NATURAL GAS

colourless, highly inflammable gaseous hydrocarbon consisting primarily of methane and ethane. It is a type of petroleum that commonly occurs in association with crude oil. A brief treatment of natural gas follows. For full treatment, see Fuels, Fossil: Natural gas; Industries, Extraction and Processing: Petroleum production. Besides methane and ethane, two volatile hydrocarbons that are gaseous under the normal atmospheric conditions at the surface of a well, natural gas may also contain heavier hydrocarbons such as propane, butane, pentane, and hexane, some of which condense at the surface and hence are known as natural-gas liquids. Other gases that frequently occur along with the gaseous hydrocarbons include carbon dioxide, hydrogen, hydrogen sulfide, nitrogen, and helium and argon. Natural gas is frequently found with or near accumulations of oil because the same geologic conditions are favourable for generating both gaseous and liquid hydrocarbons. Most of the constituents of natural gas come from organic matter (the remains of land and aquatic plants) in sediments that have been transformed through geologic processes over extremely long periods of time. The gas that eventually resulted from these natural processes has become entrapped in reservoirs distributed in the Earth's strata. Producers obtain natural gas by extracting it through wells drilled into the Earth. Some natural gases can be used as they are produced at the well, without any refinement, but most require processing. Generally, this consists of condensing the less volatile hydrocarbons such as propane and butane (yielding large quantities of liquefied petroleum gas; q.v.) and removing such undesirable constituents as water vapour, carbon dioxide, hydrogen sulfide, and other sulfur compounds. The remaining gas, consisting almost entirely of methane, is transported either in its naturally gaseous state by pipeline or, after liquefaction by cooling to -160 C (-256 F), by tankers. Liquefied natural gas (q.v.) occupies only about 1/600 of the volume of the gas. Although natural gas has been used since ancient times, it did not become an important source of energy until the 1930s, when improved pipeline technology allowed natural gas to compete with town gas, produced from coal, in the United States. Since that time, natural gas has been exploited increasingly as a residential and industrial fuel and as a chemical feedstock, particularly since the oil crises of the 1970s. Discoveries of major natural-gas fields in western Europe, Russia, North Africa, and the Middle East have contributed to this trend. colourless, highly flammable gaseous hydrocarbon consisting primarily of methane and ethane. It is a type of petroleum that commonly occurs in association with crude oil. Natural gas is often found dissolved in oil at the high pressures existing in a reservoir, and it also can be present as a gas cap above the oil. Such natural gas is known as associated gas. There are also reservoirs that contain gas and no oil. This gas is termed nonassociated gas. Joseph P. Riva, Jr. Gordon I. Atwater Additional reading James A. Clark, The Chronological History of the Petroleum and Natural Gas Industries (1963), gives a detailed chronology of both technical and human facts. Malcolm W.H. Peebles, Evolution of the Gas Industry (1980), provides international, historical, and technological developments. Arlon R. Tussing and Connie C. Barlow, The Natural Gas Industry: Evolution, Structure, and Economics (1984), examines the natural gas industry in the United States. E.L. Rawlins and M.A. Schellhardt, Back-Pressure Data on Natural-Gas Wells and Their Application to Production Practices (1935, reissued 1970), is a classic report of the U.S. Bureau of Mines describing and explaining the back-pressure method, with an analysis of data for more than 500 gas wells. Morris Muskat, Physical Principles of Oil Production, 2nd ed. (1981), and The Flow of Homogeneous Fluids Through Porous Media (1937, reprinted 1982), are fundamental works on the basic principles of gas and petroleum.Collections of scientific papers may be found in G.D. Hobson (ed.), Developments in Petroleum Geology, 2 vol. (197780); and in publications of the American Association of Petroleum Geologists, including the AAPG Bulletin (monthly), the October issue of which contains an annual review of significant exploration and production activity; and the AAPG Memoir (irregular). Basic Petroleum Data Book (three per year); and Minerals Yearbook, prepared by the U.S. Bureau of Mines, include annual statistical reviews of the petroleum industry. Each year maps, production figures, and geologic data are published in August by World Oil and in December by the Oil and Gas Journal. Joseph P. Riva, Jr. Gordon I. Atwater