Natural gas is a gas composed of methane, with other hydrocarbons, carbon dioxide, nitrogen and hydrogen sulfide. The gas occurs naturally in underground cavities in many locations on Earth (see this map). It is often found at locations where petroleum too is present.
Before it can be used as a fuel, it must undergo processing to clean the gas and remove impurities including water in order to meet the specifications of marketable natural gas. The by-products of processing include ethane, propane, butanes, pentanes, and higher molecular weight hydrocarbons, hydrogen sulphide (which may be converted into pure sulfur), carbon dioxide, water vapor, and sometimes helium and nitrogen. It is this duge diversity in products that can be made from natural gas that has made it quite popular.