natural gas

a combustible mixture of gaseous hydrocarbons that accumulates in porous sedimentary rocks, esp. those yielding petroleum, consisting usually of over 80 percent methane together with minor amounts of ethane, propane, butane, nitrogen, and, sometimes, helium: used as a fuel and to make carbon black, acetylene, and synthesis gas.

[1815-25]

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Colourless, highly flammable gaseous hydrocarbon consisting primarily of methane and ethane.

It may also contain heavier hydrocarbons, carbon dioxide, hydrogen, hydrogen sulfide, nitrogen, helium, and argon. It commonly occurs in association with crude oil (see petroleum). Natural gas is extracted from wells drilled into the Earth. Some natural gas can be used as it comes from the well, without any refining, but most requires processing. It is transported either in its natural gaseous state by pipeline or, after liquefaction by cooling, by tankers. Liquefied natural gas occupies only about 1/600 of the volume of the gas. It has grown steadily as a source of energy since the 1930s.

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Introduction

      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.

History of use

Discovery and early application

      The first discoveries of natural gas seeps were made in Iran between 6000 and 2000 BC. Many early writers described the natural petroleum seeps in the Middle East, especially in the Baku region of what is now Azerbaijan. The gas seeps, probably first ignited by lightning, provided the fuel for the “eternal fires” of the fire-worshiping religion of the ancient Persians. The use of natural gas was mentioned in China about 900 BC. It was in China in 211 BC that the first known well was drilled for natural gas to reported depths of 150 metres (500 feet). The Chinese drilled their wells with bamboo poles and primitive percussion bits for the express purpose of searching for gas in Late Triassic limestones (more than 208,000,000 years old) in an anticline west of modern Chungking. The gas was burned to dry the rock salt found interbedded in the limestone. Eventually wells were drilled to depths approaching 1,000 metres, and more than 1,100 wells had been drilled into the anticline by 1900.

      Natural gas was unknown in Europe until its discovery in England in 1659, and even then it did not come into wide use. Instead, gas obtained from carbonized coal (known as town gas) became the primary fuel for illuminating streets and houses throughout much of Europe from 1790 on. In North America the first commercial application of a petroleum product was the utilization of natural gas from a shallow well in Fredonia, N.Y., in 1821. The gas was distributed through a small-bore lead pipe to consumers for lighting and cooking.

Improvements in gas pipelines (pipeline)

      Throughout the 19th century the use of natural gas remained localized because there was no way to transport large quantities of gas over long distances. Natural gas remained on the sidelines of industrial development, which was based primarily on coal and oil. An important breakthrough in gas-transportation technology occurred in 1890 with the invention of leakproof pipeline coupling. Nonetheless, materials and construction techniques remained so cumbersome that gas could not be used more than 160 kilometres (100 miles) from a source of supply. Thus, associated gas was mostly flared (i.e., burned at the wellhead), and nonassociated gas was left in the ground, while town gas was manufactured for use in the cities.

      Long-distance gas transmission became practical during the late 1920s because of further advances in pipeline technology. From 1927 to 1931 more than 10 major transmission systems were constructed in the United States. Each of these systems was equipped with pipes having diameters of approximately 51 centimetres (20 inches) and extended more than 320 kilometres. Following World War II, a large number of even longer pipelines of increasing diameter were constructed. The fabrication of pipes having a diameter of up to 142 centimetres became possible. Since the early 1970s the longest gas pipelines have had their origin in Russia. For example, the 5,470-kilometre-long Northern Lights pipeline crosses the Ural Mountains and some 700 rivers and streams, linking eastern Europe with the West Siberian gas fields on the Arctic Circle. As a result, gas from the Urengoy field, the world's largest, is transported to eastern Europe and then on to western Europe for consumption. Another gas pipeline, shorter but also of great engineering difficulty, is the 51-centimetre line that extends from Algeria across the Mediterranean Sea to Sicily. The sea is more than 600 metres deep along some parts of the route.

Natural gas as a premium fuel

      As recently as 1960, associated gas was a nuisance by-product of oil production in many areas of the world. The gas was separated from the crude oil stream and eliminated as cheaply as possible, often by flaring. Only since the crude oil shortages of the late 1960s and early 1970s has natural gas become an important world energy source.

      Even in the United States the home-heating market for natural gas was limited until the 1930s when town gas began to be replaced by abundant and cheaper supplies of natural gas, which contained twice the heating value of its synthetic predecessor. Also, when natural gas burns completely, carbon dioxide and water are normally formed. The combustion of gas is relatively free of soot, carbon monoxide, and the nitrogen oxides associated with the burning of other fossil fuels. In addition, sulfur dioxide emissions, another major air pollutant, are almost nonexistent. As a consequence, natural gas is often a preferred fuel for environmental reasons.

