Geologicheskii Vestnik

This paper examines the lithofacies features of the Upper Frasnian (Late Devonian) Kolgan Formation in the southwestern part of the East Orenburg Arch based on detailed analysis of several well sections. The studied rocks exhibit an atypical carbonate-terrigenous composition for the Kolgan Formation, characterized by abundant marine fauna fragments and extensive bioturbation traces. A distinct suite of tempestite indicators has been identified within the succession.

Five principal lithotypes have been recognized and grouped into three lithological packages. The well sections demonstrate a gradual transition from terrigenous to carbonate-terrigenous dominated facies. Sediment accumulation occurred during marine tranWression under storm-influenced conditions, showing a facies shift from tidal flat to lower littoral environments.

The study reveals that existing lithofacies models for the Kolgan Formation fail to explain the high lithological variability and carbonate predominance observed in the southwestern EOA. This paper proposes a revised lithofacies model that accounts for the specific features of the carbonate-terrigenous deposits of the Kolgan Formation in the southwestern part of the East Orenburg Arch.

East Orenburg arch uplift, Fransky stage, Kalgan Formation, tempestites, shallow shelf, lithofacial model

• Aleksandrov V.M., Kazanskaya D.A., Belkina V.A. Features of the geological structure of tempestites in the Vikulov Formation deposits // Territoriya "Neftegaz". 2015. No. 2. P. 36–43. (In Russ.)
• Geological Structure and Petroleum Potential of Orenburg Oblast. Orenburg: Orenburgskoe knizhnoe izdatel'stvo, 1997. 272 p. (In Russ.)
• Gorozhanina E.N., Poberezhskii S.M., Gorozhanin V.M., Efimov A.G. Tectonic sedimentation model of the Upper Devonian Kolgan Formation in the western pericline of the Orenburg Arch (based on drilling data) // Aktual'nye voprosy litologii: materialy litologicheskogo soveshchaniya. Yekaterinburg: IGG UrO RAN, 2010. P. 82–84. (In Russ.)
• Zhemchugova V.A., Myachin O.M. Middle Devonian oil and gas reservoirs in the southern part of the Buzuluk Depression: structure features, formation conditions, petroleum potential // Vestnik Moskovskogo universiteta. Ser. 4. Geologiya. 2015. No. 6. P. 36–43. (In Russ.)
• Orenburg Tectonic Junction: Geological Structure and Petroleum Potential / Ed. by Yu.A. Volozh, V.S. Parasyna. Moscow: Nauchnyi mir, 2013. 259 p. (In Russ.)
• Kosmynin V.A., Kuz'min D.A. Lithofacies analysis and assessment of petroleum potential of the Kolgan Formation in the south of Orenburg Oblast // Regional'naya geologiya i metallogeniya. 2013. No. 56. P. 31–39. (In Russ.)
• Makarova S.P., Makarov G.V. Geological structure of Devonian deposits in the northern marginal zone of the Caspian Depression // Geologicheskoe obosnovanie poiskov skoplenii nefti i gaza v Prikaspii. Saratov: NVNIIGG, 1983. P. 99–104. (In Russ.)
• Marusin V.V. Trace fossils from the Vendian-Cambrian boundary deposits of the Olenek Uplift, Siberian Platform: Cand. Sci. (Geol.-Min.) Dissertation. Novosibirsk: [INGG SO RAN], 2016. 275 p. (In Russ.)
• Nikitin Yu.I., Rikhter O.V., Vilesov A.P., Makhmudova R.Kh. Structure and formation conditions of the Kolgan Formation in the south of Orenburg Oblast // Neftegazovaya geologiya. Teoriya i praktika. 2014. 9(2) P. 1–13. (In Russ.)
• Silant'ev V.V., Validov M.F., Miftakhutdinova D.N. et al. Sedimentation model of the Pashian Horizon (Devonian clastic succession) of the South-Tatar Arch, Volga-Ural Petroleum Province // Georesursy. 2022. 24(4) P. 12–39. (In Russ.)
• Staroverov V.N., Grebennikova A.V. Genetic significance of ichnofossils for lithofacial studies of Middle Devonian and Lower Carboniferous rocks in the southeastern Volga-Ural Anteclise // Nedra Povolzh'ya i Prikaspiya. 2018. No. 95. P. 3–24. (In Russ.)
•  
• Stratigraphic Scheme of Upper Devonian Deposits of the Volga-Ural Subregion. Explanatory note. Moscow: VNIGNI, 2016. 43 p. (In Russ.)
• Tugarova M.A., Zhukovskaya E.A. Atlas of Typical Facies / Sci. ed. B.V. Belozerov. Moscow; Izhevsk: Institut komp'yuternykh issledovanii, 2019. 142 p. (In Russ.)
• Yakhimovich G.D. Kolgan Petroleum Basin and the role of tectonics in its formation // Geologiya i razrabotka neftyanykh i gazovykh mestorozhdenii Orenburgskoi oblasti. Orenburg: Orenburgskoe knizhnoe izdatel'stvo, 1998. P. 72–76. (In Russ.)
• Droser M.L., Bottjer D.J. A semiquantitative field classification of ichnofabric // Journal of Sedimentary Research. 1986. 56(4) P. 558–559. https://doi.org/10.1306/212F89C2-2B24-11D7-8648000102C1865D
• Dunham R.J. Classification of carbonate rocks according to depositional texture // Classification of Carbonate Rocks: A Symposium / Ed. W.E. Ham. Tulsa: AAPG, 1962. P. 108–121. (AAPG Memoir; Vol. 1).
• Flügel E. Microfacies of Carbonate Rocks: Analysis, Interpretation and Application. 2nd ed. Berlin; Heidelberg: Springer, 2010. 984 p. https://doi.org/10.1007/978-3-642-03796-2
• Ovnatanova N.S., Kononova L.I. Frasnian conodonts from the eastern Russian Platform // Paleontological Journal. 2008. 42(10) P. 997–1166.