1. n. [Geology]

A relatively impermeable rock, commonly shale, anhydrite or salt, that forms a barrier or cap above and around reservoir rock such that fluids cannot migrate beyond the reservoir. A seal is a critical component of a complete petroleum system. The permeability of a seal capable of retaining fluids through geologic time is ~ 10^-6 to 10^-8 darcies.

Synonyms: caprock

See: evaporite,  fault,  field,  halite,  impermeable,  migration,  oil field,  permeability,  petroleum system,  pinch-out,  prospect,  reservoir,  salt dome,  shale,  structural trap,  trap