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 ^87/86Sr values of 0.7083-0.7086, eliminating coeval ( 15 myr.) seawater and marine carbonate (0.70873-0.70885) as the primary source of strontium. The ^87/86Sr 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°C (3.0-5.1 km). Oxygen isotope modeling predicts that at these depths shales would expel waters with ¹⁸O_SMOW of +5 to +9 during smectite illite conversion. Ankerite (¹⁸O_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 ^87/86Sr ratios (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°C).