The Upper Devonian carbonate deposits of the Volga-Ural oil and gas province are characterized by a wide distribution of bioherms, that control a large number of oil deposits. Deposits in the Upper Devonian carbonate strata are unevenly distributed over the area and section and are confined mainly to the territories of vast paleoshelfs. The main risk factor in prospecting for hydrocarbon deposits is the quality of seals.
The article discusses the structural features of the Bashkir and South Tatar paleoshelfs at the Middle and Late Famennian stages of development, which determined the patterns of distribution of reservoirs and seals in the Upper Devonian carbonate strata. The South Tatar and Bashkir paleoshelfs differ from each other in their internal structure. The main oil deposits on the South Tatar arch are belong to the deposits of the middle and upper Famennian, and on the Bashkirian - to the deposits of the Lower Famennian. The Famennian seals are composed of clayey-carbonate rocks. Their accumulation is associated with the stages of maximum transgression, when the shelf area totally sank below the wave base, as well as with the stages of maximum regression, when the barrier zone limited water exchange in the shelf. At that time, organogenic carbonates without clay admixtures, with bad seal properties, accumulated in the hydrodynamically active side zone. The Visean claystones are regional seals in the study area. The seals identified in the carbonate strata of the Famennian Stage are classified as zonal and local. The facies heterogeneities of the Upper Devonian carbonate shelves revealed decades ago and supplemented by us make it possible to predict the distribution of seals and reservoirs over the area. The proposed technique can be applied in neighboring regions.
1. Mkrtchyan O.M., Verkhnedevonskie rify i ikh rol' v formirovanii neftenosnykh struktur vostoka Uralo-Povolzh'ya (Upper Devonian reefs and their role in the formation of oil-bearing structures in the east of the Ural-Volga region), Moscow: Nauka Publ., 1964, 119 p.
2. Mkrtchyan O.M., Khat'yanov F.I., Shalaginova F.P., Application of seismic exploration to prospect for oil-bearing structures associated with the Upper Devonian reefs (In Russ.), Geologiya nefti i gaza, 1965, no. 2, pp. 49–53.
3. Mkrtchyan O.M., Zakonomernosti razmeshcheniya strukturnykh form na vostoke Russkoy platformy (Regularities in the placement of structural forms in the east of the Russian platform), Moscow: Nauka Publ., 1980, 134 p.
4. Khat'yanov F.I., Geologo-geofizicheskie osobennosti pogrebennykh rifovykh massivov v svyazi s problemoy ikh poiskov (Geological and geophysical features of buried reef massifs in connection with the problem of their search), Collected papers “Iskopaemye rify i metodika ikh izucheniya” (Fossil reefs and methods of studying them), Sverdlovsk: Publ. of Ural Branch of AS USSR, 1968, pp. 226–247.
5. Yunusov M.A., Timergazin K.K., Zubik I.L. et al., Novye dannye o rifovoy prirode Chermasanskogo massiva Zapadnoy Bashkirii (New data on the reef nature of the Chermasan massif of Western Bashkiria), Proceedings of AS USSR, 1971, V. 198, no. 5, pp. 1163–1166.
6. Khachatryan R.O., Trokhova A.A., O rifogennykh karbonatnykh massivakh vostoka Russkoy platformy (On reefogenic carbonate massifs in the east of the Russian platform), Collected papers “Tektonika i razmeshchenie neftegazovykh mestorozhdeniy vostoka Russkoy platformy” (Tectonics and placement of oil and gas fields in the east of the Russian platform), Moscow: Nauka Publ., 1968, pp. 152–165.
7. Fortunatova N.K., Shvets-Teneta-Guriy A.G., Gumarov R.K., Vasil'ev I.V., Stroenie i perspektivy neftegazonosnosti devonskikh i kamennougol'nykh otlozheniy vostoka Tokmovskogo svoda (Respublika Chuvashiya) (The structure and prospects of oil and gas content of the Devonian and coal deposits in the east of the Tokmovsky arch (Republic of Chuvashia)), Collected papers “Maloizuchennye neftegazonosnye regiony i kompleksy Rossii (prognoz neftegazonosnosti i perspektivy osvoeniya)” (Little-studied oil and gas regions and complexes of Russia (forecast of oil and gas potential and development prospects)), Moscow: Publ. of VNIGNI, 2001, 113 p.
8. Grachevskiy M.M., Berlin Yu.M., Dubovskoy I.T., Ul'mishek G.F., Korrelyatsiya raznofatsial'nykh tolshch pri poiskakh nefti i gaza (Correlation of different facies strata in the search for oil and gas), Moscow: Nedra Publ., 1969, 299 p.
9. Filippov B.V., Tipy prirodnykh rezervuarov nefti i gaza (Types of natural oil and gas reservoirs), Leningrad: Nedra Publ., 1967, 123 p.
10. Khitrov A.M., Il'in V.D., Savinkin P.T., Vydelenie, kartirovanie i prognoz neftegazonosnosti lovushek v trekhchlennom rezervuare (Isolation, mapping and prediction of oil and gas content of traps in a three-member reservoir), Moscow: Publ. of Ministry of Natural Resources of the Russian Federation, Ministry of Energy of the Russian Federation, VNIGNI, 2002, 84 p.
11. Khitrov A.M., Nikitin A.N., Popova M.N., Kolokolova I.V., Risk assessment of oil and gas search based on selecting and mapping the hydrocarbon accumulation caps according to geophysical techniques data (In Russ.), Vestnik TsKR Rosnedra, 2011, no. 3, pp. 22–27.
12. Khitrov A.M., Hydrocarbons seals and subsoil resource potentia (In Russ.), Aktual'nye problemy nefti i gaza, 2013, no. 1 (7), pp. 7–10.
13. Shakirov V.A., Miropol'tsev K.F., Vilesov A.P. et al., Forecast of seal rocks areal extent in Upper Devonian carbonates in Orenburg region (In Russ.), Neftyanaya provintsiya, 2018, no. 4, pp. 133–153.
14. Chikina N.N., Nikitin Yu.I., Astaf'ev E.V., Vilesov A.P., Razrabotka kompleksa kriteriev dlya otsenki kachestva flyuidouporov v otlozheniyakh famensko-turneyskoy karbonatnoy tolshchi Orenburgskoy oblasti na osnove dannykh kerna i GIS (Development of a set of criteria for assessing the quality of seals in deposits of the Famennian-Tournaisian carbonate strata of the Orenburg region based on core and logging data), Proceedings of 19th Anniversary Scientific and Practical Conference on Geological Exploration and Development of Oil and Gas Fields "Geomodel 2018", pp. 1–6.