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.
See: halite, migration, petroleum system