A review of global geologic history during seven major periods between the Cambrian and the Holocene reveals the principal factors influencing the distribution of world hydrocarbon reserves.

The Paleozoic reserves, located mainly in North America and Europe, are mostly related to stable-platform environments. The Mesozoic and Tertiary reserves are found in more diverse tectonic environments, represented mainly by the Mesogean (Tethyan) realm, passive margins, and the western American mobile belts. The Mesogea (Tethys) formed a latitudinal feature separating Gondwana and Laurasia. The Mesogean realm contains 69% of the world oil and gas reserves, divided almost equally between the Gondwana and Laurasia areas. The voluminous reserves in the Mesogean realm reflect the history of repeated opening and closing of the Mesogea ocean that resulted in several phases of basin development. The comparative isolation of the basins from the global ocean circulation favored preservation of organic matter.

Because of diagenesis, the thermal history, and migration, the ultimate hydrocarbon reserves generally decrease as the ages of the reservoir and source rocks increase. Migration is usually directed toward the higher parts of the stratigraphic section. Concentrations of reserves within particular age ranges apparently reflect development of two major categories of basins: (1) marine platform basins, which were dominant in the Devonian, Jurassic, and Cretaceous Periods, and (2) rapidly subsiding basins, which were dominant in the late Tertiary and probably during the Late Carboniferous (Pennsylvanian).

The Middle East has an exceptional volume of sediments, prolific source rocks, and well-preserved hydrocarbons, and it is not likely that another hydrocarbon province of comparable importance will be found. Moreover, these Middle East reserves do not give any anomalous weight to the total reserves of the Gondwanian areas when compared to those in the Laurasian areas.

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