The Tatar arch is the dominant structural feature of the Volga-Ural oil-gas province. Its south crest is highly productive, whereas the north crest does not have significant resources. Associated with the south crest is a ring structure, the Shentalinsko-Cheremshan, which has a diameter of 250 km. This feature is expressed clearly in the gravity and magnetic fields in the form of a ring distribution of anomalies. The outer zone of this ring structure is a belt of gravity maximums. Within this outer belt is a belt of minimums. The transition area between the two is a zone of steep gradients that coincides with faults in crystalline basement. The positive and negative belts encircle the central part of the structure, where the strong Cheremshan gravity maximum is located. This inner maximum is cut by narrow minimums, which rarely reach the outer ring.
Development of the Shentalinsko-Cheremshan structure began with formation of an isometric arch in the Earth's crust due to the introduction of mantle material. This sector is reflected in the gravity field by the strong Cheremshan maximum. This arching led to development of radial and intersecting faults and to the formation along them of synclinal troughs and narrow grabens in Late Archean and Early Proterozoic time. Subsequent inversion (downwarping) of the arch took place with development of ring faults and the ring frame.
The ring frame experienced differential movements during all the Precambrian and Phanerozoic, accompanied by various litho-facies changes in sedimentation. The final stage of development of Shentalinsko-Cheremshan paleo-arch was during Pliocene (Akchagyl) time, when in the eastern part the Romashkino segment of the block separated and rose along the ancient Altunino-Shunak fault. This basement block is the core of the south crest of the Tatar arch. To the west was a step-wise subsidence of individual blocks, passing into the flank of Melekess depression.
The paleo-arch that was present at the site of the modern
Shentalinsko-Cheremshan ring structure apparently was
oil-bearing in the Devonian clastics. Subsidence of the
western part and separation of the Romashkino block led
to break up of this deposit. Most of the hydrocarbons
became concentrated on the raised Romashkino and
Novo-Yelkhov blocks, which were separated from the
western part of the structure by graben-like downwarps.
The western part of the structure was broken up, and the
oil pools in the Devonian were preserved only where seals
were good. The rest of the oil migrated along faults into
higher parts of the section to be preserved in traps in
Carboniferous rocks or as bitumen in Permian rocks.
(Taken from Stepanov, Bogatov, and Dokuchayeva, 1981;
digested in Petroleum Geology, Vol. 18, No. 12; three maps)
Copyright 2001 James Clarke. You are encouraged to
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