News Letter No. 18

PETROLEUM PROSPECTS OF BASHKORTOSTAN (BASHKIRIA)

Internet Geology News Letter No. 18, November 8, 1999

The first mention of oil in Bashkortostan was in the Sixties of the Eighteenth Century. The first drilling, however, was in 1901, but only in 1932 was the first commercial discovery made. The region became a leading oil producer, reaching a peak in 1967 with 335 MM barrels. Cumulative production in 1998 had exceeded 10.5 BBO. A total of 174 fields have been discovered, including 142 oil, 18 gas-oil, and 14 gas fields (K. V. Antonov, Mineral'nyye Resursy Rossii, 1999, no. 2).
Although active exploration in Bashkortostan spans some seventy years, new discoveries are anticipated, particularly in 1) the Cis-Ural foredeep and 2) the southeast flank of the East European craton. The stratigraphic section that is of interest for oil and gas consists of Devonian, Carboniferous, and Permian sedimentary rocks.
1) The Cis-Ural foredeep is on the eastern border of the East European craton and immediately west of the folded Urals. It consists of a system of depressions separated from one another by transverse highs and saddles. It is one of the largest such features on the Earth. It was initiated by closure of the Ural paleo-ocean and subsequent orogeny of the Ural fold system in the concluding stages of the Middle Hercynian tectonic cycle in the Late Carboniferous and Early Permian (Petroleum Geology, vol.31, no. 1, p. 41, 1996).
Two depressions are present in the Cis-Ural foredeep of Bashkortostan: the Belaya on the south and the Yuryuzano-Sylvin on the north, separated by the Karatau allochthon. A large number of common depth point seismic surveys run there in 1985-87 have been reinterpreted, confirming earlier predicted favorability for overthrust structures and confirming new ones in a belt that extends for tens of kilometers. The overthrusts flatten out with depth to subhorizontal.
Numerous overthrusts cutting Permian and Carboniferous rocks are present in the south of the Belaya depression. Individual tectonic slices stack one upon the other from the east. Anticlines along the thrust fronts are tens of kilometers in length and 1-4 km in width. Dislocations mapped at the surface on Devonian and Carboniferous rocks are bounded at depths of 3-4.5 km by an overthrust, below which is a new set of thrust slices. Anticlines in the frontal zones are prime exploration targets.
2) The southeast flank of the East European craton is cut by overthrusts that are traced hundreds of kilometers parallel to the Urals. Within the overthrust zones are numerous low-displacement faults that dip both east and west - wedge dislocations of compression. Almost all anticlines associated with these wedge dislocations contain oil pools.
The current thinking is that the Urals are a gigantic allochthon. Strong oil-gas shows have been recorded from sediments beneath thrust sheets in the Urals. In spite of encouraging results, exploration has been discontinued. This negative assessment was due to lack of understanding of the overthrust structure. Abundant exploration targets are recognized (Petroleum Geology, vol. 29, no. 9/10, p. 325, 1995).

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