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Cap Rock Geology of the Hazlehurst Salt Dome, Copiah County, Mississippi
Charles T. Swann (1), James A. Saunders (2)
Hazlehurst Dome is a shallow, sparsely explored, salt dome near the center of the Mississippi Interior Salt Basin. Two recent sulfur exploration wells cored marine false carbonate cap rock, true calcite cap rock, and anhydrite cap rock. The marine false carbonate cap rock contains calcite, dolomite, glauconite, and marine microfossils. This unusual dolomite-enriched unit has a 13C value of -8.2(PDB), suggesting a predominantly marine source for the carbon. The true calcite cap rock contains alternating light and dark bands of calcite, the darker bands containing locally abundant sulfide minerals. The range of carbon isotope values for the banded true calcite cap rock, from -45.1 to -50.20(PDB), indicates that methane was the principal source of carbon. This cap rock is locally crosscut by steeply dipping calcite veins, which are symmetrically banded, with alternating bands of dark-brown calcite (containing fluid inclusions of liquid hydrocarbons), white calcite, and sulfide minerals. Carbon isotope values for the veins are similar to the true calcite cap rock itself.
An exotic suite of cap rock minerals, consisting of sphalerite, galena, strontianite, celestite, and barite, is locally present in both the horizontally banded cap rock and its veins. The proximity of Hazlehurst Dome to present-day oil field brines enriched in Sr-Ba-Zn-Pb suggests a genetic link between the brines and cap rock minerals containing the same elemental suite. Carbon isotope data from calcite in both true calcite cap rock and veins indicate that sulfate-reducing bacteria played a major role in the precipitation of the calcite and sulfide minerals. The presence of liquid hydrocarbons in the late-stage calcite veins implies that liquid hydrocarbons moved into the cap rock late in its developmental history, and sulfate-reducing bacteria preferentially utilized methane, thus preserving the liquid hydrocarbons. This initial study of the Hazlehurst calcite cap rock suggests that the dome warrants further evaluation for hydrocarbon potential. The anhydrite cap rock has yet to be completely penetrated and only the upper section was sampled by the sulfur exploration wells. The anhydrite cap rock contains as much as 3% sulfide minerals crudely layered in the interlocking anhydrite matrix.
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