Natural gas is a naturally occurring
hydrocarbon gas mixture consisting primarily of
methane, but commonly including varying amounts of other higher
alkanes, and sometimes a small percentage of
carbon dioxide,
nitrogen, and/or
hydrogen sulfide. It is formed when layers of decomposing plant and animal matter are exposed to intense heat and pressure supplied by existing under the surface over millions of years. The energy that the plants originally obtained from the sun is stored in the form of chemical bonds in the gas.