Properties of natural gas

Chemical composition

Hydrocarbon content

      Natural gas is a hydrocarbon mixture consisting primarily of methane and ethane, both of which are gaseous under atmospheric conditions. The mixture also may contain other hydrocarbons, such as propane, butane, pentane, and hexane. In natural gas reservoirs even the heavier hydrocarbons occur for the most part in gaseous form because of the higher pressures. They liquefy at the surface (at atmospheric pressure) and are referred to as natural gas liquids, gas condensate, natural gasoline, or liquefied petroleum gas. They may separate in some reservoirs through retrograde condensation or may be separated at the surface either in field separators or in gas processing plants by means of condensation, absorption, adsorption, or other modification. The average production of natural gas liquids in the United States is nearly 38 barrels per 1,000,000 cubic feet of produced gas.

Nonhydrocarbon content

      Other gases that commonly occur in association with the hydrocarbon gases are nitrogen, carbon dioxide, hydrogen, hydrogen sulfide, and such noble gases as helium and argon. Because natural gas and formation water occur together in the reservoir, gas recovered from a well contains water vapour, which is partially condensed during transmission to the processing plant.

Physical properties

      The physical properties of natural gas include colour, odour, and flammability. The principal ingredient of gas is methane, which is colourless, odourless, and highly flammable. However, some of the associated gases in natural gas, especially hydrogen sulfide, have a distinct and penetrating odour, and a few parts per million is sufficient to impart a decided odour to natural gas.

Measurement systems

      The amounts of gas accumulated in a reservoir, as well as produced from wells, are calculated in cubic metres at a pressure of 750 millimetres of mercury and a temperature of 15° C (or in cubic feet at an absolute pressure of 14.73 pounds per square inch and a temperature of 60° F). Since gas is compressed at high reservoir pressures, it expands upon reaching the surface and thus occupies more space. As its quantity is calculated in reference to standard conditions of temperature and pressure, however, the expansion does not constitute an increase in the amount of gas produced.

Origin of natural gas

Organic formation process

      Natural gas is more ubiquitous than oil. It is derived from both land plants and aquatic organic matter and is generated above, throughout, and below the oil window. Thus, all source rocks have the potential for gas generation. Many of the source rocks for significant gas deposits appear to be associated with the worldwide occurrence of Carboniferous coal (roughly 286,000,000 to 360,000,000 years in age).

The biological stage

      During the immature, or biological, stage of petroleum formation, biogenic methane (often called marsh gas) is produced as a result of the decomposition of organic material by the action of anaerobic microbes. These microorganisms cannot tolerate even traces of oxygen and are also inhibited by high concentrations of dissolved sulfate. Consequently, biogenic gas generation is confined to certain environments that include poorly drained swamps and bays, some lake bottoms, and marine environments beneath the zone of active sulfate reduction. Gas of predominantly biogenic origin is thought to constitute more than 20 percent of the world's gas reserves.

      The mature stage of petroleum generation, which occurs at depths of about 760 to 4,880 metres, includes the full range of hydrocarbons that are produced within the oil window. Often significant amounts of thermal methane gas are generated along with the oil. Below 2,900 metres, primarily wet gas (gas containing liquid hydrocarbons) is formed.

The thermal stage

      In the postmature stage, below about 4,880 metres, oil is no longer stable, and the main hydrocarbon product is thermal methane gas. The thermal gas is the product of the cracking of the existing liquid hydrocarbons. Those hydrocarbons with a larger chemical structure than that of methane are destroyed much more rapidly than they are formed. Thus, in the sedimentary basins of the world, comparatively little oil is found below 4,880 metres. The deep basins with thick sequences of sedimentary rocks, however, have the potential for deep gas production.

Inorganic formation

      Some methane may have been produced by inorganic processes. The original source of the Earth's carbon was the cosmic debris from which the planet formed. If meteorites are representative of this debris, the carbon could have been supplied in comparatively high concentrations as hydrocarbons, such as are found in the carbonaceous chondrite type of meteorites. Continuous outgassing of these hydrocarbons may be taking place from within the Earth, and some may have accumulated as abiogenic gas deposits without having passed through an organic phase. In the event of widespread outgassing, however, it is likely that abiogenic gas would be too diffuse to be of commercial interest. Significant accumulations of inorganic methane have yet to be found.

      The helium and some of the argon found in natural gas are products of natural radioactive disintegration. Helium derives from radioisotopes of thorium and the uranium family, and argon derives from potassium. It is probably coincidental that helium and argon sometimes occur with natural gas; in all likelihood, the unrelated gases simply became caught in the same trap.

