Epigenetic processes in the Archean-Proterozoic and Devonian terrigenous rocks of the eastern Volga-Kama anteclise

UDK: 553.98

DOI: 10.24887/0028-2448-2026-7-77-81

Key words: rock, epigenesis, mineral, porosity, permeability, reservoir, oil

Authors: R.H. Masagutov (Ufa State Petroleum Technological University, RF, Ufa; Academy of Sciences of the Republic of Bashkortostan, RF, Ufa); A.M. Nigmatzyanova (Branch of RN-GRD LLC in the city of Ufa – BashNIPIneft, RF, UFA); D.U. Komilov (Branch of RN-GRD LLC in the city of Ufa – BashNIPIneft, RF, UFA); A.A. Nikolaev (Branch of RN-GRD LLC in the city of Ufa – BashNIPIneft, RF, UFA); E.N. Savelyeva (Institute of Geology, UFA Federal Research Centre of the RAS, RF, UFA); T.V. Burikova (Ufa State Petroleum Technological University, RF, Ufa; Branch of RN-GRD LLC in the city of Ufa – BashNIPIneft, RF, UFA); V.I. Shavaliev (Branch of RN-GRD LLC in the city of Ufa – BashNIPIneft, RF, UFA)

The article presents the results of a study conducted using modern instrumental methods to examine changes in the properties of both newly formed and syngenetic minerals composing siltstone-sand terrigenous Devonian reservoirs. The study was performed in the eastern Volga-Kama anteclise, where the rocks were exposed to deep hydrothermal mineralized waters circulating through fractured zones. It was established that the resulting secondary minerals influence the reservoir properties of the productive formations. Among authigenic minerals, a positive effect was noted for microcrystalline quartz, which forms primarily on the faces of quartz grain fragments, as well as among the clay (pelitic) fraction of the rock. Syngenetic illite, which has a flaky morphology, typically impairs the filtration and capacity properties of silty-sandy rocks. However, when exposed to anoxic hydrothermal water containing iron and magnesium ions, it transforms into illite with acicular or ribbon-shaped crystals (epigenetic), characterized by higher porosity and permeability than the syngenetic type. The article also provides information on other epigenetically formed minerals. It is noted that a modified laboratory approach to studying epigenetic clays currently exists, enabling the accurate assessment of their quantity and type, which is necessary for constructing a correct petrophysical model and subsequent calculation of reservoir porosity, permeability, and oil saturation.

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DOI: https://doi.org/10.18599/grs.2021.3.19