Appraisal of the use of experimental and analogue studies in the assessment of the role of organic acid anions in diagenesis

Abstract

To fully understand and predict the processes which act to produce the diagenetic phenomena of porosity enhancement or reduction observed within sandstone reservoir rocks, it is necessary to understand the role of formation waters in mineral dissolution and precipitation reactions. Of particular interest are the mechanisms of feldspar and clay mineral dissolution/precipitation, which involve the transport of Al and Si in solution. It has been suggested that organic acid anions, known to be present within formation waters, may enhance aluminosilicate mineral solubility by complexing with Al and Si, reducing the volumes of water required for transport to take place. However, the possible significance of organic acid anion complexing still cannot be fully assessed because of limited experimental evidence and poor knowledge of the stability of dissolved organic species under appropriate pressure-temperature conditions. A series of experiments was undertaken to investigate the possible role of acetate, oxalate and citrate (representing potential complexing agents present within formation waters) in diagenetic processes involving feldspar and quartz dissolution, and clay precipitation, and to determine solute yields from organic-rich mudrocks. At 150°C, 50 MPa, albite solubility is enhanced to the greatest extent in the presence of citrate, whereas acetate enhances the rate of albite dissolution. However, in experiments with Pb-rich alkali feldspar, the rapid decay of acetate reduces its complexing potential and hence its availability for the transport of metals. Artificial maturation of organic-rich mudrocks under conditions appropriate for diagenesis yields a mixture of organic acid anions (including acetate, oxalate and citrate), but there is no evidence for solute yields significantly in excess of those predicted for mineralogical buffers of fluid composition in the absence of organic species. Domestic waste disposal sites are considered as potential analogues to diagenetic systems; the associated chemical processes show close parallels between the chemical maturation of landfill leachates and the development of sediment pore waters.