Natural gas and crude oil differ in both their chemical composition and in their natural state. Oil tends to be found in its liquid phase in underground reservoirs, whereas natural gas exists in its gaseous phase unless it can be cooled to -160 degrees Celsius.
Both natural gas and oil are composed mainly of complex organic molecules known as hydrocarbons. The major components of natural gas are methane and any combination of ethane, propane and butane. Oil varies enormously by its source, but it is primarily composed of naphthenes, paraffins and aromatic hydrocarbons. The remaining volume of crude oil is usually taken up by a class of compounds known as asphaltics.