Natural gas is a mixture of hydrocarbons (molecules that contain only carbon and hydrogen) and gases (most notably methane, ethane, propane, and butane) that exist naturally in rocks beneath the surface of the earth. It is widely used as a heating source, and in some cases, specific portions of the natural gas are used as starting materials in industrial processes. Natural gas is the product of the decaying of living matter over millions of years (as is also true for petroleum). Specific conditions, including low oxygen levels, are necessary for this to occur. The hydrocarbon gases are trapped in geological formations known as anticlines. Each of the major hydrocarbon components of natural gas is used as a fuel source.

Natural gas has its origins in decayed living matter, most likely as the result of the action of bacteria upon dead animal and plant material. In order...