Research subject. This article analyses specific features of secondary transformations of quartz grains under the influence of water-oil fluids on the example of the Tulsky and Bobrikovsky oil-saturated quartz sandstones of the Demkinskoye oil deposit in the south-east of Tatarstan.Materials and methods. The research was carried out using the methods of optical microscopy, x-ray diffraction and electron paramagnetic resonance (EPR).Results. On the example of the Tulsky and Bobrikovsky oil-saturated Visean quartz sandstones, we analysed the influence of wateroil fluids on rocks and features of the secondary transformations of quartz grains in the Demkinsky oil field in the southeast of Tatarstan Republic. In the oilcharge stage, the spatial redistribution of silica was realized. Quartz grains dissolved in the sole and middle part of the reservoir, precipitating in the form of chalcedony in the top part of the reservoir. The dissolution of quartz grains was promoted by micro defects concentrated on their periphery. The dissolution was triggered by both plastic deformations of minerals at the contacts and an increase in the alkalinity in the pore space up to pH = 9-10. Chalcedony aggregates were formed in the intergranular space of oil-saturated sandstones. Depending on the nucleation point of the siliceous substance and silica concentration in pore fluids, either single spherulitic or agate-type aggregates were formed. According to EPR, the studied chalcedony aggregates have a high content of paramagnetic E'-centres. This was caused by a deficiency of oxygen in the mineral-forming fluid during chalcedony precipitation.Conclusions. Chalcedony precipitation underwent in acidic conditions, which led to silica-type metasomatism in the top part of the reservoir followed by substitution of muscovite grains by morphological fibrous chalcedony. Metasomatism affected the grains of both detrital muscovite and muscovite present as inclusions inside quartz grains. This process indicates the manifestation of surface and bulk diffusion of silica, affecting all clastic components.

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The article discusses the structure of multilayer oil deposits of the Vereyian horizon in the territory of the Tatarstan Republic. It is shown that in the sections the reservoir rocks are different facies of limestones (mostly grainstone), sandstones and siltstones. The reservoir rocks were formed in the regressive stage of the development of the marine sedimentation basin. Oil deposits of the Vereyian horizon have experienced flooding processes with formation edgewater. The penetration of groundwater into reservoir rocks led to uneven oxidation of oils in oil-producing structures. With weak waterflooding, dolomite and pyrite were formed in the reservoir rocks, with strong waterflooding, anhydrite aggregates appear.

The article presents study results of terrigenous reservoirs of the Vereiskian horizon of Tatarstan. The sandstones and silty sandstones are composed by grains of quartz, feldspars, clay minerals and organic remains cemented with calcite. Rocks were formed under the conditions of marine paleobasin coastal part with normal salinity. The migrated aggressive solutions partially leached out the carbonate cement from sediment, realizing the reservoir-filtration potential inherent in rocks. The oil fluids that filled the pore space practically did not interact in any way with the mineral skeleton of terrigenous reservoirs at the chemical level. Thus, the influence of the rocks mineral component on the properties of oils is completely excluded. In oil-saturated reservoirs, from which good oil inflows are obtained, light fractions of hydrocarbons prevail, boiling away at temperatures of 200-320°C. In the reservoir rocks, from where the watered oils are obtained, there are medium and heavy fractions of hydrocarbons, boiling at temperatures of 320-450°C. In unproductive sandy reservoirs with immobile viscous oil, only heavy fractions of hydrocarbons, which boiling at temperatures above 450°C, prevail. Thus, the oil recovery of terrigenous reservoirs is determined not by the composition of host rocks, but by the degree of oils oxidation contained in them.