ABSTRACT: The Nature and Origin of Barite Deposits Associated with Hydrocarbon Seeps on the Gulf of Mexico Slope Offshore Louisiana
AHARON, PAUL, and BAOSHUN FU
Observations from submersibles coupled with laboratory studies over the past decade led to the generalization that the chief geologic product of hydrocarbon seepage on the Gulf of Mexico slope is the massive abiotic, chemical, carbonates whose carbon is derived from microbial breakdown of hydrocarbons under anaerobic conditions. Here we report that, in addition to carbonates, barite deposits are also intimately linked with seeps and present results of recent laboratory investigations that shed light on their nature and origin.
Indurated deposits consisting of chimneys, cups and crusts--blanketing mud volcanoes were recently recovered from hydrocarbon seep sites in Garden Banks No. 382 and Mississippi Canyon No. 929 at 510 m and 640 m water depths, respectively. Microscopy and SEM analyses reveal that chimneys and cups are dominated by barite (BaSO4) whereas the crusts are composed of barite coexisting with calcium carbonate. Chemical assays performed with non-destructive instrumental techniques indicate that Sr and Ca isomorphically replace Ba in the crystal lattice of the barite. The sulfur and oxygen isotope compositions of barite chimneys and cups are similar to those of modern seawater sulfate. In contrast, the crusts are anomalously enriched in both 34S and 18O relative to the ambient seawater suggesting partial removal of the sulfate by sulfate-reducing bacteria in a closed or semi-closed pore fluid system within the host clastic sediments. Whereas the sulfate in the barites is clearly derived from ambient seawater, the source of Ba, Sr and Ca is in fluids advecting to the seafloor and derived from formation waters ascending through fault conduits from deap-seated reservoirs.
AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana