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

Thomas R. Taylor

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 ⁸⁷86/Sr values of 0.70833 to 0.70865, eliminating coeval (~ 15 m.y.) seawater and marine carbonate (0.70873-0.70885) as a potential source. The ⁸⁷86/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°-145°C (11,000 to 14,000 ft). Oxygen isotope modeling predicts that at these depths shales would expel waters with ^dgr¹⁸O_SMOW of +5 to +8 during smectite-illite conversion. Ankerite (^dgr¹⁸O_PDB = -7.8) would be in isotopic equilibrium with the predict d waters at the temperatures derived from fluid inclusions. Ankerite cements have relatively radiogenic ⁸⁷86/Sr 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 8,500 to 11,000 ft (95°-120°C). Although the cause of calcite dissolution is unknown, these temperatures overlap with the range in which high concentrations of dissolved organic acids have been reported in Gulf Coast formation waters.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.