Petroleum

A complex mixture comprising mostly hydrocarbons, plus other organic compounds containing N, O, S, etc., that occur in Earth in liquid, gaseous, or solid form. A natural resource, petroleum is most often conceived of in its liquid form, commonly called crude oil, but, as a technical term, petroleum also refers to natural gas and the viscous or solid form known as bitumen, which is found in tar sands. The liquid and gaseous phases of petroleum constitute the most important of the primary fossil fuels.

Although oil consists basically of compounds of only two elements, carbon and hydrogen, these elements form a large variety of complex molecular structures. Regardless of physical or chemical variations, however, almost all crude oil ranges from 82 to 87 percent carbon by weight and 12 to 15 percent hydrogen. The more-viscous bitumens generally vary from 80 to 85 percent carbon and from 8 to 11 percent hydrogen.

Crude oil is an organic compound divided primarily into alkanes with single-bond hydrocarbons of the form C_nH_2n+2> or aromatics having six-ring carbon-hydrogen bonds, C^6H^6 such as benzene. Most crude oils are grouped into mixtures of various and seemingly endless proportions. No two crude oils from different sources are completely identical.