The North Caspian depression, which covers an area of 600,000 sq km on the southeast margin of the East European craton, is an oil-gas province, Sedimentary thickness in the central part of the basin according to geophysical data is 18-22 km. The surface of the Permian sub-salt sediments is at 1-10 km depth.
Three stages are recognized in the development of the basin before deposition of the Permian salt: 1) aulacogen (Riphean-Vendian, possibly Early Paleozoic also); 2) epi- continental (pre-Middle Devonian) when platformal clastics were deposited; and 3) deep-water (Middle Devonian-Artinskian) when the basin esperienced repeated subsidence.
The composition and structure of the sub-salt sediments attest to the Early Permian basin being a deep-water feature. On the extreme west and north a border step had a difference in elevation of 1000-1500 m. On the east and southeast clastics from the rising Ural fold system spread into the basin so that the increase in depth was gradual. Basin depth in the central part was several kilometers.
On the northeast the water depth in the Aksay area next to the Karachaganak reef was not less than 1200-1300 m. On the south of this reef the depth was 1500-1600 m. This reef is 9 by 18 km and has an amplitude of more than 1600 m. It is host to a giant gas-condensate field.
Large structures composed of Carboniferous and Lower Permian reefs formed along the north and west border steps of the depression. On the east and southeast they have a Carboniferous age; during the Early Permian conditions were not conducive to reef formation because of the debris apron shed from the Urals.
During Kungurian time of the Early Permian the climate became arid, and intense salt deposition began. In a comparatively short time the deep-water basin was compensated by salt deposition. Stages of downwarping were accompanied by anhydrite deposition due to drop in salinity as a result of introduction of fresher water from the Cis-Ural foredeep, which at that time connected the North Caspian basin with open ocean on the north of the Pechora depression. Potassium-magnesium salts were deposited under shallow-water conditions at the peak of salt deposition. The Kungurian was thus a fourth stage in the development of the North Caspian depression.
The Lower Permian salt-bearing sediments are subdivided into the Filippov (several hundred meters thick) and Iren (several kilometers thick) Horizons. The lower, Filippov Horizon in the west and north parts of the basin consists of carbonate-sulfate rocks and on the east and southeast contains a large amount of clsstic material. The upper, Iren Horizon is eveywhere largely salt. It contains also beds of anhydrite, potassium- magnesium salts, and clastics.
The Lower Permian section below the salt is predominantly carbonate deposited under marine conditions with normal salinity. On the east and southeast these carbonates are "overwhelmed" by clastic material from the Urals.
Modern salt deposition differs in scale from that of the ancient salt basins. In the geologic past the basins were broad marine features, whereas now they are continental and water depths are but a few tens of meters. Th greatest depth of present salt basins is 400 meters in the Dad Sea, and the greatest area is the 12,000 sq km of the Kara Bogaz Gol.
Taken from Komissarova, 1986, two cross sections and
one map. Digested in Petroleum Geology, Vol. 23, no. 7/8.
Copyright 2003 James Clarke. You are encouraged to print
out this News Letter and to forward it to others. Earlier News
Letters are available at: http://Geocities.com/internetgeology/
This News Letter is distributed without charge in the interest
of our science of petroleum geology. To be added to the mailing
list please send your e-mail address to Jamesclarke@erols.com
For information of the journal Petroleum Geology please FAX
703 759-3754 or telephone 703 759-4487.
Doubt is the vestibule which all must pass before they can
enter the temple of wisdom. Colton