TAYLOR, THOMAS R., Shell Development Company, Houston, TX; and LYNTON S. LAND, Univ. of Texas at Austin, Austin, TX

ABSTRACT: Differences in Formation Water Chemistry and Sandstone Diagenesis in Deeply Buried Miocene Reservoirs: Corsair Trend, Offshore Texas

Anomalously high porosities (20-28%) in deeply buried (16-17,000 ft) Miocene sandstones at Picaroon field (Corsair Trend, offshore Texas) are largely a result of porosity enhancement by dissolution of calcite cement. Dissolution is minor at nearby Doubloon field and porosity is much lower (15-18%). Doubloon reservoir sands contain moderately saline (TDS = 63 g/l), "NaCl-type" water with low concentrations of Ca and other cations, enriched isotope{18}O (+7.8 o/oo SMOW), and radiogenic {87/86}Sr (0.71109). Formation waters of this type are common in the Gulf Coast and are produced by dissolution of diapiric salt within the Tertiary section. In contrast, Picaroon waters have high salinity (178-243 g/l TDS), high Ca (up to 22 g/l), heavier isotope{18}O (+8.0 to +9.3 o/oo SMOW) and less ra iogenic {87/86}Sr (0.70992-0.71023). In addition, they contain unusually high concentrations of Sr (-2,000 mg/l), Ba (>1,000 mg/l), Fe (120-438 mg/l), Pb (1-6 mg/l), and Zn (25-194 mg/l). Picaroon sandstones contain late diagenetic, fracture-filling ankerite, barite, and sphalerite. Ankerite is in oxygen isotopic equilibrium with formation water at temperatures indicated by fluid inclusions (>140 degrees C) and has similar radiogenic {87/86}Sr (0.70970). With respect to major and minor cation concentrations and isotopic composition, Picaroon brines are similar to waters produced from the underlying Mesozoic section in south Texas and central Mississippi. The observed differences in water chemistry and sandstone diagenesis between two fields with nearly identical depositional and bu ial histories demonstrates the importance of fluid flow in clastic diagenesis. The association of allochthonous waters with high quality reservoirs at Picaroon suggests a potential link between deep sources of fluids and carbonate dissolution.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.