Joseph P. Riva, Jr. Ed.

The geologic environment

 Like oil, natural gas migrates and accumulates in traps (petroleum trap) (see Figure 1—> in the article petroleum). Oil accumulations contain more recoverable energy than gas accumulations of similar size, even though the recovery of gas is a more efficient process than the recovery of oil. This is due to the differences in the physical and chemical properties of gas and oil. Gas displays initial low concentration and high dispersibility, making adequate cap rocks very important.

      Natural gas can be the primary target of either deep or shallow drilling because large gas accumulations form above the oil window as a result of biogenic processes and thermal gas occurs throughout and below the oil window. In most sedimentary basins the vertical potential (and sediment volume) available for gas generation exceeds that of oil. About a quarter of the known major gas fields are related to a shallow biogenic origin, but most major gas fields are located at intermediate or deeper levels where higher temperatures and older reservoirs (often carbonates sealed by evaporites) exist.

Conventional gas reservoirs

      Gas reservoirs differ greatly, with different physical variations affecting reservoir performance and recovery. In a natural gas (single-phase) reservoir it should be possible to recover nearly all of the in-place gas by dropping the pressure sufficiently. If the pressure is effectively maintained by the encroachment of water in the sedimentary rock formation, however, some of the gas will be lost to production by being trapped by capillarity behind the advancing water front. Therefore, in practice, only about 80 percent of the in-place gas can be recovered. On the other hand, if the pressure declines, there is an economic limit at which the cost of compression exceeds the value of the recovered gas. Depending on formation permeability, actual gas recovery can be as high as 75 to 80 percent of the original in-place gas in the reservoir. Associated gas is produced along with the oil and separated at the surface.

Unconventional gas reservoirs

      Substantial amounts of gas have accumulated in geologic environments that differ from conventional petroleum traps. This gas is termed unconventional gas and occurs in “tight” (i.e., relatively impermeable) sandstones, in joints and fractures or absorbed into the matrix of shales (often of the Devonian Period [about 360,000,000 to 408,000,000 years old]), dissolved or entrained in hot geopressured formation waters, and in coal seams. Unconventional gas sources are much more expensive to exploit and have to be produced at much slower rates than conventional gas fields. Moreover, recoveries are low. In all likelihood, unconventional gas will continue to complement conventional gas production but will not supplant it.

Tight gas

      Tight gas occurs in either blanket or lenticular sandstones that have an effective permeability of less than 1 millidarcy (or 0.001 darcy, which is the standard unit of permeability of a substance to fluid flow). These relatively impermeable sandstones are reservoirs for considerable amounts of gas that are mostly uneconomical to produce because of low natural flow rates. The outlook for increased production of gas from tight sandstones has been enhanced by the use of massive hydraulic fracturing techniques that create large collection areas in low-permeability formations through which gas can flow to a producing well. A fractured well in a tight gas formation usually produces at a lower rate than a conventional gas well but for a longer time. About 2 percent of the gas production in the United States comes from tight sandstones.

Devonian shale gas

      Devonian shale gas was generated from organic mud deposited during the Devonian Period. Subsequent sedimentation and the resultant heat and pressure transformed the mud into shale and also produced natural gas from the organic matter contained therein. Some of the gas migrated to adjacent sandstones and was trapped in them, forming conventional gas accumulations. The rest of the gas remained locked in the nonporous shale. The production history of Devonian shale gas indicates that the recovered gas occurs in well-connected fracture porosity. Production is generally at low flow rates but is long-lasting. The factor of greatest importance in commercial production is the presence of natural fractures, but wells can be stimulated by explosives or by hydraulic fracturing, which sometimes enhances gas production. About 1 percent of the gas produced in the United States comes from Devonian shales.

Coal-bed gas

      Considerable quantities of methane are trapped within coal seams. Although much of the gas that formed during the initial coalification process is lost to the atmosphere, a significant portion remains as free gas in the joints and fractures of the coal seam and as adsorbed gas on the internal surfaces of the micropores within the coal itself. Since coal is relatively impermeable, any methane recovered usually must flow through existing fracture systems. Therefore, coal seams that are highly fractured appear to be the best sources of coal-bed methane. Coal-bed gas production is common in Europe, although the gas is frequently mixed with air. In the United States, coal-bed gas accounts for about 2 percent of total gas output.

