Generation and decomposition of low-molecular-weight organic acids in hydrous pyrolysis experiments with Opalinus Clay rock

Abstract

Increasing temperature in claystone formations accelerates geochemical reactions between organic matter, water and mineral phases, promoting the generation and decomposition of organic compounds such as low-molecular-weight organic acids (LMWOA). LMWOA presence in claystone formations can enhance mineral dissolution, stimulate gas generation through thermal decomposition reactions, affect metal-ion complexation, alter sorption processes, and provide feedstock for microbial life. Hydrous pyrolysis experiments (80–200 °C and 20 MPa) using Dickson-type flexible gold-titanium reaction cells were conducted to investigate the thermal generation and decomposition of LMWOA in organic matter poor (0.64 wt%) Opalinus Clay. Maximum concentrations of dominant species were 44 µmol/kg clay formate (120 °C), 310 µmol/kg clay acetate (200 °C), and 74 µmol/kg clay oxalate (120 °C). Calculated half-live ranges of the dominant compounds were between 8.88 × 1010 and 2.81 × 10−3 years for the experimental conditions. Comparison of observed and calculated reaction rates suggests that the thermal generation and decomposition of LMWOA proceed simultaneously. Therefore, the observed generation rates in these and other previously published experiments are minimum estimates that need to be corrected for parallel decomposition reactions. The new quantitative reaction rate data presented here facilitate modelling of production/decomposition and local pore water concentrations of LMWOA by the extrapolation of kinetic data to temperatures encountered, for example, in natural heating during subsidence or by thermal intrusions.