Abstract: Dissolution of Carbonates by Meteoric Water Flushing in the Lincolnshire Limestone, England: Implications for Porosity Evolution Beneath Limestone Exposure Surfaces
In the Jurassic Lincolnshire Limestone, fresh water entering at the surface moves down-dip for approx. 25 km, evolving chemically in a manner consistent with two processes: (1) limestone dissolution, resulting in porosity creation and surface erosion, and (2) dissolution and reprecipitation of carbonate, leading to porosity redistribution but not creation. Quantification of these processes has important implications for reservoir quality in karstified limestones.
Lincolnshire Limestone near-surface groundwaters are already calcite-saturated, indicating dissolution to be restricted to close to the outcrop. With increasing distance down-dip, groundwater ^dgr13C (of bicarbonate) changes gradually from -16^pmil to -7^pmil and 87Sr/86Sr changes from 0.7085 to 0.7077 due to progressive interaction with limestone calcite. Limestone calcite has constant ^dgr13C (+2^pmil) but variable 87Sr/86Sr (0.7073-0.7083). A water-rock interaction model was thus used (a) to calculate how much calcite must have interacted with each litre of groundwater to cause the observed ^dgr13C shift; and (b) to determine die 87Sr/86Sr (and thus the type) of dissolving carbonate n cessary to cause the shift in water 87Sr/86Sr. This clearly identified preferential dissolution of allochems.
Models for reservoir quality evolution beneath unconformities must thus take into account that bulk carbonate dissolution (porosity enhancement) may only operate very close to the surface, where the groundwater is undersaturated. Over a much larger area (10s of km from outcrop), porosity may be redistributed by selective dissolution of unstable grains and precipitation of meteoric cement. This may have a detrimental effect on permeability.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994