Occurrence of heavy carbon dioxide of organic origin: Evidence from confined dry pyrolysis of coal

Although CO2 is a common non-hydrocarbon gas in sedimentary basins and is significant in studying geofluid evolution, its origin is still a matter of debate. Light δ13CCO2 values have been suggested as a good indicator for discriminating CO2 of organic origin from that of inorganic origin (such as from the mantle or from carbonate mineral dissolution). However, here we present evidence suggesting that large quantities of isotopically heavy CO2 can be liberated from deeply buried source rocks. An immature coal has been pyrolyzed in confined dry gold reactors at temperatures from 250 °C to 600 °C with different heating rates and under a pressure of 30 MPa. The results suggest that isotope fractionation and origin from various oxygenated functional groups with different δ13C compositions make the late generated CO2 (over 40% of potential) enriched in 13C. The 44% of CO2 late-produced from the coal has an average δ13C value of − 6.7‰ (VPDB); the latest 20% has an average δ13C value of + 1.5‰; and the latest 10% is most enriched in 13C, with a δ13C value of + 5.0‰. This paper presents two cases highlighting organic, isotopically heavy CO2 in natural gases occurring at variable concentrations in different sedimentary contexts.