CPG Petroleum Geology & Reservoir Characterization 5

Question 1

Which property of a cap rock is MOST critical for preventing hydrocarbon migration through it?

Accepted Answer

High capillary entry pressure

Answer

High capillary entry pressure in the seal rock, resulting from small pore throats (especially in shales and evaporites), prevents buoyant hydrocarbons from displacing the wetting phase water.

Question 2

A stratigraphic trap formed by the lateral facies change from reservoir-quality sand to tight shale is classified as a:

Accepted Answer

Pinch-out trap

Answer

A pinch-out trap forms where a reservoir sand body thins and grades laterally into impermeable shale, preventing further updip migration of hydrocarbons.

Question 3

In petrophysical analysis, the Archie equation is used to calculate:

Accepted Answer

Water saturation from resistivity and porosity data

Answer

The Archie equation relates formation resistivity, porosity, and cementation/saturation exponents to calculate water saturation in clean (clay-free) reservoir rocks.

Question 4

The vitrinite reflectance value (Ro) of approximately 0.5–0.6% corresponds to which stage of organic matter maturation?

Accepted Answer

Early oil window onset

Answer

Ro values of 0.5–0.6% mark the onset of the early oil window, where sufficient thermal energy has been reached to begin cracking kerogen into liquid hydrocarbons.

Question 5

Which mechanism drives primary migration of hydrocarbons out of source rocks?

Accepted Answer

Overpressure and molecular diffusion through water-saturated pore networks

Answer

Primary migration is driven by abnormally high pore pressures in compacting source rocks and molecular diffusion, allowing hydrocarbons to move through microporosity and microfractures into carrier beds.

(CPG) Certified Petroleum Geologist Practice Test

(CPG) Certified Petroleum Geologist Practice Test

CPG Petroleum Geology & Reservoir Characterization 5