Carboxylic acid anions in formation waters, San Joaquin Basin and Louisiana Gulf Coast, U.S.A.—Implications for clastic diagenesis

Abstract

Carboxylic acid anions (CAA) in formation waters are of interest to studies of clastic diagenesis because of their ability to buffer formation water Eh and pH (and thus substantially contribute to controls on carbonate mineral stability), and their ability to complex and transport Al and Si from the site of aluminosilicate mineral dissolution during diagenesis. Carboxylic acid anions are also extremely important to the aqueous geochemistry of Ca, Fe, Mn, Pb and Zn. Some formation waters from sedimentary basins contain high concentrations of CAAs. The analyses of 20 formation waters from the San Joaquin Basin, California and 20 formation waters from the Louisiana Gulf Coast Basin presented in this study, show concentrations as high as 8100 ppm monofunctional and 370 ppm difunctional CCA; by comparison, previously reported analyses indicate monofunctional CAA occir in concentrations up to 10,000 ppm and difunctional CAA may occur in concentrations up to 2610 ppm. Analyses of drilling muds and scale soaps presented in this study show that few if any difunctional CAA in the study area can be attributed to contamination from these sources. Additionally, aqueous extracts of crude oils contain both mono- and difunctional acid anions, as do the aqueous and petroleum phases of hydrous pyrolysates. Previously unreported dissolution experiments, equilibrium computer simulations, and hydrous pyrolysis experiments support those already published and suggest that CAA are generated during thermal maturation of kerogen and expelled from the shale along oil-wet microfractures. Upon entering the water-wet sandstone pore, the hydrophilic CAA partition into the aqueous phase. Organic-inorganic reactions may occur which from CAA-metal complexes. Because the complexes are hydrophobic, they partition in the petroleum phase, where present. Carboxylic acid anions are of great importance to clastic diagenesis over the temperature range in which they dominate fluid alkalinity; certainly, no other viable mechanism has been advanced which adequately explains the observed aluminosilicate mineral dissolution with subsequent mass transfer of A1, as well as the carbonate mineral diagenetic successions observed in sand-shale systems world-wide. The utility of modeling these observations of organic-inorganic diagenesis is limited only by the ability to model the concentration and distribution of CAA through space and time.