The article represents methodical approaches to the study of clay component conductivity during resistance measurements of core samples through multiple salinity and further evaluation of cation exchange capacity by log data interpretation using specific modeling of clayey sandstones. The objective of the paper is to increase the adequacy of oil and gas (water) saturation ratio for the purpose of reliable hydrocarbon reserves appraisal in sedimentary rocks containing shaly minerals within the company’s facilities. Low permeable reservoirs of Achimov formation carrying hard-to-recover hydrocarbon reserves are the main object of analysis. The deposits have typical clinoform structure, extremely low reservoir porosity and permeability, and heterogeneous oil saturation. Substantiation of oil–and-gas saturation ratio for such deposits is essentially complicated by quantitative evaluation of shaly minerals resistivity, because analysis based on standard approach (Archie-Dakhnov equation) most commonly leads to an underestimation of hydrocarbon saturation. The authors considered Russian and foreign approaches to evaluation of shaly minerals resistivity, demonstrated application of Ellansky advanced conductivity model and worldwide used Woxman-Smits equation. Results of petrophysical resistivity modeling for fully and partially saturated samples using standard technique and specific models of shaly sands show that theoretical conclusions are validated by core analysis data under thermobaric conditions with acceptable experiment accuracy. Effect of shale content on the apparent parameters of the degree of cementation and wettability according to the Archie – Dakhnov equation depends on formation water salinity. Observed variation between mean values of water saturation for different models is up to 9 %. This variation could be the reason of mismatch between expected oil inflow calculated by log data and drill stem test interpretation.
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