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Definition of cap rock

i. Barren vein matter, or a pinch in a vein, supposed to overlie ore.

Synonym of: cap

ii. A hard layer of rock, usually sandstone, a short distance above a coal seam.

iii. A disklike plate over part of or all of the top of most salt domes in the Gulf Coast States and in Germany. It is variably composed of anhydrite, gypsum or limestone, and sometimes some sulphur.

Ref: AGI

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

Ref: AGI

v. Eng. The cap rock of the alum shale, Estuarine sandstones on the Yorkshire coast.

Ref: Arkell

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