Microfacies analysis and depositional environment of the Upper Devonian Dankovo-Lebedyansky sediments, Tatarstan, Volga-Ural Basin, Russia

The Upper Devonian, Middle Famennian Dankovo-Lebedyansky sediments in Southeast Tatarstan territory are a carbonate sequence composed of limestone and dolomite, which is important hydrocarbon reservoir units, therefore, it is necessary to conduct in-depth study on its microfacies and depositional environment. In this study, a multidisciplinary approach that combines core observation with thin section examination is used. The limestone contains abundant skeletal grains (echinoderms, foraminifera, algae, gastropods, and calcispheres), as well as non-skeletal grains (intraclasts and peloids). On the basis of detailed petrographic investigations, six sedimentary microfacies can be identified, including (i) peloidal grainstone (MF 1), (ii) cemented bioclastic peloidal grainstone (MF 2), (iii) echinoderm-concentrated packstone (MF 3), (iv) algae packstone (MF 4), (v) bioclastic wackestone (MF 5), (vi) whole-fossil wackestone (MF 6), as well as dolomite or dolostone as diagenetic facies (MF 7). Based on microfacies analysis, the Dankovo-Lebedyansky sediments were deposited in three distinct sedimentary facies belts (shoal, lagoon and open marine environment). In order to reflect dispositional energy condition, the microfacies were grouped into facies associations: (i) low-energy microfacies associations including MF 5 and MF 6, (ii) moderate energy microfacies associations including MF 1, MF 2, MF 3, and MF 4. The dolomite, or diagenetic facies (MF 7), is the result of slightly to extensively dolomitization of limestone in the Dankovo-Lebedyansky sediments. Most of frequent dolostone types are euhedral planar-e and subhedral planar-s. According to petrographic characteristic of the dolostone, a seepage reflux model can be used to explain the dolomitization process of Dankovo-Lebedyansky sediments.