Using an MST-05 NMR relaxometer, we determined the dependence of NMR parameters on the specific surface and resistivity of water-saturated shaly sandstone and siltstone core samples. The influence of the type and quantity of clay minerals was evaluated at residual water saturation, because the main contribution to the measured NMR signal in this case is made by clay content. Based on NMR relaxometry data, we obtained quantitative estimates of the specific surface with clay- and capillary-bound fluids. The surface relaxivity was estimated from thermal-desorption and NMR relaxometry data. The high degree of reliability of its values was confirmed by the agreement between the pore size distributions determined by NMR relaxometry and by capillarimetry and granulometry. We have established that the parameters of NMR spectra depend on the specific surface and resistivity, which are, in turn, a function of the surface properties of both clay and sandstone/siltstone particles.