Natural gas is a gaseous fossil fuel consisting primarily of methane, which is the simplest hydrocarbon, but including significant quantities of ethane, propane, butane, and pentane – heavier hydrocarbons which are removed prior to use as a consumer fuel – as well as carbon dioxide, nitrogen, helium and hydrogen sulfide. Natural gas is a colourless, odourless mixture of gases made up mostly of methane (CH4). Other gases that can form part of natural gas include small amounts of ethane (C2H6), propane (C3H8) and butane (C4H10). Natural Gas liquifies at a temperature of minus 161 degrees Celsius. It is found in oil fields (associated) either dissolved or isolated in natural gas fields (non associated), and in coal beds (as coal-bed 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. Natural gas has a good safety record. Methane is lighter than air and easily disperses in the event of a gas leak. Another reason Natural Gas has such a good safety record is that its use in Australia is governed by rigorous industry technical standards. Australian Natural Gas prices are among the lowest of the OECD countries and its proven and probable reserves of Natural Gas are equal to about 91 years supply at current production levels. Australia exports Liquefied Natural Gas (LNG) to the Asia-Pacific region earning over $1.6 billion in export income.