<P> 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, hydrogen sulfide, or helium . It is formed when layers of decomposing plant and animal matter are exposed to intense heat and pressure under the surface of the Earth 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 . </P> <P> Natural gas is a fossil fuel used as a source of energy for heating, cooking, and electricity generation . It is also used as a fuel for vehicles and as a chemical feedstock in the manufacture of plastics and other commercially important organic chemicals . Fossil fuel based natural gas is a non-renewable resource . </P> <P> Natural gas is found in deep underground rock formations or associated with other hydrocarbon reservoirs in coal beds and as methane clathrates . Petroleum is another resource and fossil fuel found in close proximity to and with natural gas . Most natural gas was created over time by two mechanisms: biogenic and thermogenic . Biogenic gas is created by methanogenic organisms in marshes, bogs, landfills, and shallow sediments . Deeper in the earth, at greater temperature and pressure, thermogenic gas is created from buried organic material . </P>

Most oil and natural gas is found in what type of rock