Water-Rock Interaction During Meteoric Flushing of a Limestone: Implications for Porosity Development in Karstified Petroleum Reservoirs

Abstract

ABSTRACT The Lincolnshire Limestone, comprising a succession of Jurassic wackestones, packstones, and oolitic grainstones, forms an important carbonate aquifer in eastern England. Meteoric waters enter at outcrop and penetrate between confining strata at least 25 km down-dip. This water dissolves and interacts with the limestone, and even water samples collected at or near outcrop are calcite-saturated. Net limestone dissolution is thus a process that is most dominant in the near-surface environment. Water samples taken at increasing distances from outcrop have increasing Sr and Mg contents, and 13C values of dissolved bicarbonate increase from -15 to -8 (PDB), while 87Sr/86 falls from 0.7082 to 0.7077. These data are interpreted to be the result of a coupled carbonate dissolution-reprecipitation process. Modeling of the C and Sr isotopic data indicates that primary Jurassic carbonate (ooids, micrite, fossils) is dissolving preferentially over burial cements. Isotopically light bulk-rock carbon near joint surfaces suggests that reprecipitation of calcite in the form of cement could be concentrated preferentially in and near joints. The Lincolnshire Limestone may be used as an analogue for karstified petroleum 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 ch mical data suggest that, in such reservoirs, influx of meteoric water at an unconformity creates porosity and enhances permeability through limestone dissolution, but this may be concentrated close (tens of meters) to the unconformity. This dissolution also promotes surface erosion, which limits the thickness of preserved karstified limestone. The dissolution-reprecipitation process that occurs at greater distances from outcrop has effects that are far more widespread (lens of kilometers), leading to an increase in microporosity (e.g., micrite leaching) and a possible reduction in joint or fracture porosity. Such factors should be taken into account when exploring for, and appraising, karstified petroleum reservoirs.