Thermodynamic calculations indicate that CO2 in sedimentary basins may be in stable redox equilibrium with organic acids found in basinal brines, but that CH4 is not. These calculations also show that the high concentrations of carboxylic acids are far from equilibrium with decarboxylation reactions. A compilation of recent analyses of carboxylic acids from aqueous solutions associated with petroleum reservoirs shows that there is no trend in acid concentration with temperature; however, inspection of these data at ∼100 °C indicates that acetic and propanoic acids are in homogeneous metastable equilibrium. Oxygen fugacities indicated by this metastable assemblage are consistently lower than those set by iron-oxide assemblages; this is not surprising, because both hematite and magnetite are unstable relative to siderite in the presence of detectable concentrations of acetic acid.

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