Structure, Rock Types, and Reservoir Properties of Altered Permian–Triassic Rhyodacites in Grabens of the Frolov Megadepression in Western Siberia

This article presents results of the comprehensive study (composition, reservoir properties, geophysical parameters, and age) of altered felsic volcanic rocks (rhyolites, dacites) associated with the Permian-Triassic intermediate structural level of the West Siberian. As a result of secondary alteration, oil and gas reservoirs can form in these volcanic rocks. Taking into account their occurrence depth (more than 2 km), they are available for study only from core and geophysical data. Based on the results of core studies, 25 petrographic varieties of felsic volcanics were established. The combination of such features as 1) structural and textural features, 2) degree of secondary alteration, and 3) peculiarities of logging curves made it possible to group these numerous petrographic varieties into six key rock types. At the same time, the distinguished rock types are characterized by different reservoir properties (porosity, permeability). All 443 samples representing various rock types are characterized by similar geochemical composition, and the U-Pb ages obtained by the zircons (254 ± 2–248.2 ± 1.3 Ma) indicate that the studied felsic volcanics belong to a single tectonic-magmatic stage at the Permian-Triassic boundary. According to seismic data, it was revealed that within the Frolov megadepression (the central part of Western Siberia), the studied volcanic rocks are distributed within grabens. In particular, the boundaries of a relatively large (70 km wide and 200 km long) graben-like Rogozhnikov-Nazym structure were adjusted, and several similar, but smaller structures were identified. A comprehensive analysis of core, log and seismic data made it possible to determine the morphology and spatial relationships of volcanic bodies composed of various types of felsic volcanic rocks, providing the basis for predicting intervals of the section with the best reservoir properties.