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[[file:USGSgashydratemap014.jpg|thumb|300px|Known and inferred locations of gas hydrate occurrence. [http://woodshole.er.usgs.gov/project-pages/hydrates/primer.html Map compiled by the USGS].]]

[[file:USGSgashydratemap014.jpg|thumb|300px|Known and inferred locations of gas hydrate occurrence. [http://woodshole.er.usgs.gov/project-pages/hydrates/primer.html Map compiled by the USGS].]]

Gas hydrates (also called gas clathrates) are icelike, crystalline solids composed of natural-gas molecules, principally [[methane]], trapped in rigid crystalline cages formed by frozen water molecules. They are known to be stable under conditions of high pressure and low temperature that have been recognized in polar regions at depths from 130 to 2000 m (425 to 6500 ft), where temperatures are low enough for the formation of [[permafrost]], and in the uppermost part of deep-water sediments below the sea floor at depths of more than 100-1100 m (330-3600 ft).<ref name=Collett_2001>Collett, T. S., 2001, [http://archives.datapages.com/data/specpubs/memoir74/m74ch07/m74ch07.htm Natural-gas hydrates: Resources of the twenty-first century?], ''in'' M. W. Downey et al., eds., Petroleum provinces of the twenty-first century: [http://store.aapg.org/detail.aspx?id=77 AAPG Memoir 74], p. 85-108.</ref><ref name=Collett_2002>Collett, T. S., 2002, [http://archives.datapages.com/data/specpubs/memoir74/m74ch07/m74ch07.htm Energy resource potential of natural gas hydrates]: AAPG Bulletin, v. 86, no. 11, p. 1971-1992.</ref>  

Gas hydrates (also called gas clathrates) are icelike, crystalline solids composed of natural-gas molecules, principally [http://scifun.chem.wisc.edu/chemweek/methane/methane.html methane], trapped in rigid crystalline cages formed by frozen water molecules. They are known to be stable under conditions of high pressure and low temperature that have been recognized in polar regions at depths from 130 to 2000 m (425 to 6500 ft), where temperatures are low enough for the formation of [http://ipa.arcticportal.org/resources/what-is-permafrost permafrost], and in the uppermost part of deep-water sediments below the sea floor at depths of more than 100-1100 m (330-3600 ft).<ref name=Collett_2001>Collett, T. S., 2001, [http://archives.datapages.com/data/specpubs/memoir74/m74ch07/m74ch07.htm Natural-gas hydrates: Resources of the twenty-first century?], ''in'' M. W. Downey et al., eds., Petroleum provinces of the twenty-first century: [http://store.aapg.org/detail.aspx?id=77 AAPG Memoir 74], p. 85-108.</ref><ref name=Collett_2002>Collett, T. S., 2002, [http://archives.datapages.com/data/specpubs/memoir74/m74ch07/m74ch07.htm Energy resource potential of natural gas hydrates]: AAPG Bulletin, v. 86, no. 11, p. 1971-1992.</ref>  

Most methane in gas hydrates is of microbial origin, although some is believed to be the result of thermogenic processes.<ref name=MacDonald_1990>MacDonald, G. J., 1990, The future of methane as an energy resource: Annual Review of Energy, v. 15, p. 53-83.</ref><ref name=Kvenvolden_1993a>Kvenvolden, K. A., 1993, A primer on gas hydrates, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 270-291.</ref><ref name=Kvenvolden_1993b>Kvenvolden, K. A., 1993, Gas hydrates as a potential energy resource-A review of their methane content, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 555-561.</ref> Kvenvolden<ref name=Kvenvolden_1993a /><ref name=Kvenvolden_1993b /> reports that the total worldwide amount of methane in gas hydrates is certainly large, but that all of the estimates that have been made are speculative and uncertain. He adds that <blockquote>...a convergence of current ideas suggests that the amount of methane in gas hydrates worldwide is about 7 x 10⁵ trillion cubic feet.<ref name=Kvenvolden_1993bp557>Kvenvolden, K. A., 1993, Gas hydrates as a potential energy resource-A review of their methane content, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 557.</ref></blockquote> If this estimate is correct, he states that <blockquote>...the amound of methane carbon in gas hydrates is a factor of two larger than the carbon present in known fossil fuel deposits (coal, oil, and natural gas).<ref name=Kvenvolden_1993bp559>Kvenvolden, K. A., 1993, Gas hydrates as a potential energy resource-A review of their methane content, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 559.</ref></blockquote> He concludes, however, that <blockquote>...because of unsolved technological problems in producing methane from gas hydrates . . . wide-scale recovery of methane from these substances probably will not take place until sometime in the 21st century.<ref name=Kvenvolden_1993ap279>Kvenvolden, K. A., 1993, A primer on gas hydrates, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 279.</ref></blockquote> Other authors are less optimistic about the future availability of gas from gas hydrates, although it has been estimated<ref name=Collett_1993>Collett, T. S., 1993, Natural gas production from arctic gas hydrates, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 299-311.</ref> that 183 bcf of gas has been produced from gas hydrates in the [[Messoyakha field]], in northern [http://en.wikipedia.org/wiki/Siberia Siberia], since its discovery in 1968. The field produces from both hydrates and free-gas reservoirs.

Most methane in gas hydrates is of microbial origin, although some is believed to be the result of thermogenic processes.<ref name=MacDonald_1990>MacDonald, G. J., 1990, The future of methane as an energy resource: Annual Review of Energy, v. 15, p. 53-83.</ref><ref name=Kvenvolden_1993a>Kvenvolden, K. A., 1993, A primer on gas hydrates, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 270-291.</ref><ref name=Kvenvolden_1993b>Kvenvolden, K. A., 1993, Gas hydrates as a potential energy resource-A review of their methane content, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 555-561.</ref> Kvenvolden<ref name=Kvenvolden_1993a /><ref name=Kvenvolden_1993b /> reports that the total worldwide amount of methane in gas hydrates is certainly large, but that all of the estimates that have been made are speculative and uncertain. He adds that <blockquote>...a convergence of current ideas suggests that the amount of methane in gas hydrates worldwide is about 7 x 10⁵ trillion cubic feet.<ref name=Kvenvolden_1993bp557>Kvenvolden, K. A., 1993, Gas hydrates as a potential energy resource-A review of their methane content, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 557.</ref></blockquote> If this estimate is correct, he states that <blockquote>...the amound of methane carbon in gas hydrates is a factor of two larger than the carbon present in known fossil fuel deposits (coal, oil, and natural gas).<ref name=Kvenvolden_1993bp559>Kvenvolden, K. A., 1993, Gas hydrates as a potential energy resource-A review of their methane content, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 559.</ref></blockquote> He concludes, however, that <blockquote>...because of unsolved technological problems in producing methane from gas hydrates . . . wide-scale recovery of methane from these substances probably will not take place until sometime in the 21st century.<ref name=Kvenvolden_1993ap279>Kvenvolden, K. A., 1993, A primer on gas hydrates, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 279.</ref></blockquote> Other authors are less optimistic about the future availability of gas from gas hydrates, although it has been estimated<ref name=Collett_1993>Collett, T. S., 1993, Natural gas production from arctic gas hydrates, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 299-311.</ref> that 183 bcf of gas has been produced from gas hydrates in the [[Messoyakha field]], in northern [http://en.wikipedia.org/wiki/Siberia Siberia], since its discovery in 1968. The field produces from both hydrates and free-gas reservoirs.