Distribution and significance of dicarboxylic acid anions in oil field waters

Abstract

The origin, distribution, and interactions of low-molecular-weight organic acid anions have become an intensively studied field in geochemistry since their widespread occurrence in formation waters of sedimentary basins was first documented by Carothers and Kharaka ( 1978 ). High concentrations (up to 10,000 mg 1~ ) of monocarboxylic (mainly acetate, propionate, and butyrate) and dicarboxylic (mainly oxalate, malonate, and succinate ) acid anions have been reported from many sedimentary basins, with the highest values present in relatively young (Cenozoic age) petroleum reservoir rocks at subsurface temperatures of 80-120 ° C [ see Lundegard and Kharaka (1990) for a recent review with references ]. Geochemical interest in these organic anions stems mainly from their important role in mineral diagenesis in sedimentary basins (Kharaka et al., 1986; MacGowan and Surdam, 1990). In particular, these species act as sources or sinks of protons, as a source of CO2 and as pH and Eh buffering agents (Lundegard and Kharaka, 1990). They also form complexes with cations and metals such as Ca, A1, Fe, Pb, and Zn (Kharaka et al., 1985; Harrison and Thyne, 1992). Data on the concentrations of monocarboxylic acid anions in subsurface waters from many sedimentary basins worldwide are available and are generally accepted as representing the aqueous concentrations at depth. Data on the concentrations of dicarboxylic acid anions are much more limited, some reported values are controversial, and the total values reported range widely from 0 to 2640 mg 1-~ (Surdam et al., 1984; Kharaka et al., 1986; Barth, 1987; MacGowan and Surdam, 1988, 1990). Because of the wide range in the reported concentrations and because dicarboxylic acid anions generally form stronger complexes with A1, Fe, and other cations than do monocarboxylic acid anions, we resampled four oil wells in the San Joaquin basin, California, in order to better assess the role of these acid anions in water-rock interactions in sedimentary basins.