What is crude oil?

Crude oil occurs naturally in the ground and is a complex mixture of organic liquids, formed millions of years ago. Crude oil varies from oilfield to oilfield in color and composition, from pale yellow, low viscosity fluid to heavy black “treacle” consistencies. Of little use in its natural  state, the value of crude oil lies in its bi-products including fuels, lubricating oils, waxes, asphalt and petrochemicals.
Crude oil contains a mixture of hydrocarbon compounds and relatively small quantities of other materials such as oxygen, nitrogen, sulphur, salt and water.