Petroleum - derived authigenic carbonates of the Louisiana continental slope

Seismic and echo-sounder profiles across Louisiana's continental slope have historically suggested the presence of abundant hydrocarbons in shallow subsurface sediments and in the water column. Various mineralogical types of authigenic carbonates are common in hydrocarbon seep areas. Significance of a substantial carbonate component to the sedimentary architecture of the Mississippi delta complex has never been seriously evaluated. Large topographic variations on the slope are related to salt tectonics. Close inspection of these features (geohazards data and cores) suggests that they are largely carbonate capped. These carbonates range from shell hashes through cemented clays and hardgrounds to massive mound-like buildups ten of meters in relief. Analysis of host sediments commonly reflect the presence of biogenic gas generated in situ and thermogenic gas and associated crude oil generated outside the realm of surface sediments. Numerous faults act as conduits for fluids and gases from the subsurface. Isotopic values of the authigenic carbonate (δC-13 values to -48 o/oo PDB) reflect a link with hydrocarbons. Microbial oxidation of methane and heavier hydrocarbons provides a source of CO>sup>2 in interstitial waters and triggers chemical precipitation of carbonates (primarily aragonite and Mg-calcite) characterized by extreme depletion of the C-13 isotope.