Relevance. At standard interpretation of field geophysics materials there are sometimes the problems of inconsistency of interpretation results to the results of reservoir tests. Productive oil and gas saturated intervals are interpreted as water-saturated and skipped, and this phenomenon is caused by formation of low resistivity reservoirs as a result of formation of surface electrical conductivity in the rock. In the absence of rock material, it is impossible to determine the fraction of surface electrical conductivity by the traditional method, which leads to erroneous calculation of residual water saturation. Aim. To determine an algorithm for calculating the specific electrical resistivity of the surface layer and water mixture (based on the data of geophysical well surveys) in permeable terrigenous rocks of oil-saturated low resistivity reservoirs associated with secondary geochemical transformation. Method. Development of a mathematical algorithm for calculating the specific electrical resistance of the surface layer and water mixture on the basis of the previously proposed formula of B.Yu. Wendelstein within the framework of the parallel connection model and the reduced specific electrical resistivity equation of the surface layer of epigenetically transformed sandstone. At constant salinity and interval temperature in the reduced equation of the surface layer, the parameter of relative charge concentration is a variable value. To calculate this parameter, the author has proposed a method of statistical-correlation interpretation of the materials of geophysical studies of wells. This method allows calculating the potassium content in the sandstone rock. After point-by-point determination of potassium content (C_K), a trend equation is established by negative-correlation power dependence of specific electrical resistivity (C_K), where three parameters are determined: degree value and values of maximum and minimum potassium concentrations. Based on these three values, the parameter of relative concentration of charges in the surface layer is calculated with the corresponding calculation of its specific electrical resistivity. The averaged (over the interval) value of the share of the surface layer in pores is determined from the data of logging diagrams. Results. On the basis of the obtained specific electrical resistivity equation of the mixture, theoretical curves were constructed, confirming the relevance of the proposed algorithm and validity of the results of its application. In low resistivity reservoirs of four formations of different wells the specific electrical resistivity of the mixture, incremented specific electrical resistivity were calculated and the residual water saturation and oil saturation coefficients corresponding to the results of reservoir tests were determined on the basis of the Dakhnov–Archaie equation. Conclusions. Comparisons of the results of determining the nature of reservoir saturation calculated on the basis of the proposed algorithm with empirical results of reservoir testing shown their full correspondence.