STABLE ISOTOPES OF O, H AND C IN GROUND WATERS OF PETROLEUM DEPOSITS, NORTHERN REGIONS OF THE WEST SIBERIAN SEDIMENTARY BASIN

The water isotopic composition of petroleum deposits of the Arctic sector of West Siberia was first studied in the paper. The obtained values of isotopic oxygen shifts relative to the global meteor water line (GMWL) from 2 to 9‰ indicate the ancient sedimentation origin of most samples. The primary enrichment of waters with the heavy isotope 18O due to evaporation in warm climatic conditions was subsequently further enhanced by their isotopic exchange with oxygen-containing rocks. The migration hypothesis of light isotopes of water into the overlying water-bearing horizons during the formation of condensatogenic waters is proposed, the influence of moisture burial duration on its isotopic composition is shown. The isotopic concentration characteristic of DIC is very broad and covers the range of δ13C values (from –51.8 to +21.8 ‰) and the carbon dioxide content in waters (from 0.17 to 38.6 mmol/dm3). The content of DIC in waters is primarily associated with the availability of carbon-containing substance to waters and bacteria directly involved in its processing. The isotopic composition of DIC varies non-linearly and indicates a variety of competing processes of formation and isotopic exchange of carbon dioxide with the environment. The pattern suggesting the homogenization of isotopic composition of carbon dioxide in ground waters to values close to the average values of air CO2, taking into account the fractionation coefficients is found. The revealed effect may indicate the closure of carbon cycle in the “air CO2 – organic matter – DIC” system.