Kashirskian sequence deposits of the Moscow stage are promising for replenishing the resource base of the Volga-Ural oil and gas province in conditions of depletion of the basic horizons oil reserves. In the Kashirskian sequence, 8 elementary cyclites are distinguished. These cycites have a similar lithological structure and contain signs subaeral erosion at the top. The cyclites are composed of the following lithological types of rocks: clayey mudstones (1), spongolitic siliceous limestones and silicites (2), organic-rich laminated wackestones (3), bioclastic wackestones-packstones (4), bioturbated packstones (5), foraminiferal grainstones (6), polydetritic grainstones (7), laminated bioclastic packstones (8), microcrystalline massive dolomites (9), microcrystalline laminated dolomites (10) and microcrystalline patterned dolomites (11). The main reservoirs are represented by microcrystalline dolomites with high porosity and relatively low permeability and foraminiferal grainstones, similar in properties to terrigenous reservoirs. The C2ks4 reservoir in the lower part of the Kashirskian sequence is composed of foraminiferal grainstones. It is covered by the seal of clayey limestones lithotype (1). The C2ks1 reservoir layer in the top of the Kashirskian sequence is composed of microcrystalline dolomites. The seal includes of polydetritic grainstones and laminated bioclastic packstones with sulfate inclusions. The middle part of the Kashirskian sequence is characterized by a complex interbedding of dolomite and limestone reservoirs with different filtration-volumetric properties and low-permeability rocks (possible seals). The described structural features of the Kashirskian sequence must be taken into account when calculating reserves, as well as when designing horizontal wells and hydraulic fracturing.
References
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