1. n. [Geology]

A relatively impermeable rock, commonly shale, anhydrite, or salt, that forms a barrier or seal above and around reservoir rock so that fluids cannot migrate beyond the reservoir. It is often found atop a salt dome. The permeability of a caprock capable of retaining fluids through geologic time is ~ 10^–6–10^–8 darcies.

Synonyms: seal

See: halite,  migration,  petroleum system