Carbonate Cementation and Dissolution in Miocene Sandstones from Corsair Trend, Offshore, Texas: ABSTRACT

Abstract

Middle Miocene sandstones occur in Picaroon field (Corsair trend; offshore Texas Gulf Coast) at depths of approximately 13,000-17,000 ft (3.9-5.1 km). These deltaic sandstones contain evidence of the following sequence of diagenetic events: (a) precipitation of chlorite coatings on detrital grains; (b) partial dissolution of feldspar; (c) quartz cementation; (d) calcite cementation; (e) dissolution of calcite cement; (f) ankerite cementation. The reservoir quality of the sandstones is largely a function of porosity enhancement due to calcite dissolution. Calcite cement was emplaced at depths of approximately 3,000 to 8,500 ft. The calcite has /sup 87/86/Sr values of 0.70833 to 0.70865, eliminating coeval (/approximately/15 m.y.) seawater and marine carbonate (0.70873-0.70885) as a potential source. The /sup 87/86/Sr composition of calcite cement is consistent with mass transfer of calcite from older marine sources to younger sediments. Fluid inclusion measurements indicate that ankerite cement formed at temperatures of approximately 120/degree/-145/degree/C (11,000 to 14,000 ft). Oxygen isotope modeling predicts that at these depths shales would expel waters with /delta//sup 18/O/sub SMOW/ of +5 to +8 during smectite-illite conversion. Ankerite (/delta//sup 18/O/sub PDB/ = /minus/7.8) would be in isotopic equilibrium with the predicted waters at the temperatures derived from fluid inclusions. Ankerite cements have relatively radiogenicmore » /sup 87/86/Sr ratios (/approximately/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 8,500 to 11,000 ft (95/degree/-120/degree/C).« less