natural gas - naturally occurring mixture of gaseous saturated hydro carbons, consisting of 80-95% methane (CH4), lesser amounts of propane, ethane and butane, and small quantities of non-hydrocarbon gases (e.g., nitrogen, helium). Natural gas is found in sandstone, limestone, and other porous rocks beneath the earth's surface, often in association with crude oil. Because of its high heating value and clean-burning characteristics, natural gas is widely used as a fuel. The heavier hydrocarbons in natural gas can be extracted, through compression or absorption processes, to yield LPG (propane or butane), natural gasoline, and raw materials for petrochemical manufacture. See gas engine, natural gas liquids.