Impact of microfacies and diagenesis on the reservoir quality of Upper Devonian carbonates in Southeast Tatarstan, Volga-Ural Basin, Russia

Abstract

The results of integrated sedimentology, petrography, and petrophysical study of the Upper Devonian (Middle Famennian) Dankovo-Lebedyansky carbonates from Southeast Tatarstan of the Volga-Ural Basin revealed a variety of microfacies and diagenetic events that impacted the reservoir quality. Although our earlier study documented microfacies analysis and depositional environments, none of the studies focused on diagenesis, microfacies interaction, and their controls on the studied sediment's reservoir quality. Based on petrographic and microfacies analyses, the seven identified microfacies types are peloidal grainstone MF 1, cemented bioclastic peloidal grainstone MF 2, echinoderm-concentrated packstone MF 3, algae packstone MF 4, bioclastic wackestone MF 5, whole-fossil wackestone MF 6, and dolomite MF 7. For the investigated sediments, a gently deepening carbonate ramp depositional model with an inner, middle, and outer ramp setting is proposed. The observed diagenetic events in this study include micritization, calcite cementation (six cement types), dolomitization (six dolomite types), dissolution (fabric and non-fabric-selective dissolution), compaction, and microfracturing. The identified microfacies were classified into three distinct classes based on their petrophysical characteristics. MF 1 and MF 7 are microfacies types with the best reservoir quality. MF 3 and MF 4 are microfacies types of moderate reservoir quality. MF 2, MF 5, and MF 6 are microfacies types with poor or non-reservoir quality. Calcite cementation, micritization, and compaction are the primary diagenetic modifications responsible for porosity reduction. Moldic pores created by dissolution are a significant porosity-improving process. Porosity is locally enhanced by stylolite and microfractures. Dolomitization improved reservoir quality by creating intercrystalline and vuggy porosity. Understanding the impact of microfacies and diagenesis on reservoir quality is crucial for understanding reservoir properties in nearby fields with similar settings.