Fergana Basin - Uzbekistan, Kyrgyzstan, and Tadzhikistan

Internet Geology News Letter No. 13, October 4, 1999

The Fergana basin is an intermontane graben bordered by reverse faults and overthrusts. It is the westernmost of a group of similar features that formed to the front of the Indian plate when it docked against the Asian land mass during Tertiary time. The Fergana Valley is about 300 km long, 175 km wide, and has an area of some 38,000 sq km. At the mouth of the valley is the city of Khuzbad (formerly Leninabad), which was founded in 329 B. C. by Alexander. In the 13th Century the army of Genghis Khan is said to have wintered in the valley and collected a waxy substance to lubricate the wheels of its carts. The mine that supplied the uranium and radium ore for the Curies is located in this valley (Clarke, U.S.G.S. Open-File Report 84-513). Most of the Fergana Valley is in Uzbekistan; however, parts are within Kyrgyzstan and Tadzhikistan. The Uzbeks are an agricultural people and occupy the flat land, whereas the Kyrgyzians and Tadzhiks are shepherds and live in the hilly parts of the valley. Paleozoic rocks form the mountains surrounding the Fergana basin and are exposed through the sedimentary fill in places along its borders. These rocks are thick miogeosynclinal limestones, shales and phyllites, sandstones, and volcanics. Total measure thickness of the Paleozoic section is given at 10-15 km. I wonder if some of this thickness is due perhaps to repetition from overthrusting. I have seen these rocks along the southern margin of the basin, where they are Silurian phyllites. Granite plutons invaded this sedimentary pile during the Permian in the final phase of the Hercynian orogeny.

Following the Hercynian orogeny the region entered a platform stage. In some areas, however, sedimentary rocks of Late Permian and Early Triassic age form an intermediate complex between the miogeosynclinal rocks beneath and the platformal deposits above. The Middle and Late Triassic were times of emergence and erosion. The region then began to subside to become a Mesozoic-Cenozoic basin of deposition, a part of the Tethyan Sea. The basin was bordered on the north by the Kazak Plateau, which was the principle source area for sediment. The sea extended thence southward, interrupted by an island that coincides with the present Alay Mountains. These mountains gained their present high elevation only in the Late Tertiary with the Alpine orogeny. Some fifty small and medium fields had been discovered in the Fergana basin, when in 1992 Mingbulak well no. 5 blew out and caught fire. Uncontrolled flow continued for about two months at rates estimated at 35,000 to 150,000 barrels of oil per day. More than 2 million barrels were diked by emergency earthworks before the flow stopped naturally. This blowout was from a deeper Miocene sandstone reservoir in the central basin graben. Disclosure of this deeper play has been the basis for greatly increased optimism for the basin. A large number of pay zones are recognized in the Fergana basin: 3 in the Upper Permian-Triassic, 7 in the Jurassic, 16 in the Cretaceous, 9 in the Paleogene, and 2 in the Neogene. Most of the discovered oil is in the Tertiary section, whereas the gas reserves are in Cretaceous and Eocene-Paleocene reservoirs. Most of the fields of Fergana basin consist of multiple, stacked reservoirs. The basin has not been thoroughly explored, and small discoveries continue to be made along the basin flanks. Potential of the deep Tertiary plays was made obvious by the blowout at Mingbulak. Undiscovered oil in the basin is assessed at 3 billion barrels and gas at 3 tcf (Sanders, Long, and Clarke, 1995, Oil and gas resources of the Fergana basin: DOE/EIA-TR/0575, 147 p.) This EIA report has an extensive list of references. Copyright 1999 James Clarke. You are encouraged to print out this News Letter and to forward it to others.