Geochemical comparison between the gas in bulk fluid inclusions and reservoir gas produced in Paleozoic formation, Ordos Basin, China

•A method for geochemical analysis of gases captured in inclusions is presented.•The gases pooled in P-C sandstones experienced weak or no secondary alteration.•The gases sourced from Lower Paleozoic share a characteristic δ13C1 value of <− 38‰.•TSR contributes to heavy δ13C2 of gas pooled in the Lower Paleozoic formation.

Taking the reservoir sandstones of natural gas pools in the Upper Paleozoic and the calcite veins in Ordovician carbonate collected in the Ordos Basin, China, as samples, a unique method for geochemical analysis of gases captured in fluid inclusions (FIs) is presented in this study. Geochemical comparison between FI gases and reservoir gases in this basin is conducted. The results show that the gases from FIs developed in Upper Paleozoic reservoir sandstones and from the corresponding reservoirs share similar geochemistry in hydrocarbon gaseous compositions, although there is a high concentration of non-hydrocarbon gases (e.g. CO2) in FIs. The occurrence of FIs in sandstones and the value of the carbon isotopic ratio of carbon dioxide (δ13CCO2 > − 8‰) suggest that non-hydrocarbon gas in FIs originated from primary FIs formed during hosted mineral (quartz) crystallization from magma. Similar geochemistry in hydrocarbon gases from FIs and from reservoirs indicates that the gases pooled in Upper Paleozoic sandstones represent the original state charging from the source rocks and thus experienced a weak or no secondary alteration. Dominating hydrocarbon gaseous components, such as wells Lt1 and Yu9, in FI gases at a secondary formation in calcite veins of Ordovician carbonate imply that the FIs are related to oil and gas charging. The gases originated from the Lower Paleozoic source rock is characterized by δ13C1 < − 38‰, although there is a great difference in geochemistry of gases from FIs of different samples and from FIs and reservoirs. The heavier δ13C2 value of reservoir gas in the Lower Paleozoic reservoir compared with that of FI gases might be attributed to thermochemical sulfate reduction (TSR) after oil and gas charging into the reservoir.