Evolution of the composition of Н, О and C stable isotopes in the groundwaters of oil and gas bearing sediments in the northern regions of West Siberia

Abstract

The results of a study of the isotopic composition of oxygen and hydrogen of groundwater and dissolved inorganic carbon in oil and gas deposits of the northern regions of Western Siberia, covering a wide stratigraphic range – Mesozoic and Paleozoic are presented. The obtained values are very diverse, covering the intervals from –120 to –50‰ (δD) and from –17 to –2‰ (δ18O). Such a variable isotopic composition of waters indicates the absence of a unified mechanism for the accumulation of waters and their transformation during the geological evolution of the West Siberian sedimentary basin. The main feature of most of the studied waters is the pronounced values of oxygen isotope shifts relative to GMWL, which generally increase with the age of water-bearing deposits and reach 9‰, which indicates their ancient sedimentary origin. Variations in the isotopic composition of DIC (from –51.8 to +21.8‰) and its content in waters (from 0.2 to 38.6 mmol/I) indicate a wide range of water burial conditions and processes involved in the conversion of water-soluble carbon dioxide. A general pattern of changes in the isotopic composition of carbon in CO2 with time, expressed in general form by damped oscillations of the “isotope oscillator”, is revealed. A model of transformation of the carbon isotope composition in carbon dioxide in the closed cycle “atmosphere-soils-deep aquifers” for oil and gas deposits of the northern regions of Western Siberia based on the obtained isotope data is proposed.