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[http://www.glossary.oilfield.slb.com/en/Terms/n/natural_gas.aspx Natural gas] is known to occur under many different conditions. Gas [[accumulation]]s have been assigned to two general groups depending on the feasibility of the recovery of their contained natural gas: Conventional natural-gas accumulations are those in reservoirs with a [[permeability]] high enough to allow gas to flow at rates that make its production profitable at current development and marketing costs. They are generally supported by water. Unconventional accumulations, on the other hand, are those from which gas is not profitably recovered for one reason or another at a certain time. Schmoker<ref name=Schmoker_2002>Schmoker, J. W., 2002, [http://archives.datapages.com/data/bulletns/2002/11nov/1993/1993.htm Resource-assessment perspectives for unconventional gas systems]: AAPG Bulletin, v. 86, no. 11, p. 1993-1999.</ref> describes unconventional gas accumulations as "continuous accumulations . . . that exist more or less independently of the water column," and discusses several methods for their assessment.

[[Natural gas]] is known to occur under many different conditions. Gas [[accumulation]]s have been assigned to two general groups depending on the feasibility of the recovery of their contained natural gas: Conventional natural-gas accumulations are those in reservoirs with a [[permeability]] high enough to allow gas to flow at rates that make its production profitable at current development and marketing costs. They are generally supported by water. Unconventional accumulations, on the other hand, are those from which gas is not profitably recovered for one reason or another at a certain time. Schmoker<ref name=Schmoker_2002>Schmoker, J. W., 2002, [http://archives.datapages.com/data/bulletns/2002/11nov/1993/1993.htm Resource-assessment perspectives for unconventional gas systems]: AAPG Bulletin, v. 86, no. 11, p. 1993-1999.</ref> describes unconventional gas accumulations as "continuous accumulations . . . that exist more or less independently of the water column," and discusses several methods for their assessment.

Some of the sources of natural gas labeled unconventional in the past have produced gas profitably in the United States for at least a decade or two and will undoubtedly become major sources of energy in the decades to come. Not only in the United States but in many other countries of the world, such unconventional sources of natural gas are known to occur, and although they are not given serious recognition now, they will certainly be developed sometime in the future. Foremost in this category of former unconventional sources of natural gas in the United States are gas in coal beds (the [[coalbed methane]]), gas in low-permeability sandstones ([[tight sands]]), [[basin-centered gas]] accumulations, and gas in organic black shales (also known as fractured shales). As of 2002, they contributed 26% of the total United States gas production.<ref name=LawandCurtis_2002>Law, B. E., and J. B. Curtis, 2002, [http://archives.datapages.com/data/bulletns/2002/11nov/1851/1851.htm Introduction to unconventional petroleum systems]: AAPG Bulletin, v. 86, no. 11, p. 1851-1852.</ref>

Some of the sources of natural gas labeled unconventional in the past have produced gas profitably in the United States for at least a decade or two and will undoubtedly become major sources of energy in the decades to come. Not only in the United States but in many other countries of the world, such unconventional sources of natural gas are known to occur, and although they are not given serious recognition now, they will certainly be developed sometime in the future. Foremost in this category of former unconventional sources of natural gas in the United States are gas in coal beds (the [[coalbed methane]]), gas in low-permeability sandstones ([[tight sands]]), [[basin-centered gas]] accumulations, and gas in organic black shales (also known as fractured shales). As of 2002, they contributed 26% of the total United States gas production.<ref name=LawandCurtis_2002>Law, B. E., and J. B. Curtis, 2002, [http://archives.datapages.com/data/bulletns/2002/11nov/1851/1851.htm Introduction to unconventional petroleum systems]: AAPG Bulletin, v. 86, no. 11, p. 1851-1852.</ref>

* [[Historical sources of energy]]

* [[Historical sources of energy]]

* [[Natural-gas liquids]]

* [[Natural-gas liquids]]

* [[Natural gas]]

* [[Natural gas: history and forecast]]

* [[Gas hydrates]]

* [[Gas hydrates]]

* [[Coalbed methane]]

* [[Coalbed methane]]