Date of Award

1990

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

The Smackover Formation in the southeastern Mississippi salt basin has been buried to maximum depths of 3 to over 6 km and subjected to maximum temperatures of 100 to over 200 $\sp\circ$C. This formation contains a lower laminated mudstone member, a middle mudstone to wackestone member, and an upper grainstone member. In Clarke County, the upper Smackover member exhibits a north-south gradient in dolomite content, original mineralogy of ooids, and lithology. These regional gradients resulted from an increase in Mg/Ca concentrations of Smackover sea water northward across the carbonate platform. Shallow burial diagenesis ($<$4.5 km) was dominated by calcite cementation. Calcite cements in Clarke County are luminescently zoned and exhibit a consistent sequence of luminescent zonation. However, the intensity of luminescence and number of zones in the calcite cements vary across the county. This is due to variation in the chemical composition of the host carbonate rock and local hydrology. Deep burial diagenesis ($>$4.5 km and $<$7 km) is dominated by calcitization of anhydrite, precipitation of post-bitumen calcite cement, and formation of elemental sulfur. The chemical composition of post-bitumen calcite cement suggests precipitation from a pore water similar to the present Smackover brine. The high temperature replacement of anhydrite by calcite, the presence of post-bitumen calcite cement, the sulfur isotopic composition of sulfates and elemental sulfur, and the high concentration of hydrogen sulfide indicate that rapid thermochemical sulfate reduction of solid sulfate is taking place in the deep part of the Mississippi salt basin at temperatures of $>$150 $\sp\circ$C. Availability of large volumes of pore water during shallow burial and lack of a light carbon source resulted in progressive $\delta\sp{18}$O depletion and constant $\delta\sp{13}$C values in shallow burial calcite cements. Limited volumes of pore water during deep burial and incorporation of hydrocarbon-derived-light carbon generated by thermochemical sulfate reduction resulted in progressive $\delta\sp{18}$O enrichment and $\delta\sp{13}$C depletion in deep burial calcite cements. Trace element composition of shallow and deep burial calcite cements indicates that the Sr/Ca and Mg/Ca ratios of associated pore waters increased and decreased, respectively, during burial.

Pages

286

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