The influence of calcite dissolution on reservoir porosity in Miocene sandstones, Picaroon Field, offshore Texas Gulf Coast

Abstract Middle Miocene sandstones occur in Picaroon field (Corsair trend; offshore Texas Gulf Coast) at depths of approximately 4.9 to 5.2 km (16-17,000 ft). These deltaic sandstones contain evidence of the following sequence of diagenetic events: 1) formation of chlorite coatings on detrital grains, 2) partial dissolution of detrital feldspar, 3) quartz cementation, 4) calcite cementation, 5) dissolution of calcite cement and grains, 6) ankerite cementation. The reservoir quality of the sandstones is largely a function of porosity enhancement by calcite dissolution. Calcite cement was emplaced at depths of approximately 1.8 to 2.6 kin. The calcite has (super 87/86) Sr values of 0.7083-0.7086, eliminating coeval ( nearly equal 15 myr.) seawater and marine carbonate (0.70873-0.70885) as the primary source of strontium. The (super 87/86) Sr composition of calcite cement implies that mass transfer of calcite from older marine sources to younger sediments has occurred. Fluid inclusion measurements indicate that ankerite cement formed at minimum temperatures of 120-188 degrees C (3.0-5.1 km). Oxygen isotope modeling predicts that at these depths shales would expel waters with delta 18 O SMOW of +5 to +9 during smectite --> illite conversion. Ankerite (delta 18 O PDB = -7.8) would be in isotopic equilibrium with these waters at temperatures similar to those derived from fluid inclusions. Ankerite cements have relatively radiogenic (super 87/86) Sr ratios ( nearly equal 0.7097) which are consistent with their formation from shale-derived fluids. Calcite dissolution occurs between the precipitation of calcite and ankerite. It is therefore concluded that calcite cement dissolution occurred at burial depths of 2.6 to 3.0 km (107-120 degrees C).