Oil and gas traps, sometimes referred to as petroleum traps are below ground traps where a permeable reservoir rock is covered by some low permeability cap rock. This combination of rock can take several forms, but they all prevent the upward migration of oil and natural gas up through the reservoir rock. Once oil and natural gas are in the reservoir rock, they continue to migrate upwards through the pore spaces of the rock until blocked by some sort of seal with a cap rock. The low permeability cap rocks are generally shale or low permeability sandstones and carbonate rocks.
There are two major categories of traps, and are classified based on how the petroleum accumulates. The two main groups are structural traps and stratigraphic traps.