Petroleum is essentially a mixture of hydrocarbons particularly alkanes, cycloalkanes and aromatic hydrocarbons. The tarry residue which is left when the volatile components have been boiled off, consists of resins and asphaltenes. This non-volatile component contains very large organic molecules. Apart from hydrocarbons it may also contain compounds containing oxygen, nitrogen and sulphur. The exact composition of crude oil varies from one place to another and also from one oil field to another.