Isotope composition of organic and carbonate carbon of surface horizons of bottom sediments in the area of the Shtokmanovskoe deposit and “pockmarks field” in the Barents Sea

Abstract

One of the methods applied in searching hydrocar� bon (HC) deposits is the geochemical one, which is based on study of stable isotopes of organic (Corg) and carbonate (Ccarb) carbon in bottom sediments. In the second half of the twentieth century, a global natural phenomenon was discovered: the formation of isoto� pically light Corg and Ccarb in bottom sediments during aerobic and anaerobic microbe oxidation of migrating hydrocarbons [1]. The results of investigation of deposits of the Norway Sea neighboring the Barents Sea in the area of the caldera of the transpolar mud volcano Haakon Mosbi and pockmarks on the Vest� nese Ridge may be considered as an example of appli� cation of the isotope data. The isotope composition of Corg in these sediments was depleted in heavy isotope 13 C by 8–10‰ in comparison with Corg of normal marine sediments. Carbon of carbonates was also depleted in the 13 C isotope at the expense of incorpo� ration of isotopically light carbon from methane formed by methane oxidation by microorganisms [1]. Thus, the discovery of Corg and Ccarb depleted in the 13 C isotope in comparison with the isotope composi� tion of Corg and C carb of marine genesis in sediments may provide evidence for the release of fluids contain� ing hydrocarbons at the bottom and their partial microbe oxidation with the formation of isotopically light carbonate compounds. In this study we wanted to solve the opposite prob� lem and to determine the participation of gas–water fluids in the formation of overlying sedimentary series in the area of a known HC deposit and in the field of assumed pockmarks using analysis of the stable iso� topes of organic and carbonate carbon.