The paper presents the results of a study of groundwater saturation degree in relation to the minerals of kimberlite breccia based on the physicochemical modeling of equilibrium in the brine-kimberlite system. Highly concentrated Ca Cl brines with salinity of up to 400 g/L were discovered through boreholes drilled in the deep horizons of the diamond-bearing deposit of the Udachnaya pipe (Western Yakutia, Russia). The sampling depth of kimberlite was 1034-1145 m and the depth of the water sample from the ore body was 869 m. The pipe rocks are kimberlite breccias that consist of minerals such as serpentine, calcite, chlorite, mica, and quartz. The simulation results, using HydroGeo software, showed that the brines in the deep horizons of the ore body of the Udachnaya pipe remain unsaturated in relation to almost all groups of kimberlite-forming minerals (carbonate, sulfate, chloride, clay, and aluminosilicate) at any temperature and pressure in the reservoir. The nature and degree of equilibrium in the brine-kimberlite system, formed in the course of their long geochemical evolution, were influenced by the processes of both dissolution-precipitation of the chemical components of the system and the dilution of ground brines with low-salinity or fresh waters.