Subsurface reservoir of petroleum. The oil is always accompanied by water and often by natural gas; all are confined in porous rock. The natural gas, being lightest, occupies the top of the trap and is underlain by the oil and then the water. A layer of impervious rock, called the roof rock, prevents the escape of the petroleum. The part of the trap that is actually occupied by the oil and gas is called the petroleum reservoir.
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A trap is a device or tactic intended to harm, capture, detect, or inconvenience a human or animal intruder, or animal pest or game. Traps may be physical objects, such as cages or snares, or metaphorical concepts.