The article presents an analytical solution to the direct problem of the method of spontaneous polarization of rocks in a well crossing an electrically inhomogeneous reservoir of limited thickness with a drilling mud penetration zone. The analytical solution of the problem by the employees of Schlumberger-Doll Research (M.R. Taherian, at all) for an impermeable reservoir in the absence of a penetration zone is a special case of the analytical solution of the direct problem of the spontaneous polarization (PS). The accuracy of the interpretation of the analytical and numerical solution of the inverse PS problem does not exceed the permissible accuracy of measuring the PS potential. The static potential of the ESP calculated on the basis of the analytical solution of the direct and inverse PS problem is used to construct the correlation dependence of the «core-GIS» static potential of the ESP on the volumetric humidity W, equal to the product of the porosity coefficient KP by the coefficient of residual water saturation KOV. An example of the interpretation of low-permeable, low-resistance oilsaturated reservoirs of Jurassic sediments for the operational assessment of reserves of the Yegansky, Martovsky and Zaozerny deposits of Western Siberia is given. The algorithm proposed in the article can be used for automated interpretation of the PS method and multi-probe induction logging in the corporate software of PJSC OC Rosneft.