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The world's 932 giant oil and gas fields are considered those with of ultimately recoverable oil or gas equivalent.〔Halbouty, M (2001). "(Giant Oil and Gas Fields of the Decade 1990-1999: An Introduction )." Retrieved Dec. 13, 2007.〕 Geoscientists believe these giants account for 40 percent of the world's petroleum reserves. They are clustered in 27 regions of the world, with the largest clusters in the Persian Gulf and Western Siberian Basin. The past three decades reflect declines in discoveries of giant fields.〔Mann, P., M. Horn, I. Cross. "(Emerging Trends from 69 Giant Oil and Gas Fields Discovered from 2000-2006 )." Presentation on April 2, 2007, at the 2007 Annual Meeting of the American Association of Petroleum Geologists in Long Beach, California. Retrieved Dec. 13, 2007.〕 The years 2000–2011 reflect an upturn in discoveries and appears on track to be the third best decade for discovery of giant oil and gas fields in the 150-year history of modern oil and gas exploration.〔Brown, David. "World Fields Study Shows Trends: Giants Like Stable Environments." AAPG Explorer (March 2007): 36-38, 51.〕 According to analysis led by Paul Mann of the University of Houston, almost all of the 932 giant oil and gas fields cluster within 27 regions, or about 30 percent of Earth's land surface. Since 2003, Mann and colleagues M.K. Horn and Ian Cross have tracked the giants on a map that highlights the tectonic and sedimentary basin maps of the 27 key regions.〔Airhart, M. and P. Mann (2007). "(Location, Location, Location: Mapping the World’s Oil & Gas Giants )." Retrieved Jan. 12, 2015. IHS Energy Group provided pre-1980 data for the map, which is an ongoing collaborative effort between Mann, M.K. Horn, and IHS Vice President Ian Cross.〕 Recent work in tracking giant oil and gas fields follows the earlier efforts of the late exploration geologist Michel T. Halbouty, who tracked trends in giant discoveries from the 1960s to 2004. == Tectonic settings == Geophysicists and exploration geologists who look for oil and gas fields classify the subsurface characteristics, or tectonic setting, of geological structures that contain hydrocarbons. Any one oil and gas field may reflect influences from multiple geological periods and events, but geoscientists often attempt to characterize a field based on the dominant geological event that influenced the structure's ability to trap and contain oil and gas in recoverable quantities. A majority of the world's giant oil and gas fields exist in two characteristic tectonic settings—passive margin and rift environments. Passive margins are found along the edges of major ocean basins, such as the Atlantic coast of Brazil where oil and gas has been located in large quantities in the Campos basin. Rifts are oceanic ridges formed when tectonic plates separate and a new crust is created. The North Sea is an example of a rift setting associated with prodigious hydrocarbon reserves.〔Mann, P., Gahagan, L., and Gordon, M. "Tectonic Setting of the World's Giant Oil Fields." ''World Oil'' 222.10 (October 2001): 78-79.〕 Geoscientists theorize that both zones are especially conducive to forming giant oil and gas fields when they are distant from active tectonic areas. Stability appears to be conducive to trapping and retaining hydrocarbons under the subsurface.〔Brown: 38.〕 Four other common tectonic settings, including collisional margins, strike-slip margins, and subduction margins, are associated with the formation of giant oil and gas fields, though not to the dominant extent of passive margin and rift settings.〔Mann, P. (Tectonic Setting of the World's Giant Oil & Gas Fields ). Retrieved Dec. 13, 2007.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Giant oil and gas fields」の詳細全文を読む スポンサード リンク
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