Seven major periods between the Cambrian and the Holocene; Paleozoic reserves (North America and Europe) are related to stable-platform environments; Mesozoic and Tertiary reserves are in the Mesogean (Tethyan) realm, passive margins, and the western American mobile belts. The Mesogean realm contains 69% of the world oil and gas reserves and reflectsthe history of repeated opening and closing of the Mesogea ocean that resulted in several phases of basin development. Comparative isolation favored preservation of organic matter. Hydrocarbon reserves generally decrease as the ages of the reservoir and source rocks increase. Concentrations of reserves within particular age ranges 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.--Modified journal abstract.