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There are the structural-material complexes of the rocks in the crystalline basement of the Volga-Ural oil and gas province, which characterized by different composition and structural position. The biggest structure, which occupies a central position in the region, is the Tatar arch. The Tatar arch is a large megablock of the lithosphere (granitegneiss core) bounded by mobile zones (graben-shaped deflections). They are associated with pre-metamorphic basic magmatism. The areas of the crust bounding and separating the granite-gneiss domes are zones of a repeated diaphtoresis process. In the Archean- Proterozoic, the rock complexes underwent a regional metamorphism of the granulite facies and at the regressive stage the amphibolite facies. Rocks of mobile belts were drilled by deep parametric wells. They are represented mainly by biotite-, amphibole-, pyroxene-containing gneisses, plagiogneisses, and rarely crystal shales. The layer of gneisses and plagiogneisses is heterogeneous in petrographic composition. It was probably formed by various rocks of predominantly basic composition. In the Archean period, they underwent a metamorphism of granulite facies. In the early Proterozoic, rocks underwent ultrametagenic process with partial granitization. Their modern morphology was formed as a result of subsequent diaphtoresis in the conditions of the amphibolite facies. The findings of relict pyroxene grains in amphibole and biotite crystals and relict phaneritic texture indicate that the original rocks were primary magmatogenic and sedimentary-volcanogenic rock complexes. These rock complexes formed from ancient ultrabasic-basic substrate. They are characterized by an appropriate composition of trace elements. Elevated concentrations of such elements as Cr, Ni, Zr, indicate the participation of ultrabasic rocks in the formation of the complex. Almost throughout the all section, the rocks are broken by a rare network of cracks, filled with red oxide material and black chlorite-containing clayey substance, which indicates the occurrence of hydrothermal processes. [ABSTRACT FROM AUTHOR]

The crystalline basement of the East European Platform is composed of metamorphic rocks of the Archean-Proterozoic age. They are interesting as products of fluidmagmatic differentiation of the primary crust, which led to the formation of the early continental crust. The Volga-Ural segment of the crystalline basement is located in the east of the platform. It is covered by a sedimentary cover up to 1.5-2.0 km and was opened by the deep oil drilling. Based on the study of the obtained materials it was established that the crystalline basement within the Volga-Ural segment is composed of metamorphic complexes of granulite and amphibolite facies. The rocks have different genesis and age. They were formed in environments with different tectonic regimes and metamorphosed. Granite-gneiss domes located at the central part of the arch uplifts. Their formation is associated with the processes of metamorphic and metasomatic processing of the primary crust of the basic-ultrabasic composition. Within the crystalline basement of the Tatar arch granite-gneiss complexes are composed of highly metamorphosed, partially migmatised mafic and high-alumina rocks. Mafic rocks are metamorphic altered associations of igneous rocks of predominantly basic composition (Otradnenskaya series). The high-alumina rocks are composed of metamorphosed primary sedimentary rocks (Bolshecheremshanskaya series). The original nature of these rock complexes causes debate. Sensitive indicators of geological processes are the minerals of the garnet group. Their formation in the rocks of the crystalline basement is associated with the anatectic melting of the original rocks. All of the investigated garnets belong to the almandine series, but they are characterized by variability of the chemical composition. The different chemical composition of almandines notes to the different chemical composition of the substrate from which the almandines crystallized. Thus, the composition of almandine reflects the chemical characteristics of the original rocks and can be used for the reconstruction of their primary genesis. The data of mineralogical studies are confirmed by the results of carbon isotope analysis. The carbon presents in the rocks in the composition of rock-forming minerals and graphite. The predominance of the heavy carbon isotope indicates the magmatogenic origin of the rocks of the Otradnenskaya series, and the light one notes to the biogenic nature of carbon in the rocks of the Bolshecheremshanskaya series. The development of the earth's crust in its early stages has a very complex history. Its reconstruction is a fundamental geological problem. [ABSTRACT FROM AUTHOR]

