ABSTRACT

The Black Bayou Field is associated with a salt dome which pierces Miocene sediment and rises to within 900 ft 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 4155 to 6145 ft. Distances of samples from the edge of the dome range from 82 to 820 ft.

During the late Oligocene a widespread regression exposed the top of the dome. It was subsequently buried, and throughout Miocene time the salt dome probably remained at shallow depths. Upward movement of the dome occurred contemporaneously with deposition of Miocene sediment. Diagenesis of Miocene sediment began with formation of quartz overgrowths. This was followed by precipitation of calcite and pyrite by meteoric water at shallow depths.

Today, secondary porosity is abundant near the dome. Dissolution of calcite and framework grains has occurred. Precipitation of kaolinite, in primary and secondary pore spaces, followed this dissolution. Kaolinite is absent at distances greater than 325 ft from the edge of the dome. Analysis of clays reveals that alteration of smectite to illite 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; burial temperatures are less than 175°F (80°C).

Present fluid circulation patterns around the east flank of the dome are controlled by both density- and pressure-induced flow. Meteoric water, with density enhanced by dissolved salts, migrates down along the east flank of the dome, and warm, geopressured fluids, originating in Oligocene shales, flow up along the flank into Miocene sediment. These vertically migrating fluids are acidic which may result from sulfate reduction of salt-dome associated minerals. Also, deep-heated fluids could contain organic acids originating in the shaley interval below the base of the Miocene section.