Natural gas is a combustible mixture of hydrocarbon gases.

While natural gas is formed primarily of methane, it can also include ethane, propane, butane and pentane.

The composition of natural gas can vary widely, but is often composed of the following: methane (70-90%), Ethane, propane (0-20%), Butane, Carbon Dioxide (0-8%), Oxygen (0-0.2%), Nitrogen (0-5%), Hydrogen Sulphide (0-5%) and sometimes traces of other rare gases (A, He,Ne,Xe). In its pure form it is almost pure methane. Methane is a molecule made up of one carbon atom and four hydrogen atoms, and is referred to as CH4.

When you talk about gas composition you are in fact referring to its calorific value