Although CO 2 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 δ 1 3 C C O 2 values have been suggested as a good indicator for discriminating CO 2 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 CO 2 can be liberated from deeply buried source rocks. An immature coal has been pyrolyzed in confined dry gold reactors at temperatures from 250 o C to 600 o C with different heating rates and under a pressure of 30MPa. The results suggest that isotope fractionation and origin from various oxygenated functional groups with different δ 1 3 C compositions make the late generated CO 2 (over 40% of potential) enriched in 1 3 C. The 44% of CO 2 late-produced from the coal has an average δ 1 3 C value of -6.7%% (VPDB); the latest 20% has an average δ 1 3 C value of +1.5%%; and the latest 10% is most enriched in 1 3 C, with a δ 1 3 C value of +5.0%%. This paper presents two cases highlighting organic, isotopically heavy CO 2 in natural gases occurring at variable concentrations in different sedimentary contexts.