Posted: 3 November 2000

A mixture of naturally occurring hydrocarbons, that may exist in a solid (e.g. bitumen), liquid (e.g. crude oil) or gaseous state (e.g. natural gas). It commonly contains variable amounts of other chemicals such as sulphur (S) and nitrogen (N). Petroleum is the end-product of the partial decay of living organisms which once inhabited the world's oceans. As they died they sank to the bottom of the oceans, where the anaerobic conditions allowed them to be preserved.