C 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. 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.