crude oil - complex, naturally occurring fluid mixture of petroleum hydrocarbons, yellow to black in color, and also containing small amounts of oxygen, nitrogen and sulfur derivatives and other impurities. Crude oil was formed by the action of bacteria, heat, and pressure on ancient plant and animal remains, and is usually found in layers of porous rock such as limestone or sandstone capped by an impervious layer of shale or clay that traps the oil (see reservoir). Crude oil varies in appearance and hydrocarbon composition depending on the locality where it occurs, some crudes being predominantly naphthenic, some paraffinic, and others asphaltic. Crude is refined to yield petroleum products. See asphalt, distillation, hydrocarbon, sour crude, sweet crude, naphthene, paraffin.