Natural gas is a combustible, gaseous mixture of simple hydrocarbon compounds, usually found in deep underground reservoirs formed by porous rock. Natural gas is a fossil fuel composed almost entirely of methane, but does contain small amounts of other gases, including ethane, propane, butane and pentane. Methane is composed of a molecule of one carbon atom and four hydrogen atoms.
Natural gas is used extensively in residential, commercial and industrial applications. Use is also rapidly increasing in electric power generation and cooling, and as a transportation fuel.
The prevailing scientific theory is that natural gas was formed millions of years ago when plants and tiny sea animals were buried by sand and rock. Layers of mud, sand, rock and plant and animal matter continued to build up until the pressure and heat from the earth turned them into petroleum and natural gas. Natural gas reserves worldwide are thought to be about the same as oil -- around 130 billion tonnes of oil equivalent (toe).