Oil-Bearing Basins on Eastern Edge of the Russian Platform1 So-Called Second Baku or Ural-Volga Region
Approximately 600,000 barrels of oil daily, or about one half of all Soviet oil production, at present comes from the so-called Second Baku or Ural-Volga region.4 This new oil-bearing province lies along the eastern edge of the Russian platform adjacent to the Ural mobile belt. Oil occurrence in the Second Baku is related to structural development along the eastern portion of the Russian platform, and is controlled primarily by movements of separated blocks of the crystalline basement. These movements occurred during Caledonian, Hercynian, and Alpine time, and they resulted in the formation of arches and depressions in the platform.
In this eastern portion of the Russian platform, the following oil-bearing basins may be distinguished—Ural pre-mountain trough of Hercynian age, Ural-Volga arch basin, and the pre-Caspian depression.
Along the Ural pre-mountain trough, well-developed reefs of early Permian age are the principal oil-bearing reservoirs. The reefs were formed along the eastern edge of the platform facing the Ural trough. They developed on transverse uplifts along the faulted edges of grabens in the transitional zone between the platform and the pre-mountain trough. The Ural pre-mountain trough developed in late Hercynian time by "capture" of the Caledonian and early Hercynian platforms. Evidence of this is the existence of platform oil-bearing facies in the lower and middle Paleozoic deposits in the present Ural trough belt of Hercynian or late Paleozoic age.
In the oil-bearing basin of the Ural-Volga arch the oil occurs in the Devonian, Carboniferous, and Permian. The Devonian oil-bearing series in some places rests on the crystalline basement. In other places it is underlain by deposits of the lower Paleozoic. Volcanism of the platform type developed locally during pre-Devonian and Devonian time.
In the pre-Caspian oil-bearing depression which occupies the southeastern corner of the Russian platform there are more than 1,500 salt domes. The salt is of lower Permian age. Devonian salt may be present in the western portion of the pre-Caspian depression. A mobile belt of Caledonian and Hercynian time circles the southeastern edge of the Russian platform, connecting the Caledonian and Hercynian structure of the Urals and Caucasus. This belt lies buried beneath the Caspian depression and may have had considerable influence on oil occurrence in the depression.
Study and analysis of the structure of the eastern edge of the Russian platform show that there was movement of the edges opposite the mobile belt. Sharp-angle wedgelike projections of the platform edge were extremely mobile. At the same time, they preserve their platform-like character. The Ural-Caucasus Paleozoic mobile belt encircles the southeastern edge of the Russian platform like a garland.
Many examples of such platform edges facing the deep trough of the mobile belt might be cited from all parts of the world. Some of these platform edges or "hingebelts" and their garlands of mobile belts are richly petroliferous.
The Second Baku region, discovered about 25 years ago, is a comparatively new petroliferous province of the Soviet Union. Unfortunately, only a general regional geologic picture can be presented in this paper, as no specific data on production and reserves are available outside the Soviet Union.