An impervious layer of rock that overlies a reservoir rock, thus preventing hydrocarbons from escaping to the surface.

Impermeable rock overlying an oil or gas reservoir that tends to prevent migration of the reservoir fluids from the reservoir.

an impermeable layer of rock that traps or halts the upward movement of oil and gas, forming the top or cap of a reservoir.

Cap rock or seal. Impermeable rock overlying an oil or gas reservoir that tends to prevent the upward migration of fluids form the reservoir.

A hard rock layer capping a softer rock unit (such as basalt capping a soft sandstone).