Organic acid metastability in sedimentary basins

Abstract

Thermodynamic calculations indicate that CO{sub 2} in sedimentary basins may be in stable redox equilibrium with organic acids found in basinal brines, but that CH{sub 4} is not. These calculations also show that the high concentrations of carboxylic acids are 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 concentrations with temperature; however, inspection of these data at {approximately} 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.