WATER-ROCK INTERACTION DURING METEORIC FLUSHING OF A LIMESTONE - IMPLICATIONS FOR POROSITY DEVELOPMENT IN KARSTIFIED PETROLEUM RESERVOIRS

The Lincolnshire Limestone, comprising a succession of Jurassic wackes tones, packstones, and oolitic grainstones, forms an important carbona te aquifer in eastern England. Meteoric waters enter at outcrop and pe netrate between confining strata at least 25 km down-dip. This water d issolves and interacts with the limestone, and even water samples coll ected at or near outcrop are calcite-saturated. Net limestone dissolut ion is thus a process that is most dominant in the near-surface enviro nment. Water samples taken at increasing distances from outcrop have i ncreasing Sr and Mg contents, and deltaC-13 values of dissolved bicarb onate increase from -15 parts per thousand to -8 parts per thousand (P DB), while Sr-87/Sr-96 falls from 0.7082 to 0.7077. These data are int erpreted to be the result of a coupled carbonate dissolution-reprecipi tation process. Modeling of the C and Sr isotopic data indicates that primary Jurassic carbonate (ooids, micrite, fossils) is dissolving pre ferentially over burial cements. Isotopically light bulk-rock carbon n ear joint surfaces suggests that reprecipitation of calcite in the for m of cement could be concentrated preferentially in and near joints. T he Lincolnshire Limestone may be used as an analogue for karstified pe troleum reservoirs, specifically those which have been buried and lost their unstable carbonate minerals (aragonite, high-Mg calcite) prior to uplift and karstification. The present water chemical data suggest that, in such reservoirs, influx of meteoric water at an unconformity creates porosity and enhances permeability through limestone dissoluti on, but this may be concentrated close (tens of meters) to the unconfo rmity. This dissolution also promotes surface erosion, which limits th e thickness of preserved karstified limestone. The dissolution-repreci pitation process that occurs at greater distances from outcrop has eff ects that are far more widespread (tens of kilometers), leading to an increase in microporosity (e.g, micrite leaching) and a possible reduc tion in joint or fracture porosity. Such factors should be taken into account when exploring for, and appraising, karstified petroleum reser voirs.