Giant oil and gas fields

The world's 932 giant oil and gas fields are considered those with 500 million barrels (79,000,000 m3) of ultimately recoverable oil or gas equivalent. 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. The years 2000–11 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. 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. 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. 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. 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. 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.