is the 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.
Also see asphalt,
distillation,
sour crude,
sweet crude, naphthene,
paraffin.