William R. Leger
The Black Bayou field is associated with a salt dome that pierces Miocene sediment and rises to within 900 ft (275 m) of the surface. The Louisiana Gulf Coast regional geothermal gradient is locally affected by the salt dome. The gradient increases to values greater than the regional gradient, 1.26°F/100 ft (23°C/km), near the dome. Local effects of the salt dome on clastic diagenesis have been determined by studying sandstone samples adjacent to and away from the salt dome within Miocene sediment. Sample depths range from 4,155 to 6,145 ft (1,266 to 1,873 m). Distances of samples from the edge of the dome range from 82 to 820 ft (25 to 250 m).
During the late Oligocene, a widespread regression exposed the top of the dome. Throughout the Miocene, the salt dome probably remained at shallow depths, and upward movement of the dome occurred contemporaneously with deposition of Miocene sediment. Diagenesis of Miocene sediment began with formation of quartz overgrowths. These were followed by deposition of early calcite and pyrite from meteoric waters at shallow depths. Secondary porosity is abundant near the dome where dissolution of calcite and framework grains has occurred. Precipitation of kaolinite, in primary and secondary pore spaces, followed this dissolution, but kaolinite is absent at distances greater than 325 ft (100 m) from the edge of the dome. Clay analysis revealed that alteration of expandable to nonexpandable cla s has not occurred. Although higher than regional thermal gradients and possible Na-rich pore fluids exist near the dome, plagioclase grains have not been albitized.
Fluid circulation patterns around the east flank of the dome are controlled by both density- and pressure-induced flow. Density-enhanced meteoric water containing dissolved salts migrates down along the east flank of the dome, and warm geopressured fluids from Oligocene shales flow up along the flank into Miocene sediment. These vertically migrating fluids are acidic, perhaps resulting from sulfate reduction of salt-dome associated minerals. Also, deep, heated fluids could contain organic acids originating in the shaly interval below the base of the Miocene section.
AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.