Offshore production platform for natural gas. This is a fossil fuel, a fuel that is formed by the breakdown of organic matter. It occurs mainly at the top of oil wells, although it may exist in wells of its own. It consists largely of the gases methane and ethane. Heavier gases, such as butane, are generally removed before the gas is used as fuel. A drilling rig is used to drill below the seabed to reach the natural gas. The rig is then replaced by a production platform, usually anchored to the seabed, which brings the natural gas to the surface. Photographed in the Irish Sea.