Naturally occurring hydrocarbon systems found in petroleum reservoirs are mixtures of organic compounds which exhibit multiphase behavior over wide ranges of pressures and temperatures. These hydrocarbon accumulations may occur in the gaseous state, the liquid state, the solid state, or in various combinations of gas, liquid, and solid. These all are produced from the earth either in liquid or gaseous form and these materials are known as crude oil or natural gas depending upon there composition. Crude oil is the most desire product of petroleum industries but natural gas is commonly produced along with crude oil.
Chemically petroleum consists of about 10 to 15 wt % of hydrogen and 85 to 90% of carbon. Oxygen, sulfur, nitrogen and helium may be found in fewer amounts.
Since petroleum is mostly composed of hydrocarbon, the molecular constitution of crude oils differs widely. It has wide range of hydrocarbon series. Of these series the most commonly encountered are the paraffin, the olefins, the polyethylene, the acetylenes turpenes and benzenes. Natural gas is predominantly composed of lower –molecular weight hydrocarbons of the paraffin series.