This book provides leading edge research on Natural gas which is a gaseous fossil fuel consisting primarily of methane but including significant quantities of ethane, propane, butane, and pentane -- heavier hydrocarbons removed prior to use as a consumer fuel -- as well as carbon dioxide, nitrogen, helium and hydrogen sulphide. It is found in oil fields (associated) either dissolved or isolated in natural gas fields (non associated), and in coal beds (as coalbed methane). When methane-rich gases are produced by the anaerobic decay of non-fossil organic material, these are referred to as biogas. Sources of biogas include swamps, marshes, and landfills (see landfill gas), as well as sewage sludge and manure by way of anaerobic digesters, in addition to enteric fermentation particularly in cattle.