The influence of hydrothermal processes in the crystalline basement on the oil-bearing capacity of the sedimentary cover of the Volga-Ural region (Russia)

Abstract

© SGEM2017. All Rights Reserved. The Volga-Ural oil and gas province is one of the oldest oil-producing regions in Russia. The central position in its borders is occupied by Tatarstan, in the territory of which is explored the largest oil deposits of the region. Oil deposits belong to the Devonian sediments, and in a lesser degree to the Carboniferous and Permian deposits. Despite the high degree of research of the region, the origin of oil deposits still one of the most controversial issues. Most researchers consider that the source of oil is the organic-rich strata of the Devonian - domanikites, which mostly fill depressed areas on the surface of the crystalline basement. Their largest capacities are confined to the zone of the Melekess depression, the northern part of which is the most elevated and located on the territory of Tatarstan. However, the thickness of sedimentary rocks here mostly does not exceed 2 km. Based on the geothermal gradient, this depth is not enough to start the processes of oil formation. According to the data from field researches, conducted on oil wells, the maximum temperature difference at the absolute level of -1000 meters in the Volga region reaches 30°C. Temperatures range from 14.25°C to 48.20°C. The structure of the thermal field is closely related to the tectonic structure of the territory and is characterized by relatively low temperatures in the area of the North Tatar arch and their increase in the areas located within the South Tatar arch and the adjacent sections of the Melekess depression. Studies of the catagenetic maturity of organic matter in domanikites and the upper carbonaceous coal, carried out on the reflectivity of vitrinite, have showed that their organic matter is characterized by a greater degree of conversion than can be achieved at modern temperatures. Thus, it can be assumed that the formation of oil deposits in the region occurred as a result of the effect of heat fluxes of deep origin. In favor of this point of view says the result of a study of the crystalline basement of the Volga-Ural region. The crystalline basement on the Volga-Ural segment of the earth's crust is composed by high-metamorphosed rocks of the Archaean-Proterozoic age (up to 2-2.6 billion years) mainly mafic and high alumina composition. Based on the results of deep drilling, there are fixed in some parts of the crystalline basement traces of later hydrothermal activity. Such areas are noted within the Melekess depression, where in sedimentary cover located the thick strata of the domanikites. In the crystalline basement there are rocks broken by a rare net of open cracks with burgundy-red crust of iron oxides on the surface. Some of the cracks are filled with barite, which is a typical hydrothermal mineral. These results may be interesting in the connection with the oil content of the overlying sedimentary strata. Signs of hydrothermal activity may indicate increased heat flux, contributed the formation of oil in organic-rich (domanikoid) sediments and its migration to higher horizons (Carboniferous and Permian).