In this paper, we present an application of the resistivity model based on a modified Archie's law (Glower et al., 2000), and accounting for a conductivity of clay material, to the Kotlin clayey formation (Vendian age, St. Petersburg, Russia). We compare theoretical prediction of the model with experimental data based on the well logging and laboratory measurements of the core porosity and granulometric composition. We found that the experimental data are in agreement with the model prediction for the case of salty formation water (about 35 g/l). This fact is in accordance with sea conditions occurring in the region in Quaternary period. Considering an abundance of sandy aquifers with fresh water in the formation, we believe that because of the salt diffusion, the formation water salinity in clayey sequences must be low in the vicinity of aquifers, and much larger in the middle of the sequences. Because the swelling pressure increases with decrease of the pore water salinity, clay with fresh water can be of high porosity. With this hypothesis we can explain an appearance of a seismic reflector at the depth of 60 m, which coincides with any lithological boundary, and which can be related to the high porosity zone.