Petroleum Potential of Fergana Intermontane Depression Internet Geology News Letter No. 34, February 28, 2000

The Fergana intermontane depression of the Tyan-Shan orogen of Central Asia measures 350 by 130 km, and the area within it that is favorable for petroleum discovery covers 34,100 sq km. The central part of the depression is characterized by block subsidence to depths of 6-7 km. The faults are upthrusts and overthrusts. Anticlines associated with these faults are traps for oil and gas. The sedimentary section of the depression ranges in age from Permo- Triassic to Quaternary. The Jurassic and a large part of the Cretaceous are continental deposits with some carbonates. The Turonian Stage of the Upper Cretaceous and the Paleogene are marine carbonate-clay deposits. The stratigraphic section is most complete in the central and east and least complete in the west, where the Jurassic, Cretaceous, and part of the Paleogene are missing. Discoveries have now been made along the entire section from Permo-Triassic to Quaternary and in 52 fields. Distribution of assessed undiscovered oil is 66.2 percent in the Paleogene and 22.9 percent in the Neogene. For gas, 47.0 percent is placed in the Cretaceous, and 22.3 percent in the Jurassic. About 12.4 percent of the oil of the Paleogene has been found, and 2.06 percent of the total gas assessed. The principal structural features of the Fergana depression are a North Overthrust Belt, Central Mega-Syncline, South Transition Belt, South Structural Step, and Maylisu-Karagunday High on the northeast. The North Overthrust Belt and the South Transition Belt are very interesting for exploration for anticlinal traps. Eighteen anticlines have been outlined in the South Transition Belt. Their maximum dimensions are 6 by 2.5 km, and depth to Paleogene ranges from 4100-4700 m on the west to 2500-4900 m on the east. Overthrusts on both the north and south borders of the depression have vertical displacements of 4-5 km and horizontal displacements of 5 km and more. Anticlines occur in both autochthons and allochthons. Closures are in the 150-250 m range. The boundary between the Central Mega-Syncline and the North Overthrust belt has now been drawn more precisely, and as a result the favorable area of the Central Mega-Syncline has been broadened. The area of maximum subsidence of the Paleogene at 7 km has been found to be to the north of where it had been placed previously. Folds of the Central Mega-Syncline and Maylisu-Karagunday High are of particular interest for oil and gas. Nine fields including Mingbulak have been discovered in the deeply subsided areas, and five fields on the Maylisu-Karagunday High. Pools are present in Neogene, Paleogene, and Cretaceous, and also in the Jurassic on Maylisu-Karagunday High. Most of the initial resources (47.7 percent) occur in the central part of Fergana depression. This area accounts for 33.4 percent of the Paleogene oil and 13.4 percent of the Neogene oil. The main exploration targets for the coming years are deep anticlines of the Central Mega-Syncline and traps in the South Transition Belt and North Overthrust Belt. In most regions of the world the large fields are discovered first, and then medium and smaller fields are found. In the Fergana depression most of the discoveries have been small, but there are possibilities for discovery of larger fields as suggested by recent discovery of Mingbulak and Severo-Karakchikum-Niyabek in the Central Mega-Syncline (See Abidov and others, 1992, digested in Petroleum Geology, vol. 29, no. 1/2, 1995; one map showing main structural features and two cross sections). See also News Letter No. 13. Copyright 2000 James Clarke. You are encouraged to download this News Letter and to forward it to others. Earlier News Letters available at: 1