Role of Organics as Agents of Dissolution

The role of organic acids and their anions as agents of dissolution and secondary porosity generation has been much debated since the late 1970's when it was recognized that oilfield waters can contain large quantities of these acids. Recent references include, among others, Helgeson et al. (1993), Hanor et al. (1993), Barth and Bjø rlykke (1993), Lundegard and Land (1993), Surdam et al. (1993), Lundegard et al. (1992), Harrison and Thyne (1992), and Boles (1992). The crux of the debate is whether it is possible for sufficient quantities of dissolved organic acids to be expelled from shales during thermal maturation of kerogen, migrate several kilometers into sandstones, and still have the potential to affect diagenetic reactions. The petrographic evidence of enhanced porosity due to such reactions remains equivocal, and there is considerable disagreement as to their importance in preserving or destroying porosity.

Most recently, the debate over the role of organics has evolved to consider the possibility that in higher temperature reservoirs, organic acids can be produced from the reservoired hydrocarbons themselves as a result of hydrolysis of those hydrocarbons at the oil-water interface (e.g. Helgeson, 1993; Surdam et al., 1993; Lundegard et al., 1992; and Boles, 1992). Evidence for this process includes local plagioclase dissolution associated with hydrocarbons reservoired in the San Joaquin Basin (Boles, 1992), and bleaching of redbeds associated with the presence of hydrocarbons (Surdam et al., 1993). Thermodynamic calculations by Helgeson et al. (1993) suggest that light hydrocarbons react irreversibly with HO at the oil-water interface to produce CO and other organic acids, which can then act as agents of diagenesis. These calculations are supported by thermal alteration experiments conducted on two oils of widely different initial oxygen content (Lundegard et al., 1992). As expected, the oil with the higher initial oxygen content generated significantly more organic acid anions than the lower oxygen oil. However, mass balance calculations suggested that the volume of organic acids produced was unlikely to be diagenetically significant. A resolution of this question will probably be found in more careful experimental studies tied to field tests in hydrocarbon reservoirs.

In contrast to arguments relating to the role of organics in dissolution and secondary porosity formation, a few recent papers have begun to discuss whether bacteria play a prominent role in catalyzing precipitation of carbonate and other cements (e.g. Folk, 1993; McMahon et al., 1992). Results from these studies suggest that processes of microbial organic-acid production in clays and microbial organic-acid consumption in sands have resulted in precipitation of significant diagenetic cements in some localities. Quantification of the role these effects play in overall reservoir quality evolution awaits further study.