Terrigenous Devonian deposits of the Volga-Ural oil and gas province are characterized by features of geological structure and oil content different from the overlying deposits of the Paleozoic sedimentary cover. By combining new and traditional types of laboratory research in the study of quartz sandy-siltstone reservoir rocks of the Devonian terrigenous strata, new data on the material composition and size of minerals contained in the pelitic fraction were obtained. In addition to clay and other most common minerals of this group, microcrystalline quartz and other non-radioactive minerals of other origins are found in certain lithological-facial and tectonic conditions. They are often indeterminable by standard field geophysical surveys. The percentage of the clay component increases when carrying out particle size distribution. Their account in the general balance of the pelitic fraction distorts the forecast of filtration-capacitive and other properties of reservoirs, pointing to a number of existing limitations in determining petrophysical criteria by laboratory methods. Quartz microcrystals have been identified in sections of the terrigenous Devonian for the first time. Their formation is presumably associated with dynamic movement and the subsequent epigenetic impact of mineralized and hydrothermal waters moving along tectonic faults. Thus, the mineralogical composition of the pelitic fraction, determined from granulometry data using the sieve method, has no relationship with the measured filtration and capacitance properties. In the case of post-sedimentary transformations of sandy reservoirs, in particular the Pashi horizon, methods for determining the granulometric and mineral composition, using X-ray phase analysis equipment and scanning electron microscopy are most effective.
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