natural gas

natural gas The term ‘fossil fuels’ is commonly used for tar sands, oil, gas condensates, natural gas, coal, and oil shale, and their products. Natural gas is a petroleum and one of the fossil fuels.

Petroleums have a wide range of compositions but can be classified into three major types depending on the relative proportions of gaseous hydrocarbons (C₁–C₅) and components with more than six carbon atoms (C₆₊). Natural gases are petroleums that are in the gas phase in the subsurface and are composed of hydrocarbon mixtures dominated by methane, with contributions from ethane, propane, butanes, and the pentanes, and traces of inorganic gases, principally carbon dioxide and nitrogen. Gases by definition contain no C₆₊ components. Natural gases can exist in the reservoir as a gas phase or in solution in a liquid petroleum (see petroleum). Natural gas does not condense at standard temperature and pressure and is usually dominated by methane.

Natural gas contains other compounds than hydrocarbons, including significant quantities of gases such as carbon dioxide, nitrogen, and hydrogen sulphide and traces of inert gases such as helium. These non-hydrocarbon gases can be the dominant components in some reservoirs. Gases high in carbon dioxide are usually associated with basins, such as the Pattani basin in Thailand, that have high geothermal gradients and deep carbonates.

Natural gas has the same origin as the other forms of petroleum and natural gas, and can be thought of as a petroleum with an extreme composition. Most natural gases are derived from the action of heat on organic matter in sediments, and all types of kerogens (insoluble organic matter in sediments) generate natural gas. Most commercial accumulations of natural gas are associated with source rocks that have been heated to temperatures of over 160 °C. Coals do not generate as much gas per weight of organic matter as organic matter in oil source rocks, but they are prolific source rocks for natural gas since they are rich in organic matter and rarely expel any generated oil, which remains in the source rock and is converted to natural gas at higher temperatures. Most natural gases migrate and accumulate in reservoirs in the same types of rocks, and by the same processes, as other petroleums. Coals may act both as a source and reservoir for natural gas. This is because the methane is readily absorbed in the coal matrix and may be desorbed by reducing the pressure in the coal by pumping out the water. The natural gases in the San Juan basin in the USA are principally of this type.

Natural gas is also derived from the thermal breakdown of crude oil at temperatures above 160 °C, either in reservoirs or from oil remaining in a source rock. Natural gas may also be generated by biological activity in shallow sediments when bacteria reduce carbon dioxide to methane to produce so-called ‘biogenic methane’. Natural gases derived from biogenic methane can also be produced during the biodegradation of oil. This is termed ‘secondary biogenic gas’. Natural gases may also be bacterially altered; this has happened in several of the large North Sea accumulations that have large gas caps, such as the Frigg and Troll fields. Usually when this happens bacterial activity destroys the ethane and propane in the gases, leaving a gas in which methane predominates.

The reserves of natural gas greatly exceed those of crude oil.

Steven Larter