Central North Caspian

Deep Subsurface of Central Part of North Caspian Depression

Internet Geology News Letter, No. 2, July 19, 1999

The North Caspian depression is largely onshore north of the Caspian Sea. It extends oven an area greater than 500,000 sq km. The section is divided by thick Lower Permian Kungurian salt into sub-salt and supra-salt parts (Gabrielyants and others, 1990, Regional Geology of oil-gas regions of USSR, in Russian).

Although geophysical study of the North Caspian depression has extended over several decades, there has been a paucity of information on the deep subsurface of its central part, where depth to the top of the salt is 10-12 km. The sub-salt section had been depicted as dipping gently and smoothly toward the central part of the depression. Common depth point seismic surveys in recent years, however, have disclosed quite a different picture. The sub-salt section is divided into a pre-platform mega-complex, which is pre-Late Devonian, and overlying platform mega-complex, which is Late Devonian to Early Permian in age. The structure on the tops of both these complexes turns out to be very complex - a sag complicated by faults, highs, and downwarps.

The Sarpin downwarp in the southwest part of the North Caspian depression extends some 250 km in a northeast direction and is 100 km wide. To its northeast is the Aralsor basement high, and then farther to the east in the central part of the depression is the Kushum arch. Eleven well-defined highs have been mapped on the top of the sub-salt section on the Kushum arch and other parts of the depression (Petroleum Geology, vol. 33, no. 3 p. 245 and Errata in vol 33, no. 4).

What is the petroleum potential of these rocks? Upper Devonian source beds such as the Domanik and older bituminous carbonates and shales are certainly present in the section. Clastic and carbonate reservoirs are available, and seals are regional. But would the oil be over-cooked? Maybe not. The thick salt is a good conductor and will have drained off much heat, thereby permitting liquid oil to survive at greater depths. But even if oil has not survived the high temperatures at depth, gas and gas condensate can be present in great amounts. Perhaps there is a herd of elephants down there.

A possible analog of the North Caspian depression is the current exploration play in the Gulf of Mexico, where high resolution seismic surveys have been important in recent major oil discoveries below the salt (Dr. James A. Peterson, personal communication).

If you make a computer search on "North Caspian", you should also search on the terms "Peri-Caspian", "Pri-Caspian", and "Pre-Caspian".