--> ABSTRACT: C13 Depleted Authigenic Carbonate Buildups from Hydrocarbon Seeps, Louisiana Continental Slope, by Harry H. Roberts, Roger Sassen, Robert Carney, and Paul Aharon; #91029 (2010)

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C13 Depleted Authigenic Carbonate Buildups from Hydrocarbon Seeps, Louisiana Continental Slope

Harry H. Roberts, Roger Sassen, Robert Carney, Paul Aharon

Geohazard and geochemical survey data consisting of high-resolution profiles, side-scan sonographs, drop cores, dredge samples, and borings have substantiated the consistent association between carbonate buildups and hydrocarbon seeps on the Louisiana continental slope.

Analyses of lithified bottom samples indicate a range of carbonate mineralogies including aragonite, Mg-calcite, and dolomite that are extremely depleted in the C13 isotope (^dgrC13 values to - 48 ^pmil PDB). Microbial oxidation of methane (biogenic and thermogenic) and crude oil creates a source of pore-water CO2 containing isotopically light carbon which triggers carbonate precipitation. Geophysical and geochemical evidence suggests that both surface and subsurface lithification is taking place. Recent observations and samples collected using a Pisces class research submersible confirm the abundance of C13 depleted sedimentary carbonates and massive authigenic buildups associated with the tops and flanks of shallow salt diapirs and gas hydrate hills. Although chemosynthetic communities (including tube worms and bivalves) with isotopically light carbon in their tissues have been described from gas seeps, bacterial mats sampled from several seep areas using a submersible have ^dgrC13 values of - 28 to - 31 ^pmil PDB, suggesting a crude oil contribution to microbial biomass. Lithoherms 15 m in vertical dimension are common to dome crests. These features dominate mesoscale sea-floor topography on the slope and have important short-term impacts on platform locations and pipeline routing. They are of long-term importance as sites for low sea level reefs. Moreover, these observations provide new insight into the earliest stages of salt dome caprock evolution.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.