A body of porous rock holding a large quantity of oil or natural gas.

a rock formation in which oil is present throughout without interruption, forming a single reservoir (cf. oilfield, n.).

rock that is or may be a reservoir for oil or natural gas.

An oil-producing area or formation; spec. = oil pool n. at oil, n.1 compounds 5.

Geology. An underground rock formation in which an accumulation of oil or gas is trapped; so oil trap.

Geology. A geological formation which is a source of oil or gas. Cf. sense 4a.