Extremely positive δ13C and subsurface CO2 phase separation isotope fractionation

Extremely positive δ¹³C is found in calcite veins for the first time in the Huangqiao CO₂-gas reservoir, eastern China, with ¹³C-enriched values as high as +16.9 ‰. The calcite cannot be explained as a product of isotopic evaporation fractionation and/or microbial methanogenesis, given the high formation temperatures of 100–160 °C. Consequently, we propose a new model that attributes the extremely δ¹³C enrichment to subsurface vapor–liquid CO₂ phase separation. In the model, deep CO₂ was initially dissolved in the pore water, and then phase-separation occurred when CO₂ underwent supersaturation. Light ¹²CO₂ preferentially escaped, leading to the enrichment of heavier ¹³CO₂ in the pore water. During tectonic movements, the ¹³C-enriched pore water was injected into the gas reservoir strata and the high-δ¹³C calcite veins were precipitated along fractures. Numerical simulation supports this hypothesis, suggesting that phase separation can induce considerable C isotope fractionation. At temperatures below 140 °C, δ¹³C in fluid systems can rise to 17 ‰ through the phase separation of approximately 4.53 × 10¹¹ m³ CO₂, aligning with that of the current CO₂ reserves in the Huangqiao CO₂ gas field. Simultaneously, the δ¹³C values of the escaped CO₂ remain relatively stable and fall within the δ¹³C range of the current-day CO₂. Our results provide important insights into the isotopic fractionation in subsurface fluid systems.