Geopressured fluids

      Geopressured reservoirs exist throughout the world in deep, geologically young sedimentary basins in which the formation fluids (which usually occur in the form of a brine) bear a part of the overburden load. The fluid pressures can become quite high, sometimes almost double the normal hydrostatic gradient. In many cases the geopressured fluids also become hotter than normally pressured fluids, because the heat flow to the surface is impeded by insulating layers of impermeable shales and clays. Geopressured fluids have been found to be saturated with 0.84 to 2.24 cubic metres of natural gas per 0.159 cubic metre of brine, or 30 to 80 cubic feet of gas per barrel. To produce this gas, high flow rates of the hot geopressured fluids must be maintained from formations of high porosity and permeability. Because very large amounts of formation water must be produced to recover commercial quantities of the associated gas, there is no commercial gas production known to be derived from a geopressured deposit.

World distribution of natural gas

Location of major gas fields

      The largest natural gas fields are the supergiants, which contain more than 850,000,000,000 cubic metres of gas, and the world-class giants, which have reserves of roughly 85,000,000,000 to 850,000,000,000 cubic metres. Supergiants and world-class giants represent less than 1 percent of the world's total known gas fields, but they originally contained, along with associated gas in giant oil fields, approximately 80 percent of the world's reserves and produced gas.

Russia

 Some of the world's largest gas fields occur in Russia in a region of West Siberia east of the Gulf of Ob on the Arctic Circle (see Figure 3—> in the article petroleum). The world's largest gas field is Urengoy, which was discovered in 1966. Its initial reserves have been estimated at 8,087,000,000,000 cubic metres. Nearly 6,230,000,000,000 cubic metres of this gas are in the shallowest reservoir, 1,100 to 1,250 metres deep, which is Upper Cretaceous in age (from about 66,400,000 to 97,500,000 years old). In all, Urengoy has 15 separate reservoirs, some in Lower Cretaceous rocks (those that are approximately 97,500,000 to 144,000,000 years old). The deepest is a gas condensate zone in Upper Jurassic strata (from about 144,000,000 to 163,000,000 years old). Urengoy began production in 1978. Its maximum output is expected to be as much as 250,000,000,000 cubic metres of gas per year, which would considerably exceed the production from any other gas field in the world.

      Yamburg, Russia's second largest gas field, was discovered north of the Arctic Circle and north of Urengoy. Its original reserves were estimated at 4,700,000,000,000 cubic metres of gas, mostly from Upper Cretaceous reservoir rocks at depths of 1,000 to 1,210 metres. Development of Yamburg began in the early 1980s. Bovanenkovskoye, discovered in 1971 on the Yamal Peninsula in northwestern Siberia, is Russia's third largest field. It has reserves estimated at 4,102,000,000,000 cubic metres in Lower Cretaceous reservoir rock at depths of 1,190 to 1,475 metres. Bovanenkovskoye has not yet been developed. Some of the gas in these huge, shallow fields may be of biogenic origin and capped by permafrost.

       Orenburg, discovered in the Volga-Urals region in 1967, is the largest Russian gas field outside of West Siberia. It had initial reserves of 1,778,300,000,000 cubic metres of gas and is now under production.

Europe

 The largest natural gas field in Europe is Groningen (see Figure 3—> in the article petroleum), with original recoverable reserves of about 2,270,000,000,000 cubic metres. It was discovered in 1959 on the Dutch coast. The discovery well was drilled through evaporites of Permian age (about 245,000,000 to 286,000,000 years old) into a thick basal Permian sandstone that was gas-productive. Subsequent drilling outlined a broad anticline about 24 kilometres wide by 40 kilometres long, which has a continuous basal Permian sandstone reservoir capped by evaporites. The reservoir contains natural gas at depths between 2,440 and 3,050 metres. It overlies the truncated and strongly faulted coal-bearing Pennsylvanian sequence (the Pennsylvanian Period extended from about 286,000,000 to 320,000,000 years ago), which is considered to be the main source of the gas.

North America

 In the United States, Hugoton, discovered in 1927 in Kansas and found to extend through the Oklahoma and Texas panhandles, is a gas field with an estimated ultimate recovery of 1,986,000,000,000 cubic metres (see Figure 4—> in the article petroleum). More than 7,000 wells have been drilled in this extensive field, which produces from a series of Permian limestones and dolomites. The gas accumulations are stratigraphically controlled by variations in lithology. The productive area extends along a 400-kilometre trend. Canada has a significant estimated endowment of natural gas, of which only about 17 percent has been produced. Its undiscovered resource potential is almost equal to that of the United States. The largest gas field is Elmworth. Discovered in Alberta in 1976, Elmworth contained some 560,000,000,000 cubic metres of gas in a Cretaceous sandstone reservoir. Mexico's largest gas accumulation is associated with the supergiant Bermudez oil field. Located in 1958 in the Chiapas-Tabasco region, Bermudez originally contained 490,000,000,000 cubic metres of associated gas in a Cretaceous dolomite reservoir. Although Mexico's estimated gas endowment is less than half that of Canada, natural gas is underutilized in Mexico, and only 11 percent of that country's estimated total recoverable gas has been produced.

