Petroleum is known as crude oil. It is a thick, dark brown or greenish liquid. Petroleum exists in the upper strata of some areas of the Earth's crust. It consists of a complex mixture of various hydrocarbons, largely of the alkane series, but may vary much in appearance, composition, and purity.
Petroleum is used mostly, by volume, for producing fuel oil, which is an important "primary energy" source. Petroleum is also the raw material for many chemical products, including solvents, fertilizers, pesticides, and plastics.