The crystalline basement of the Tatar arch located in the eastern part of the East European Platform (Russia) is composed of the metamorphic rocks of different composition and genesis. The Tatar arch consists of two blocks of the basement. The North Tatar and the South Tatar blocks are separated by a faults system. The South Tatar arch is a granite-gneiss core of the basement. It is composed of Archaean- Proterozoic metamorphic complexes of rocks (granite-gneisses, gneisses, migmatites and granulite-gneisses). The moving zones that separate them are composed of metabasite complexes. Metamorphic rocks of the crystalline basement of the Tatar arch are formed under the conditions of the granulite facies of a regional metamorphism. The associations of rocks are combined into structural and material complexes, the largest of which are the Otradnenskaya and the Bolshecheremshanskaya series. Otradnenskaya series is composed of rocks of mafic composition (metamorphosed volcanogenic, volcanic-sedimentary and intrusive formations), while the Bolshecheremshanskaya series is composed of the rocks of high-alumina composition. Many researchers identify them with metapelites. Granulite rock complexes are migmatized and granitized in various degrees. The rocks of the crystalline basement of the Volga-Ural oil and gas province have been drilled by great number of boreholes. The most profound is the 20009 Novo-Elkhovskaya well as deep as 5.5 km. One of the most important tasks is the stratigraphic dismemberment of the metamorphic complexes of the basement. Because of the absence of paleontological remains for their dismemberment, the material composition of the rock complexes can be used (mineralogical and geochemical features). In this connection, the radiation samples were estimated in terms of the content of natural radionuclides by emitting elements - 232Th, 40K, 226Ra with the determination of their activity (A, in Bq / kg). There are 5 packs of rocks in the section of Well 20009 Novo-Elkhovskaya correspond to the rocks of the high-alumina (Bolshecheremshanskaya series) and mafic (Otradnenskaya series) composition. Analysis of the distribution of natural radionuclides K, Th, Ra along the borehole section showed their close dependence on the mineralogical composition of the rocks. The highest values of radionuclide activity are associated with granitoids in zones of migmatization of high-alumina rocks. The concentrations of these radionuclides are related to minerals such as potassium feldspars, garnets, zircon and monazite. Thus, the distribution of K, Th, Ra in metamorphic rocks depends on their petrographic composition, and also clearly reflects the zones of migmatization. The regular distribution of radionuclides in the studied rocks can be used for the dismemberment and comparison of metamorphic rock complexes. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

Conducting of quantitative mineralogical analysis of metamorphic rocks penetrated by deep wells in the territory of the South-Tatar arch (East of the Russian Plate) provided an opportunity to examine and identify the leading formational types of rocks or metamorphic formations. According to the results of long-term mineralogical and petrographic research, followed by petrochemical analysis, the rocks have been grouped into two major formations. The first one - mafic-silicate formation is characterized by a predominance of mafic and mafic-silicic rock complexes. Ortho and clinopyroxene, amphibole and biotite are leading femic minerals. The second formation - high-alumina formation is characterized by a wide development of cordierite, sillimanite, garnet, biotite. The content of pyroxene, amphibole among the rocks of this formation is reduced dramatically. Leucocratic mineral component of these two formations is the same. However, the relation between minerals within the formation is different. Formation of buried weathering crust is located on the border of the sedimentary cover and the surface of crystalline basement. It was found that the material composition of the weathering crust is associated with mineralogical and petrographic features of leading metamorphic complexes. The mineral composition of weathered rocks, peculiarities of the development of finely dispersed and clay minerals in various areas of the crystalline basement are also variable and related to the original composition of the substrate in the studied territory. [ABSTRACT FROM AUTHOR]

Due to the prospects of hydrocarbons content of the deep horizons of the east of the Russian plate the study of the mineral-geochemical features of the rocks of the basement is actual. The great information is given by carrying out deep drilling and analysis of the core material of the bore holes, which revealed the buried weathering crusts on the border of the sedimentary cover and the basement. [ABSTRACT FROM AUTHOR]