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Carbonate-Cemented Zones in the Lower Tuscaloosa Formation (Upper Cretaceous), Southwestern Mississippi
Lisa L. Minter (1), Christopher P. Cameron (2), Andrew R. Thomas (3), William C. Ward (4)
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.
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