EFFECTS OF CO2 DEGASSING IN NATURAL SETTINGS – NATURAL EXAMPLES AND RELEVANCE TO IMMISCIBLE H2O-CO2 RELATIONS

Examples of mineralization resulting from CO₂ degassing of natural pore fluids include carbonate-filled fractures in fault zones, scaling in oil field production tubing, and speleothems formed in cave and tunnel environments. In some cases, the precipitation appears to be a rapid response to degassing, where the carbonate is out of isotopic equilibrium with the pore water and ambient temperature. In other cases, precipitation appears to be a slow void-filling process resulting in isotopic equilibrium between the pore fluid and mineral. Evidence for rapid crystallization and the resulting isotopic disequilibrium (i.e. kinetic fractionation) is the co-variation of carbon and oxygen isotopes within a sample (Hendy, 1971). The best examples showing co-variation are from speleothems formed in some tunnels and cave environments. Surprisingly, carbonate-filled fractures in faults, although often cited to be a result of degassing, typically do not show isotopic evidence for rapid carbonate crystallization. The reason for the difference between the shallow and deep environments is not clear, but it may be related to the state of CO₂ in the higher pressure environments of the more deeply buried samples. In an example of carbonate scaling in production tubing from the San Joaquin basin of California, the degassing conditions, which are relatively well known, suggest that the CO₂ in the pore fluid is close to the critical state. This condition may cause anomalous isotopic behavior during the degassing process.

Experiments are needed to replicate the degassing process and the effects of critical state CO₂ on the isotopic system of carbonate minerals.