Petroleum (from Greek petra – rock and elaion – oil or Latin oleum – oil ) or crude oil, sometimes colloquially called black gold, 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 and composition. Petroleum is used mostly, by volume, for producing fuel oil and gasoline (or petrol), both important "primary energy" sources (IEA Key World Energy Statistics). Petroleum is also the raw material for many chemical products, including solvents, fertilizers, pesticides, and plastics.