Fig. 2: The δ¹³C of CO₂ in the Olla (CO₂ injected field) samples.

a, The δ¹³C of CO₂ as a function of the CO₂/³He ratio. The dashed lines show endmember methanogenesis and dissolution (pH 7) fractionation trajectories. The tick marks represent the total amount of CO₂ trapping within the system, relative to sample O5. The shaded region represents trapping by the combination of both microbial methanogenesis and dissolution. The upper and lower methanogenesis:dissolution ratios (M:D) are 0.33 and 0.19, respectively, showing that dissolution accounts for approximately three times more CO₂ removal (M:D = 0.26) than microbial methanogenesis. The lines labelled ‘consumption’ show the portion of original injected CO₂ that has been removed by net microbial methanogenesis. b, The δ¹³C of CO₂ as a function of the δ¹³C of CH₄. The shaded region represents the CO₂ isotopic composition of the injectate from the Black Lake Oil Field into the Olla system. The Olla data are consistent with thermal re-equilibration with both reservoir temperatures (solid lines) and microbial methanogenesis (dotted lines). The 1σ level of uncertainty is within the symbol size.