North Africa

      In North Africa the central basin of Algeria is the location of the Hassi R'Mel (Hassi RʾMel) gas and condensate field. Discovered in 1956 in a large anticline, the field is estimated to have originally contained about 2,520,000,000,000 cubic metres of recoverable gas in reservoirs of permeable Triassic sandstone (from about 208,000,000 to 245,000,000 years old) capped by salt beds. Hassi R'Mel is under development and is reported to have the capacity to produce 59,000,000,000 cubic metres of gas per year.

Middle East

      There is an enormous gas potential in the Middle East associated with the major structures in the Arabian-Iranian basin. The Permian Khuff formation underlies most of the region and is an important gas-bearing horizon. It forms the reservoir of the supergiant North Field of offshore Qatar and also of other smaller nonassociated gas fields in the region. There also is great potential for nonassociated gas accumulations in Lower Cretaceous (as well as in the Permian) strata should the demand for Persian Gulf gas rise, either for domestic use or for export.

Asia

      The largest gas field in Asia is Arun, which was discovered in 1971 in the North Sumatra basin of Indonesia. The gas reservoir is a reef limestone of middle Miocene age (from about 11,000,000 to 16,000,000 years old). Original reserves have been estimated at about 383,000,000,000 cubic metres. The gas is liquefied for export.

Status of world gas reserves

       Recoverable Natural Gas Resources of the World*, TableWhen the generation and migration of gas is considered, the extensive vertical gas-generation zone includes shallow biogenic gas, the intermediate dissolved gas of the oil window, and deeper thermal gas. This large vertical habitat for gas and the additional availability of source material indicate that considerable gas may have been formed and still remains undiscovered. The Table (Recoverable Natural Gas Resources of the World*, Table), derived from an assessment of the U.S. Geological Survey and other estimates in the technical literature, shows the broad distribution of world natural gas. It is estimated that 45 percent of the world's recoverable gas remains undiscovered and that, on the basis of energy content, the world's ultimate recoverable resources of natural gas will approach those of oil. Because the utilization of gas in large volumes lags behind the use of oil, the world's stock of gas is expected to last longer than that of oil. However, if the consumption of gas approaches that of oil on an equivalent basis, it, too, will be short-lived as a major energy resource.

      About 14 percent of the world's estimated total gas endowment has been consumed or flared. The flaring of associated gas has long been a practice connected with oil production. As recently as 1980, approximately 10 percent of world annual gas production was lost at the wellhead by this procedure. Historically, Middle Eastern and African oil-producing countries have flared the most gas. Much of the gas yielded is reinjected, but what cannot be reinjected has often been flared because the remote location of many oil wells makes the recovery of gas expensive. As the value of gas has appreciated, however, conservation efforts have increased and gas flaring has been reduced.

       Recoverable Natural Gas Resources of the World*, TableThe Table (Recoverable Natural Gas Resources of the World*, Table) shows that the estimated total world endowment of natural gas is more than 344,000,000,000,000 cubic metres. About one-third of this gas was originally located in the Soviet Union, which, prior to its dissolution in 1991, had surpassed the United States to become the world's leading producer of natural gas. Together, the Soviet Union and the Middle East originally accounted for more than half of the world's natural gas endowment. The United States also possessed a significant endowment of natural gas, but it has already consumed more than half of its resources. U.S. gas production has been projected to fall by as much as 10 percent by the end of the 20th century because of the declining resource base.

      The total gas endowments of Latin America, western Europe, Africa, and Asia and the Pacific region, while significant, are thought to be considerably smaller than those of North America, the former Soviet Union, and the Middle East. However, past gas production in these regions has been somewhat limited; therefore much of the original gas is still available for use.

      Russia had the world's largest original gas endowment—more than 98,000,000,000,000 cubic metres. The United States and Iran both had original gas endowments of more than 33,000,000,000,000 cubic metres. The gas endowments of the following countries were in excess of 2,800,000,000,000 cubic metres in descending order: Saudi Arabia, Canada, China, Turkmenistan, Norway, Mexico, the United Arab Emirates, Nigeria, Qatar, Kazakhstan, Venezuela, Indonesia, Kuwait, Australia, Algeria, Uzbekistan, Malaysia, The Netherlands, and Ukraine. These countries originally possessed more than 90 percent of the world's total recoverable natural 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. (1977–80); 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

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