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

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

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

Analyses 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 chemosyn hetic communities (including tube worms and bivalves) with isotopically light carbon in 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 not unusual on dome crests. These features dominate mesoscale sea-floor topography on the slope and have important short-term impacts on platform locations as well as 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 cap-rock evolution.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.