Satellite photography is used to evaluate petroleum prospects of intermontane depressions of Kirgizia and Eastern Kazakstan. The main neotectonic features are recognized, particularly large lineaments, which correspond with deep faults.
Several systems of depressions are recognized: the South Tyan-Shan system, which includes the depressions of the south flank of the western part of the Tyan-Shan orogen; the Central Tyan-Shan system; and the East Kazakstan system, which includes the depressions on the flanks of the Dzhungar and Altay orogens and the Kazakh Shield.
The South Tyan-Shan system of depressions is characterized by considerable thickness of the sedimentary section. Deposition began with Permo-Triassic redbed molasse. Coal-bearing clastics were then deposited under humid climatic conditions during Early and Middle Jurassic time. In the Late Jurassic the climate again became dry, and some evaporites were deposited. The region was a marine basin during Cretaceous and Paleogene time. Then redbeds collected in Oligocene and Miocene time under arid conditions. Finally, great thicknesses of molasse accumulated during the Pliocene-Anthropogene in connection with the Alpine orogeny. Basins of this system are the Fergana, Afghan-Tadzhik, Alay, Cis-Kun Lun, and Kuchar.
The Central Tyan-Shan system of depressions includes the Talas, Vostochno-Chuy, Issyk-Kul, and Naryn depressions. These contain some Paleozoic sediments, remaining after extensive Permian and Cretaceous erosion. The Jurassic consists of coal-bearing conti- nental deposits; the Cretaceous, however, is practically absent, represented by a weathered zone. A peneplain existed until the end of the Oligocene. Then tectonic movements resulted in deposition of lake sediments followed by molasse in Pliocene-Quaternary time.
The East Kazakh system of depressions is related largely to zones of Hercynian tectogenesis. Upper Paleozoics were deposited in isolated depressions under humid climatic conditions. Middle Carbon- iferous-Permian sediments contain bituminous shales. Coal was deposited in the Triassic and Jurassic. The Cretaceous consists of variegated lake beds. The orogenic stage, beginning at the end of the Paleogene, was a time of deposition of Oligocene-Miocene continental-lake redbeds. Pliocene-Quaternary molasse is not thick in these depressions. The depessions of this system are the Zaysan, Alakol, Iliy, Yuzhno-Balkhash, and Severo-Dzhungar.
Oil and gas fields have been discovered in those depressions that have a large area, a thick sedimentary section, and coal-bearing or evaporite deposits. These are the Afghan-Tadzhik, Fergana, Dzhungar, Cis-Kun Lun, and Kuchar. Primary exploration areas are the Alay, Zaysan, and possibly the Alakol depressions.
Alay depression is located in the vicinity of other oil-gas-bearing depressions of the South Tyan-Shan system and has a complete sedimentary section beginning with Triassic-Jurassic sediments. The Cretaceous, Paleogene, and Neogene deposits are practically the same as in the Fergana and Afghan-Tadzhik depressions. The Alay depression is characterized by considerable faulting. The Trans-Alay Range abuts against it along an overthrust that has a displacement of up to 7000 m. Source beds are present in the Jurassic and Cretaceous-Paleogene sections. Thickness of these sediments is more than 5 km, thus providing an environment sufficient for maturation. A negative factor is hydrodynamic openness at depth. However, seismic surveys clearly show very complicated structure in the lower structural stage. The Alay stratigraphic test is planned for 5000 m depth.
In the Zaysan depression the thickness of the sedimentary section exceeds 5000 m. A lower structural stage consists of Carboniferous- Permian coal-bearing sediments and an upper composed of Triassic and Lower Jurassic coal-bearing sediments. These sediments are source beds across the border in China. A strati- graphic well to 5000 m is now (1986) being drilled on Sarybulak high in this depression.
Taken from Aksenov and others, 1986; digested in Petroleum
Geology,Vol. 23, No. 5/6, one tectonic map.
Copyright 2003 James Clarke. You are encouraged to print out this
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