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caprock

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1. n. [Geology]
A relatively impermeable rock, commonly shaleanhydrite 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
Alternate Form: cap rock