Petroleum is recovered mostly through oil drilling. It is a naturally occurring, smelly, yellow-to-black liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds as well as sulfur, oxygen, nitrogen, phosphorus and vanadium and other elements. It is soluble in various organic solvent instead of water. But it can form emulsion with water. Crude oil can be classified in to paraffin base crude, naphthenic crude and intermediate base crude. it can also categorized into ultra low-sulfur crude, sweet crude, sour crude and high-sulfur crude oil in terms of sulfur content；it can also be grouped into light crude, intermediate crude, heavy crude in terms of relative density.