60‰) and nearly pure.
Question 2: In petroleum systems analysis, what is the 'critical moment' for a petroleum system?
- The single point in geologic time when the system is best evaluated, typically at maximum generation and migration (Correct answer)
- The moment when the trap was formed
- The time of first commercial discovery in the basin
- The depth at which oil transforms to gas
Correct answer: The single point in geologic time when the system is best evaluated, typically at maximum generation and migration
The critical moment is the point in time chosen to reconstruct a petroleum system's geometry and processes, typically selected to represent peak hydrocarbon generation and migration.
Question 3: What does a high Tmax value (>470°C) on a Rock-Eval pyrolysis measurement indicate about a source rock?
- The source rock is overmature and has largely exhausted its generative potential (Correct answer)
- The source rock is immature with abundant remaining potential
- The rock contains predominantly Type III kerogen
- The sample was contaminated with drilling mud
Correct answer: The source rock is overmature and has largely exhausted its generative potential
Tmax represents the temperature of maximum S2 generation; values above ~470°C indicate that kerogen is overmature, meaning most hydrocarbons have already been generated and expelled.
Question 4: Which of the following best describes the process of 'secondary cracking' in petroleum geochemistry?
- Thermal cracking of oil already in a reservoir to gas and condensate at high temperatures (Correct answer)
- Migration of oil from source to trap
- Biodegradation of crude oil by bacteria near the surface
- Mechanical fracturing of source rock during burial
Correct answer: Thermal cracking of oil already in a reservoir to gas and condensate at high temperatures
Secondary cracking occurs when previously generated oil trapped in the reservoir or still in the source is further cracked to gas and condensate at temperatures typically above 160–180°C.
Question 5: What is the significance of the 'Production Index' (PI = S1/(S1+S2)) in source rock evaluation?
- PI indicates the transformation ratio showing what proportion of original generative potential has already been converted to hydrocarbons (Correct answer)
- PI measures the permeability of the source rock
- PI quantifies the amount of biodegradation in a crude oil sample
- PI represents the ratio of oil to gas generation
Correct answer: PI indicates the transformation ratio showing what proportion of original generative potential has already been converted to hydrocarbons
The Production Index (also called transformation ratio) ranges from 0 (immature, all potential intact) to 1 (fully spent), and values above 0.4 in shallow samples may indicate contamination.
Question 6: In basin modeling, what does 'Easy%Ro' refer to?
- A kinetic algorithm that calculates vitrinite reflectance equivalent from time-temperature history using a simple first-order reaction model (Correct answer)
- A simplified map of vitrinite reflectance data across a basin
- The percentage of Type II kerogen in a source rock interval
- An empirical estimate of recovery factor in unconventional plays
Correct answer: A kinetic algorithm that calculates vitrinite reflectance equivalent from time-temperature history using a simple first-order reaction model
Easy%Ro (Sweeney & Burnham, 1990) is a widely used kinetic model that computes vitrinite reflectance equivalent from burial temperature history using a parallel first-order reaction scheme.
CPG CPG Geochemistry & Source Rock Analysis 2
What is the primary geochemical distinction between thermogenic and biogenic natural gas?