Abstract: Geology and Environmental Factors Affecting Giant Fields

By

Michel T. Halbouty, Robert H. Dott, Sr., and A. A. Meyerhoff

At least 187 giant oil fields and 79 giant gas fields are known in the world today. Giant fields are those that contain 500 million barrels or more of recoverable oil, or 3.5 Tcf or more of recoverable gas, or an equivalent combination of gas and liquids which has a calorific value equivalent to 500 million barrels of oil. Altogether these fields contain an estimated, minimum ultimate recoverable reserve of 638.77 billion barrels of oil and 1,180 Tcf of gas or approximately 30-40% of the total known world recoverable gas. Most giant fields (190=71%) are in the Eastern Hemisphere; only 76 (29%) are in the Western Hemisphere; 81% of those in the Eastern Hemisphere (58% of the world's total) are in a U-shaped belt 6500 miles long and 475 to 800 miles wide that extends from Algeria to the Arctic Ocean at the longitude of the Polar Urals.

Giant accumulations show a distinct preference for certain geologic environments. Platform, semi platform (parageosyncline), and platform-margin areas contain 83% (211 fields) of all giants; only 17% (45 fields) are in other geologic environments (e.g., fold belts, actively subsiding grabens, etc.). The numerous giant fields in platform-related areas suggests that giant fields are more likely to be preserved in tectonically stable environments.

Of the reserves in giant oil fields, 58% are in sandstone and 42% are in carbonate reservoirs (an unusually large percentage of carbonate reservoirs are in the Middle East); of the reserves in giant gas fields 75% are in sandstone and only 25% in carbonate reservoirs. A total of 29% of the oil and 10% of the gas are in Tertiary strata; 63% of the oil and 65% of the gas are in Mesozoic beds; and 8% of the oil and 25% of the gas are in Paleozoic reservoirs. The abrupt increase in the number of giant fields in Mississippian and younger beds very possibly reflects the sudden proliferation during Late Devonian and Mississippian times of plant life in the terrestrial and, particularly, in marine environments. However, giant accumulations are not restricted to marine sediments. Of the 266 giant fields 6% (5 oil fields, 10 gas fields) are in rocks of nonmarine origin and 15% contain major oil and/or gas reserves of probable nonmarine origin.

Giant hydrocarbon accumulations require (1) abundant organic source materials; (2) depositional and post depositional environments suitable for accumulating, preserving, and converting the organic materials into mobile hydrocarbons; (3) efficient carrier beds; (4) voluminous and/or high-quality reservoir rocks; and (5) a giant trap, ideally syndepositional. Unconformities, though important in some fields, are not important in most. Geothermal gradient can be of importance in controlling the types of hydrocarbons present and their degree of mobility during migration.

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