Based on geological data accumulated during geological exploration activities (GEA) for oil and gas, we can see the following trend. The share of non-productive small fields (sites) of a complicated structure is gradually increasing in the discoveries, up to the complete replacement in this process of large- and medium-sized fields (not to mention giant ones) with high-quality reservoirs. This trend has common features with the process of involving oil and gas fields in the development, where two varieties of this trend can be noted. On the one hand, there is a decrease in the share of giant, large and medium hydrocarbon fields. Accordingly, there is an increase in the share of developing small fields, the oil-bearing capacity of which is concentrated in layers of a complex structure, which are often disjunctively disturbed and have low filtration properties. On the other hand, when developing mature fields, the role of poorly developed reservoirs with low filtration properties is growing. In oilfield practice and science, such fields and reservoirs with degraded geological and physical parameters are called sites with hard-to-recover reserves. New sites with hard-to-recover reserves found out at the late GEA period are discovered with increasing methodological difficulties, with a high degree of risk and, for this reason, they should be described as hard-to-
discover reserves. Their geological criteria are consistent with those of hard-to-recover reserves sites; however, in the GEA practice, for the sake of convenience, it is reasonable to classify them as hard-to-discover. Upon introducing this concept, the classification of oil and gas fields according to the reserve quality criterion will have a complete structure.
References
1. Khalimov E.M., Klassifikatsiya trudnoizvlekaemykh zapasov nefti (Classification of hard-to-recover oil reserves), In: Geotekhnologii razvedki i razrabotki neftyanykh mestorozhdeniy (Geotechnologies for exploration and development of oil fields), Moscow: Publ. of IGiRGI, 2001, pp. 119–24.
2. Khalimov E.M., Krylov N.A., Baturin Yu.N., Azamatov V.I., Structure and qualitative characteristics of oil resources in Western Siberia (In Russ.), Geologiya nefti i gaza, 1993, no. 9, pp. 4–9.
3. Ovanesov G.P., Formirovanie zalezhey nefti i gaza v Bashkirii, ikh klassifikatsiya i metody poiskov (Formation of oil and gas deposits in Bashkiria, its classification and search methods), Moscow: Gostoptekhizdat Publ., 1962, 296 p.
4. Mirchink M.F., Khachatryan R.O., Gromeka V.I., Tektonika i zony neftenakopleniya Kamsko-Kinel’skoy sistemy progibov (Tectonics and oil accumulation zones of the Kama-Kinel trough system), Moscow: Nauka Publ., 1965, 214 p.
5. Mirchink M.F., Mkrtchyan O.M., Khat’yanov F.I. et al., Rify Uralo-Povolzh’ya, ikh rol’ v razmeshchenii zalezhey nefti i gaza i metodika poiska (Reefs of the Ural-Volga region, its role in the location of oil and gas deposits and search methods), Moscow: Nedra Publ., 1974, 152 p.
6. Egorova N.P., Khalimov E.M., Ozolin B.V. et al., Zakonomernosti razmeshcheniya i usloviya formirovaniya zalezhey nefti i gaza Volgo-Ural’skoy oblasti (Regularities of location and conditions for the formation of oil and gas deposits in the Volga-Urals region), Part 4. Bashkirskaya ASSR (The Bashkir ASSR), Moscow: Nedra Publ., 1975, 233 p.
7. Afanas’ev V.S., Yunusov N.K., Bulgakov R.B. et al., Kartirovanie devonskikh grabenoobraznykh progibov, gorstovidnykh struktur i svyazannykh s nimi neftegazoperspektivnykh ob»ektov Bashkirii (Mapping of Devonian graben-like troughs, horst-like structures and associated oil and gas prospects in Bashkiria), In: Prognozirovanie geologicheskogo razreza i poiski slozhno ekranirovannykh lovushek (Prediction of the geological section and the search for complexly screened traps), Moscow: Nauka Publ., 1986, pp. 39–45.
8. Lisovskiy N.N., Afanas’ev V.S., Lozin E.V., Nadezhkin A.D., Features of prospecting and exploration work in old oil-producing areas (In Russ.), Geologiya nefti i gaza, 1985, no. 9, pp. 1–6.
9. Lozin E.V., Masagutov R.Kh., Khamzin A.Z., O poiskakh i razvedke rukavoobraznykh i drugikh strukturno-litologicheskikh i litologicheskikh zalezhey v Bashkirii (On prospecting and exploration of sleeve-shaped and other structural-lithological and lithological deposits in Bashkiria), In: Metodika poiskov i razvedki neftegazonosnykh ob»ektov netraditsionnogo tipa (Methodology for prospecting and exploration of oil and gas objects of an unconventional type): edited by Aleksin A.G., Moscow: Nauka Publ., 1990, pp. 110–117.
10. Krasnevskiy Yu.S., Lozin E.V., A new type of oil deposits: annular, encircling reef body (In Russ.), Oil&Gas Journal Russia, 2015, no. 1–2, pp. 38–42.
11. Lozin E.V., Geologiya i neftenosnost’ Bashkortostana (Geology and oil content of Bashkortostan), Ufa: Publ. of BashNIPIneft’, 2015, 704 p.
12. Lozin E.V., Oil and gas perspectives of Bashkir (Southern) Urals in comparison with Appalachians and Rocky Mountains in North America (In Russ.), Neftyanoe khozyaystvo = Oil Industry, 2019, no. 2, pp. 8–12, DOI: 10.24887/0028-2448-2019-2-8-12
13 Khalimov E.M, Fursov A.Ya., Experience in exploration of small oil fields (In Russ.), Geologiya nefti i gaza, 1987, no. 11, pp. 40–46.
14. Slovar’ po geologii nefti i gaza (Dictionary of
