Distribution and Occurrence of Organic Acids in Subsurface Waters

Abstract

Summary This chapter provides a summary of data on the occurrence of dissolved organic acid anions in subsurface waters, especially in formation waters associated with petroleum. In addition, it discusses general aspects of the origin and survivability of organic acid anions in the subsurface. Published data on the concentration of dissolved organic acid anions can collectively be characterized as showing considerable variation. Maximum concentrations and the highest average concentrations of organic acid anions in a basin commonly occur at reservoir temperatures between 80 and 140 °C, but temperature alone is a poor predictor of the concentration in individual samples. Furthermore, it is clear that organic acid anion concentrations greater than about 3000 mg/1 acetate-equivalent are rare. Acetate is by far the most abundant species contributing to the organic alkalinity of formation waters in sedimentary basins, although in some formation waters from subsurface temperatures less than about 80 °C other monocarboxylic organic acids may occur in higher concentrations. Much fewer data are available on the concentrations of dicarboxylic acid anions and some reported results are contradictory. In general, dicarboxylic acid anions are much less abundant than monocarboxylic acid anions and typically occur at concentrations less than 100 mg/1. Succinate and methyl-succinate appear to be the most abundant dicarboxylic species. The concentrations of some dicarboxylic acid anions are probably limited by the low solubility of their calcium salts and their greater susceptibility to thermal decomposition. The dominant species contributing to the organic alkalinity of formation waters have been identified, but waters exist in which organic acid anions do not account for all of the dissolved organic carbon. Little work has been done to characterize and quantify these other species contributing to the dissolved organic carbon. Organic alkalinity, contributed mainly by short-chain monocarboxylic acid anions, is responsible for a variable fraction of total formation water alkalinity. The fraction of total alkalinity contributed by organic acid anions varies with respect to temperature and from basin to basin. In Cenozoic formations it is common for organic alkalinity to exceed bicarbonate alkalinity in the temperature range from 80 to 140 °C. At temperatures less than 80 °C and greater than 140 °C bicarbonate generally dominates the alkalinity of formation water. Formation waters in Miocene reservoirs in the San Joaquin Basin, California, are exceptional in their very high concentrations of organic acid anions and in the dominance of organic alkalinity over bicarbonate alkalinity. Present-day concentrations of organic acid anions are influenced by factors that control the competing processes of organic acid production and destruction. Together, field and experimental data allow identification of the important factors and provide insight into the processes they control. Field data discussed here as well as experimental data discussed in other chapters demonstrate that thermal alteration of kerogen in shales is the dominant process by which aqueous organic acid anions are produced in basins. Consideration of the effects of burial on shale porosity and thermal history shows that high geothermal gradients will increase the rate of organic acid production from kerogen, whereas low geothermal gradients will tend to produce higher organic acid concentrations. In situ thermal alteration of reservoired oil may be locally significant in producing moderate concentrations of aqueous organic acids but is unlikely to be a process of diagenetic significance. The association of biodegraded oils with lower concentrations of organic acid anions and the generally low concentrations of organic acid anions in shallow formation waters indicate that biological processes are important in the destruction of these species. Field studies indicate that organic acid anions undergo destruction with progressive thermal exposure, and at temperatures of roughly 100 °C, acetate has a half-life of tens of millions of years.