ABSTRACT: Carbonate-Cemented Zones in the Lower Tuscaloosa Formation (Upper Cretaceous), Southwestern Mississippi

MINTER, LISA L., ARCO Oil and Gas Company, Houston, TX, CHRISTOPHER P. CAMERON, University of Southern Mississippi, Hattiesburg, MS, ANDREW R. THOMAS, Texaco, USA, Houston, TX, and WILLIAM C. WARD, University of New Orleans, New Orleans, LA

Zones of preferential ankerite cementation are common within nonmarine and marine Lower Tuscaloosa sandstone reservoirs of southwestern Mississippi. These carbonate-cemented zones occur as nodules (average radius 6 cm), streaks (average thickness 1 cm), and patches that reduce and often completely impede porosity.

Petrographic comparison of sandstone compositions and textures inside and outside the cemented zones revealed that the precipitation of the ankerite was a late-diagenetic event. With the exception of carbonate cement infilling porosity, the overall mineralogy and textural relationships both inside and outside of the carbonate-cemented zones are alike. Stable carbon and oxygen isotope analyses of whole-rock samples also indicate that the cemented zones are products of late diagenesis. Light isotope {18}O values of -9 o/oo to -12 o/oo indicate that the carbonates were precipitated after considerable burial. Negative isotope {13}C values of -11 o/oo to -14 o/oo indicate that carbonate precipitation was from pore waters enriched in {12}C because of thermal decomposition of interbedded org nic-rich zones.

AAPG Search and Discovery Article #91014©1992 AAPG GCAGS and GC-SEPM Meeting, Jackson, Mississippi, October 21-23, 1992 (2009)