The southwest border area of the North Caspian basin has been studied extensively by seismic, gravity, electrical, and geochemical surveys; yet, in spite of a high assessment of this area no commercial fields have been discovered. Of the wells that have been drilled, only a few have reached the Middle Carboniferous Bashkirian, and not one has passed through the entire carbonate section. It should be kept in mind that the largest discoveries in the North Caspian basin have been made in rocks of the Upper Devonian-Bashkirian section.
The Karasal monocline in the study area is expected to host pools associated with atoll-like bodies or shelf carbonates of Upper Devonian and Carboniferous age close to the deep-water zone on the northwest toward the center of the basin. Presence of large domal highs has been suggested. The main seismic reflecting horizons that characterize the Carboniferous and Devonian rocks on Karasal monocline are P-II in the Bashkirian and P-III in the Middle Devonian. Reflections from below P-II in the Upper Devonian-Bashkirian section are not parallel with the main reflecting horizons. This is attributed to fore- and back-reef flanks of carbonate buildups or to irregular surfaces of organic masses.
In spite of a paucity of data, structure and isopach maps have been compiled for the southern part of Karasal monocline, which disclose carbonate terraces similar to those present on the borders of the Kama-Kinel system of downwarps of the Volga-Ural oil gas province. See News Letter No. 11, September 20, 1999.
The Upper Devonian-Bashkirian carbonate complex rests on Devonian clastics. The top of these Devonian clastics in the south of the Karasal monocline dips monoclinally to the east and is cut by faults into large blocks. In the overlying carbonates, just as everywhere else in the border zones of the North Caspian basin, there are sedimenta- tional bodies associated with these blocks. They are not so clearly defined here because of partial compensation. Complete compensation of the relief on the Bashkirian carbonates was accomplished by the end of the Late Carboniferous.
The carbonate terraces of the Upper Devonian-Bashkirian complex are a newly recognized feature of the regional structure of the Karasal monocline and should be exploration targets for petroleum. The southern part of Karasal monocline is located close to the zone of junction of the North Caspian basin and Karpinskiy Ridge. Super-giant oil and gas fields - Tengiz, Orenburg, and Karachaganak - have been discovered in similar structural positions in the southeast and northeast parts of the North Caspian basin.
Regional studies along with continued exploration are recommended for the southern part of Karasal monocline. High-accuracy gravity studies are also recommended. This model of the geology of the sub- salt section of Karasal monocline is a basis for reassessment of the petroleum potential of the southwest part of the North Caspian basin. (Taken from Kal'nov and Obryadchikov, 1997; digested in Petroleum Geology, vol. 32, no. 1, four seismic sections, three structure maps, and one cross section.)
Copyright 2000 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 distribution list, please send your e-mail address to: jamesclarke@erols.com