Hydrogeology of the northwestern margin of the West Siberian Artesian Basin

The available hydrogeological data collected over the past four decades in the northwestern West Siberian Artesian Basin (WSAB) have been synthesized for the first time in this study. The inventory of the applied methods includes review of local geological and stratigraphic data; analyses of major and minor ions, salinity, and isotope ratios; paleohydrogeological reconstructions; and hydrogeological modeling of oil and gas fields. As a result, a refined high-resolution hydrostratigraphic section below the Turonian-Oligocene regional seal has been obtained, which comprises five major aquifer systems and several intervening aquitards. Groundwaters in the study area, corresponding to the northern Fore-Ural petroleum province, are of three distinct types (Na–Cl, Na–Cl–HCO 3 , and Na–HCO 3 –Cl geochemical facies), with total dissolved solids (TDSs) from 1.0 to 18.6 g/L. The dissolved gas is predominantly methane throughout the area, with minor contents (to 2.2 vol%) of heavy hydrocarbons, while nitrogen has a significant contribution along the basin margins only. The formation water has recharged at the eastern slope of the Ural mountains through the whole WSAB history, this flow regime being responsible for the vertical and lateral changes in hydrochemistry and salinity. Groundwaters systematically increase in salinity as the aquifers become deeper in the southeastern direction away from the recharge area (the Urals), i.e., have salinity profiles typical of many artesian basin margins worldwide. The compositions of waters and their variations are controlled by the presence of a large recharge source in the west of the study area and coexistence of water types of different origins.