Geochemical characteristics, origin and carbon isotope reversal of the presalt natural gas in the Lower Paleozoic Ordovician carbonates, Ordos Basin, China

Abstract

The origin and potential source of the presalt natural gas in the Lower Paleozoic Ordovician carbonates within the Ordos Basin, China, is heavily debated, and the causes of carbon isotope reversal are currently unclear. This study analyzes the source and origin of the presalt natural gas and determines the reasons for carbon isotope reversal. The results show that the nonhydrocarbons contain CO2, H2S, and N2 and some gas samples have high H2S contents of 9.02%–23.58%. The dryness coefficient (C1/C1+) of 91.06%–99.99% (ave. 98.34%) suggests that the presalt gas is mainly a dry gas with some wet gas and that it originates from a high-to over-mature source. However, the results show that the presalt natural gas is primarily a wet-gas-cracking gas, rather than a kerogen-cracking gas, with a certain amount of coal-type gas mixing and it mainly originated from oil-prone marine carbonate source rocks rather than the Permian–Carboniferous coal measures. The gas has different types of carbon isotope reversal patterns, and the δ13C2 values become negative at Roequ > 1.5%. The mixing between the oil-cracking and wet-gas-cracking gas at Roequ > 1.5% results in carbon isotope reversal (δ13C1 > δ13C2), rather than Rayleigh and migration fractionation. The occurrence of thermochemical sulfate reduction (TSR) is supported by the high abundance of H2S in the presalt gas, reservoir bitumen in dissolved pores, secondary calcite with negative δ13Ccarbonate of −21.1‰ to −9.7‰, and diagenetic cubic pyrite. It is evident that the heavy δ13C2 values (greater than −28‰) and the carbon isotope reversal of δ13C2 > δ13C3 of the presalt natural gas at Roequ < 1.5% are related to TSR. This study also establishes a new model of the carbon isotope reversal of the natural gas, which is primarily controlled by its thermal maturity and TSR. Six carbon isotope patterns can be well explained and are supported by real gas samples from the presalt strata, and two types of carbon isotope reversal are predicted.