subsurface reservoir of petroleum. The oil is always accompanied by water and often by natural gas; all are confined in porous rock, usually such sedimentary rocks as sands, sandstones, arkoses, and fissured limestones and dolomites. 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 upward or lateral escape of the petroleum. That part of the trap actually occupied by the oil and gas is called the petroleum reservoir.

Many systems have been proposed for the classification of traps; one simple system divides them into structural traps, stratigraphic traps, and combinations of the two. A structural trap has a concave (viewed from below) roof caused by the local deformation (by faulting or folding) of the reservoir rock and the impervious roof rock. In this case, the intersection of the oil-water contact with the roof rock determines the edges of the reservoir. In a stratigraphic trap, variations in the rock strata themselves (e.g., a change in the local porosity and permeability of the reservoir rock, a change in the kinds of rocks laid down, or a termination of the reservoir rock) play the important role. The stratigraphic variations associated with the reservoir rocks are the main influence on the areal extent of the reservoirs in these traps. A complete gradation between the two varieties is possible.

The oil and gas pool will rise to the top of the trap if the underlying water is stationary, and the resulting oil-water contact will be level. When the water is moving, however, the pool is displaced down the trap's side in the direction of flow because of hydrodynamic pressure. In some traps the pool may be displaced great distances or may even be completely flushed out. See also salt dome.