Which one of the following acts as a cap rock to retain oil in the reservoir rock?

Correct option is C

Shale acts as a cap rock in oil reservoirs due to its low permeability and fine-grained structure. A cap rock is an impermeable layer that prevents the upward migration of hydrocarbons trapped in porous reservoir rocks such as sandstone or limestone.
· Key Characteristics of Shale:
· Shale is composed of fine particles of clay and silt.
· It is highly impermeable, making it effective at trapping oil and gas beneath it.
· Its ability to retain fluids makes it an ideal seal for hydrocarbon reservoirs.
· Role in Oil Reservoirs:
· Oil and gas, being less dense, tend to migrate upwards through porous reservoir rocks.
· A cap rock like shale prevents these hydrocarbons from escaping to the surface.
Other options:
· Sandstone (1): Sandstone is a reservoir rock with high porosity and permeability, not a cap rock.
· Jointed Limestone (2): Although limestone can act as a reservoir rock, its jointed structure makes it unsuitable as a cap rock.
· Conglomerate (4): Similar to sandstone, conglomerates are porous and permeable and function as reservoir rocks.
Information Booster: 1. Cap Rock:
· A geological layer that prevents hydrocarbons from escaping.
· Typically composed of impermeable rocks like shale or salt.
2. Reservoir Rock:
· A porous rock that stores hydrocarbons (e.g., sandstone, limestone).
3. Oil and Gas Trap:
· A combination of reservoir rock and cap rock forms a trap for hydrocarbons.
4. Hydrocarbon Migration:
· Movement of oil and gas due to buoyancy forces.
· Stopped by impermeable cap rocks.
Additional Knowledge: · Shale:
· Shale contains organic material and can sometimes act as a source rock for hydrocarbons.
· Sandstone:
· Known for its ability to store large amounts of oil and gas.
· Conglomerates:
· Act as reservoir rocks due to their high porosity but are rarely associated with cap rock roles.
· Jointed Limestone:
· Jointed nature allows fluid movement, limiting its sealing capability.