Knowledge of mineral composition is helpful in the interpretation of well-logging data, where the hydrocarbon saturation is expected as the final result. It is calculated from the electric resistivity, cation exchange capacity (CEC) and porosity. Porosity is calculated from the bulk density, which is measured directly in the well, and the matrix (mineralogical, grain) density, which can be calculated precisely only when the mineral composition is known. In the shale-sand formations, which are the subject of this study, the rock matrix can conduct electric current. The Waxman Smits formula is applied to calculate the water saturation of such rocks. This formula combines rock porosity, water saturation, water resistivity and the CEC with the total rock resistivity measured in the well. CEC is introduced to this formula as the parameter determining the shale ability to conduct electricity. Typically, CEC is controlled by the smectite content of the rock. In order to evaluate the input data needed in the log interpretation the extended mineralogical (quantitative mineral composition, CEC) and chemical (major, trace and REE) analyses were made for over 200 core samples obtained from four gas fields in the Miocene formation of the Carpathian Foredeep. The most important clay component: the sum of mixed-layer illite-smectite and discrete illite has been quantified. The detailed recognition of clay minerals allowed for constraining the density-neutron cross-plot, which is a well log data interpretation technique for the determination of both porosity and shale content in shale-sand formations. The statistical multivariate analysis of all data helped to set up a comprehensive petrophysical model. A reliable correlation (r2 = 0.96) of the thermal neutron absorption cross-section (Σama) and the total natural radioactivity (GR) with CEC was established. Such good correlation allows for a continuous on-line CEC determination and therefore a reliable application of the Waxman-Smits water saturation model. Experimental equations with similar level of correlation were established for quartz, clay and carbonate contents, allowing the calculation of matrix density, required in the porosity calculation. The high correlation of Sa mawith CEC reflects the presence of boron in the illite-smectite clays.