Crude oil and natural gas are naturally occurring substances present in rock amidst the earth's crust. The origin of oil and gas is organic material. The remains of plants and animals compressed in sedimentary rock such as sandstone, limestone and shale. Sedimentary rock is a product of sediment deposits in ancient oceans and other bodies of water. As layers of sediment were deposited on the ocean floor, decaying remains of plants and animals were integrated into the forming rock. This organic material eventually transformed into oil and gas after being exposed to a specific temperature and pressure range deep within the earth's crust. Because oil and gas are less dense than water, which occurs in huge quantities in the earth's subsurface, oil and gas migrate through relatively porous sedimentary source rock toward the earth's surface. When the hydrocarbons are trapped beneath relatively less-porous cap rock, an oil and gas reservoir is formed. These reservoirs, which are simply layers of rock containing relatively large quantities of oil and gas, are our source for crude oil and gas.