Geochemistry of Organic Acids in Subsurface Waters

Abstract

Integration of field observations, experimental data, and computer simulations provides valuable insights into the origin and inorganic interactions of organic acids and organic acid anions in subsurface waters. Present-day concentrations show significant variations with respect to temperature, basin, and reservoir age, reflecting complex interaction of biological, geological, and geochemical factors that influence the production or destruction of these species. Hydrous pyrolysis experiments demonstrate that most dissolved organic acids are produced by thermal degradation of kerogen, and that variation in kerogen chemistry can cause relative generating capacities to vary by a factor of two or more between samples of equal thermal rank. A mathematical model of the kinetics of organic acid generation from kerogen shows that variations in thermal gradients can cause significant variations in maximum organic acid concentrations, and in trends of organic acid concentrations with depth and temperature. Experimental and field studies of acetate decarboxylation rates indicate that they are strongly affected by temperature and catalysis. In nature, halflives of acetate are on the order of 20 to 60 million years at 100° C. Geochemical models predict that major organic acid species 1) are important proton sources in subsurface waters, 2) buffer pH, but increases in pCO2 are still likely to favor calcite undersaturation in most subsurface waters, 3) do not form important complexes with Pb, Zn, and U, and 4) do form important complexes with Fe, Ca and possibly Al.