Model for the origin of carboxylic acids in basinal brines

Abstract

Carboxylic acids are ubiquitous in basinal brines from petroleum-producing sedimentary basins. Although the absolute concentrations of individual short-chain acids vary systematically over many orders of magnitude, relative abundances are characterized by approximately constant ratios. Laboratory experiments have demonstrated that oxidation of aqueous n-alkanes proceeds through a sequence of reactions involving alkene, alcohol, ketone, and carboxylic acid reaction intermediaries. The highly specific nature of these reactions allows the relative distribution of carboxylic acids produced during oxidation of n-alkanes to be predicted quantitatively as a function of petroleum composition. A model based on the results of laboratory experiments was developed to account for the thermogenic production of aqueous carboxylic acids in sedimentary basins.Model predictions are highly consistent with the distribution of carboxylic acids observed in nature. This result suggests that hydrocarbon oxidation reactions may be pervasive during petroleum maturation in sedimentary basins. Likely oxidizing agents include ferric iron-bearing aluminosilicates, oxides, and hydroxides, pyrite, sulfate-bearing minerals, and water. Such chemical interactions allow inorganic sedimentary components to act as sources of oxygen for the formation of oxygenated organic alteration products. Accordingly, the absolute amount and timing of carboxylic acid generation may not be limited by the compositional evolution of kerogen, as suggested by previous models.