GEOLOGY CITED IN UNEVEN AREAL, STRATIGRAPHIC OIL, GAS EXTENT

Sharply uneven areal and stratigraphic distribution of the world's oil and gas reserves is a fundamental fact of petroleum geology, not a result of incomplete knowledge, two authors write in a paper published in the December 1991 issue of the American Association of Petroleum Geologists Bulletin. Analysis of maps that show facies, structural forms, and oil and gas source rocks indicates that several primary factors controlled the areal distribution of source rocks their geochemical type,

Dec. 23, 1991

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Analysis of maps that show facies, structural forms, and oil and gas source rocks indicates that several primary factors controlled the areal distribution of source rocks their geochemical type, and their effectiveness.

The factors are geologic age, paleolatitude of the depositional areas, structural forms in which the deposition of source rocks occurred, and the evolution of biota, wrote H. Doug Klemme, GeoBasins Ltd., Bondville, Vt., and Gregory F. Ulmishek, U.S. Geological Survey, Denver.

The maturation time of these source rocks shows the young age of most of the discovered oil and gas. Almost 70% of the world's original reserves has been generated since Coniacian, and nearly 50% has been generated and trapped since Oligocene.

Petroleum was meant in this paper to exclude bitumen and other solid and semisolid derivatives, the authors noted.

Klemme and Ulmishek wrote that six stratigraphic intervals contain petroleum source rocks that have provided more than 90% of the world's discovered original reserves of oil and gas in barrels of oil equivalent. The six intervals represent one third of Phanerozoic time, the period during which abundant life appeared in the geologic record. The intervals are:

Silurian 9% of the world's reserves.

Upper Devonian-Tournaisian 8%.

Pennsylvanian-Lower Permian 8%.

Upper Jurassic 25%.

Middle Cretaceous 29%.

Oligocene-Miocene 12.5%.

"This uneven distribution of source rocks in time displays no obvious cyclicity and the factors that controlled the formation of source rocks vary from interval to interval," the paper said. Neither is the distribution mainly a function of uneven exploration maturity as previously thought.