Carbonates of Aleksinsky horizons in platform part of Bashkortostan: tectonics, petroleum potential

Positive and negative regional structures are identified with the use of paleostructural imaging. Among the negative structures, the Ust-Aisky-Goncharovsky Trough is established, which was running in north-north-eastern direction across the entire platform part of Bashkortostan, and in terms of tectonics, this trough divided the ancient structural plan into two zones - Western and Eastern. During the course of formation, the Western zone had the greatest influence on accumulation and distribution of carbonate and terrigenous reservoirs. Dolomite and magnesium limestone, which reservoirs in С1аl carb bed are associated with, owe their origin to the process of calcium ions substitution by magnesium ions in limestone mud minerals being formed in shallow-marine basin; in Middle Aleksinsky time, bottom of the basin was subject to erosion (up to complete disappearance) on a considerable part of platform Bashkortostan. These rocks are observed in the wells drilled on the South Tatrsky Arch, its eastern and south-eastern slopes, almost all over the territory of the Blagoveschensky Depression and the Birsky Saddle; they are replaced by tight rocks in the Verkhnekamsky, Bymsky-Kungursky, and Salmyshsky depressions, and also in the Urals Trough. The present-day and ancient tectonic features of the Aleksinsky horizon are different. Reversal of individual block movement in the subsequent stages of geological evolution caused the development of positive forms instead of negative and vice versa. The Aleksinsky Horizon is divided into three sequences; their sections begin with the same-type impermeable limestone used as marker beds. Oil accumulations discovered therein are confined to the structures draping the Upper Devonian organic buildups, and may also be found in the structures draping the Lower Carboniferous organic buildups. There are numerous fields in the area of organic buildup occurrence, the section of which has not been studied at all in terms of oil and gas content. These fields are classified as highly promising, and the remaining territories of the area - promising, and territories outside the area - unpromising