The word Petroleum is derived from two words Petra meaning rock and oleum meaning oil. The oil occurs naturally in porous rocky strata in liquid form is the crude oil. If it is in semi hard state mixed in sand then it is bitumen. The crude oil can be black, brown, green or even orangish yellow. It does contain oxygen, sulphur, nitrogen and other items in minute quantities which determines it suitability for specific application.The crude oil or its derivatives are marketed by volume or weight, gallon, litre, barrel, are the volumetric measurements while metric tonnes (mt or MT) are for the weight.