Natural gas has a similar origin to oil and is usually found forming a layer or deposit on the top of oil fields. It is formed of a mixture of gases, of which the main one is methane (CH₄). Natural gas is a good substitute for coal as a fuel, due to it being easy to transport and having a high gross calorific value and the fact that it is less polluting than other fossil fuels.
The following picture shows a methane molecule, the main component of natural gas.