Fergana oil-gas basin is largely in Uzbekistan; parts, however, are in Tadjikistan and Kirgizistan. This basin is on a lithospheric block that is part of the Turan micro-plate. It is bounded on the south by the Pamir micro-plate, on the northeast by the Chu-Sarysuy micro-plate, and on the northwest by the Syr-Dar'ya block of the Turan micro-plate.
Formation of the Fergana basin was predetermined during the Paleozoic by inter-plate movements of these micro-plates and blocks. In the Late Paleozoic with collision of micro-continents and island arcs, all of Central Asia was combined into a continental block (Central and Southern Tyan-Shans) that docked onto the Kazak continent. In the south of Fergana basin a passive margin then formed where oil-source beds were deposited.
The Mesozoic geodynamic evolution of Fergana basin was char- acterized by intra-plate movements as rifts in the Early-Middle Jurassic and supra-rift depressions in Late Jurassic-Eocene time. In Oligocene- Anthropogene time as a result of collision of the Pamir with the Amu- Dar'ya plate, Paleozoic-Mesozoic mountainous terranes were thrust onto the sedimentary pile within the basin. The basin is thus a graben bounded on north and south by overthrusts.
The South Fergana overthrust zone is an east-northeast-trending entity. A large volume of seismic surveys during the last four decades has disclosed a complex character of this zone, particularly its south- west part. The thrust planes are complicated by numerous normal and wrench faults. These faults are accompanied by anticlines in the allochthon. For example, in Severnyy Rishtan field on the southwest border area of the basin the drill penetrated Jurassic and Cretaceous sediments beneath Paleozoic rocks. The plane of the overthrust there is almost horizontal. Similar relationships are found on several profiles across this South Fergana overthrust zone.
The North Fergana zone of overthrusts extends from southwest to northeast and corresponds with the North Fergana deep fault. Numerous seismic profiles have determined up to 17 km of overlap of the border of the depression by masses of Paleozoic folded terrane relared to overthrusting on this North Fergana fault. Some geophysical studies indicate that the horizontal component of this overthrusting is as much as 25 km. Such a large displacement of the allochthon was probably facilitated by plastic salt-bearing sedments in the Paleogene section.
Study of the structure beneath the overthrusts of the North Fergana and South Fergana zones has increased the area regarded as favorable for discovery of oil and gas. Thermodynamic environments in these rocks have been favorable for maturation of their disseminated organic matter. These Mesozoic-Cenozoic sediments that are overridden by Paleozoics are of great interest in the search for traps for oil and gas. (Taken from Peshkova, 1990; digested in Petroleum Geology, vol. 32, no. 4, 1998, one map and five cross sections)
Copyright 2001 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Ealier News Letters are available at our web page: http://geocities.com/internetgeology/ This News Letter is distributed at no 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