Petroleum Geology & Reservoir Characterization Flashcards

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

Answer: High capillary entry pressure

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.

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

Answer: Pinch-out trap

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

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

Answer: Water saturation from resistivity and porosity data

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

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

Answer: Early oil window onset

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.

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

Answer: Overpressure and molecular diffusion through water-saturated pore networks

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.

A reservoir exhibits 'dual porosity' behavior during pressure transient testing. This MOST likely indicates:

Answer: A naturally fractured reservoir with matrix and fracture porosity systems

Dual porosity behavior in pressure transient analysis is diagnostic of naturally fractured reservoirs, where fluid transfers between high-permeability fractures and low-permeability matrix blocks.

The geothermal gradient in a sedimentary basin is influenced MOST strongly by:

Answer: Crustal heat flow and thermal conductivity of lithologies

The geothermal gradient depends on basal heat flow from the mantle/crust and the thermal conductivity of the overlying rocks; low-conductivity shales trap heat, raising gradients.