CPG CPG Geochemistry & Source Rock Analysis 2

Question 1

What is the primary geochemical distinction between thermogenic and biogenic natural gas?

Accepted 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

Answer

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?

Accepted Answer

The single point in geologic time when the system is best evaluated, typically at maximum generation and migration

Answer

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?

Accepted Answer

The source rock is overmature and has largely exhausted its generative potential

Answer

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?

Accepted Answer

Thermal cracking of oil already in a reservoir to gas and condensate at high temperatures

Answer

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?

Accepted Answer

PI indicates the transformation ratio showing what proportion of original generative potential has already been converted to hydrocarbons

Answer

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.

(CPG) Certified Petroleum Geologist Practice Test

(CPG) Certified Petroleum Geologist Practice Test

CPG CPG Geochemistry & Source Rock Analysis 2