Super-giant Romashkino oil field and other fields occur on a gigantic ring structure in the basement rocks. This feature, the Shentalinsko-Cheremshan structure, has a diameter of 250 km and 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 the crystalline basement. Positive and negative belts encircle the central part of the structure, where the intensive Cheremshan maximum is located. This inner maximum is cut by narrow minimums, which rarely reach the outer rings.
Development of the Shentalinsko-Cheremshan ring structure began with formation of an isometric arch in the Earth's crust due to introduction of mantle material. This sector is reflected in the gravity field by the strong Cheremshan maximum. The 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. Subsequently 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 lithofacies changes in sedimentation. The final stage of the Shentalinsko- Cheremshan arch was during Akchagyl time of the Pliocene, 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 individual blocks subsided step-wise, passing into the flank of the Melekess depression.
The paleo-arch that was present in the sedimentary cover at the site of the modern Shentalinsko-Cheremshan ring structure apparently was oil-bearing in the Devonian clastic sediments. Subsidence of the western part and separation of the Romashkino block led to break-up of this field. Most of the hydrocarbons became concentrated on the raised Romashkino and Aktashsko-Novo-Yelkhov blocks, which were separated from the western part of the structure by the Kuzaykin and Altunino-Shunak graben-like downwarps. The western part of the structure was broken up, and the oil deposits in the Devonian clastics were preserved only where seals were good. The rest of the oil migrated along faults into the higher parts of the section and was preserved in favorable traps in Carboniferous rocks or as bitumen deposits in Permian rocks.
Taken from Stepanov, Bogatov, and Dokuchayeva, 1981; digested in
Petroleum Geology, vol. 18, no. 12.
Copyright 2001 James Clarke. You are encouraged to print out this
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