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

ABSTRACT Zones of preferential ankerite cementation are common within non-marine 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 which 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 18O values of -9 to -12 indicate that the carbonates were precipitated after considerable burial. Negative 13C values of -11 to -14 indicate that carbonate precipitation was from pore waters enriched in 12C because of thermal decomposition of interbedded organic-rich zones.