Oil and Gas Potential of the Pre-Caspian Depression

GABRIELIANTS, G. A., A. H. ZOLOTOV, E. S. VOTSALEBSKII, and B. A. SOLOVIEV, Ministry of Geology, Moscow, USSR

The Pre-Caspian depression is one of the largest basin of the Soviet Union measured by sedimentary volume. The area of basin is more than 500,000 km2, and the thickness of sedimentary mantle is 20 km in the central part of depression (Central-Pre-Caspian graben). About one-tenth of the initially recoverable hydrocarbons in the Soviet Union hook up with this basin. The sequence of sedimentary mantle of the depression is divided by salt massives of Kungarian age on subsalt (Riphean-Aptian) and supersalt (Upper Permian-Quaternary) structural-formational complexes. The first complex contain a large part (96%) of the initial recoverable hydrocarbons of the region; the remaining resources are in the supersalt complex.

Deposits from Middle Devonian to Aptian age are productive in the subsalt complex. More 30 fields of hydrocarbons (including giant fields: Astrakhan gas-condensate, Tengiz oil and gas-condensate, and Karachaganak oil and gas-condensate) have discovered in this deposits.

Complicated gas-fluid mixing are characteristic of deep subsalt deposits. Free gas and condensate, oil, and dissolved gas amalgamate in equal parts. The part fluid hydrocarbons compose more than 50% of the total oil and gas potential of the Pre-Caspian depression (geological resources). Concentration of hydrocarbon resources has been proceeding in the near-edge part of the depression in the oil and gas accumulation area of the subsalt complex, bound with tectonic sedimentary structures.

Most commercial oil and gas reserves have been found in the carbonate reservoirs, but development of capacious terrigenous reservoirs have been predicted for the subsalt complex. Terrigenous reservoirs have received exceptional development in the supersalt complex, but large oil and gas accumulation areas are absent there.

Discovered commercial oil fields (about 50) have, usually, small reservoirs in this complex.

On the whole, the Pre-Caspian depression is characterized by poorly known oil and gas potential. This permits us to predict discoveries of new, large hydrocarbon accumulations.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)