Results of the pioneering numerical physicochemical modeling of interactions in the water - rock system on the example of Oxfordian deposits in southern regions of the Ob-Irtysh interfluve (southeastern regions of Western Siberia) are presented.It has been established that, despite the groundwaters having low salinity (up to 46 g/dm3) of and their persistent interactions with rocks (during tens, or probably hundreds of millions of years), they practically have no equilibrium with primary (endogenous) minerals: albite, anortite, microcline.At the same time, groundwater is in equilibrium with micaceous minerals, Ca-, Na - and Mg-montmorillonites, kaolinite, Mg-chlorite. There is always a carbonate barrier in the way of groundwater attaining an equilibrium with primary aluminosilicate minerals. As a result of the detailed petrographic studies, the genetic series of authigenic minerals of the diagenetic and catagenetic phases were established in the studied silty-sandy rocks. Relict diagenetic minerals are represented by glauconite, pyrite, siderite, calcite of the first generation of precipitation, kaolinite, anatase and leucoxene. Catagenetic minerals are considerably dominated by calcite of the second and third generations of precipitation, quartz, kaolinite, whereas siderite, chlorite, albite, dolomite are developed in minor amounts. The results of petrographic studies have corroborated the thermodynamic calculations.