APPRAISAL OF THE USE OF EXPERIMENTAL AND ANALOG STUDIES IN THE ASSESSMENT OF THE ROLE OF ORGANIC-ACID ANIONS IN DIAGENESIS

To fully understand and predict the processes which act to produce the diagenetic phenomena of porosity enhancement or reduction observed wi thin sandstone reservoir rocks, it is necessary to understand the role of formation waters in mineral dissolution and precipitation reaction s. Of particular interest are the mechanisms of feldspar and clay mine ral dissolution/precipitation, which involve the transport of Al and S i in solution. It has been suggested that organic acid anions, known t o be present within formation waters, may enhance aluminosilicate mine ral solubility by complexing with Al and Si, reducing the volumes of w ater required for transport to take place. However, the possible signi ficance of organic acid anion complexing still cannot be fully assesse d because of limited experimental evidence and poor knowledge of the s tability of dissolved organic species under appropriate pressure-tempe rature conditions. A series of experiments was undertaken to investiga te the possible role of acetate, oxalate and citrate (representing pot ential complexing agents present within formation waters) in diageneti c processes involving feldspar and quartz dissolution, and clay precip itation, and to determine solute yields from organic-rich mudrocks. At 150-degrees-C, 50 MPa, albite solubility is enhanced to the greatest extent in the presence of citrate, whereas acetate enhances the rate o f albite dissolution. However, in experiments with Pb-rich alkali feld spar, the rapid decay of acetate reduces its complexing potential and hence its availability for the transport of metals. Artificial maturat ion of organic-rich mudrocks under conditions appropriate for diagenes is yields a mixture of organic acid anions (including acetate, oxalate and citrate), but there is no evidence for solute yields significantl y in excess of those predicted for mineralogical buffers of fluid comp osition in the absence of organic species. Domestic waste disposal sit es are considered as potential analogues to diagenetic systems; the as sociated chemical processes show close parallels between the chemical maturation of landfill leachates and the development of sediment pore waters.