Carbonate buildups on the continental slope off Central Louisiana

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Conference Proceeding

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Hydrocarbon exploration and production on the Louisiana continental slope have produced data derived from high-resolution geohazards surveys which indicate that enormous volumes of authigenic carbonates are present. Geochemical analyses of these carbonates show a wide range of mineralogies as well as 6 13C values (-25 to -55 • / •• PDB) indicating an origin from microbial oxidation of hydrocarbons. Broad variation in 6 C values is thought to pnmar-i1y reflect various parent hydrocarbons ranging from biogenic methane to thermogenic gas and crude oil. Recent research submersible work in two areas known for extensive carbonate seafloor, Green Canyon Blocks 52-53 and 184-185, has confirmed considerable variability of seafloor features related to authigenic carbonate production. Authigenic carbonates range from small nodules In unconsolidated slope sediments to massive buildups in excess of 60 ft (-20 m) vertical relief. A complete spectrum of intermediate forms between these extremes was observed during submersible operations, suggesting a wide range of seafloor conditions and geotechnical and/or engineering properties. In addition to the fact that these carbonates are geologically important because they represent salt dome cap rocks in early stages of development, slope carbonates present substantial challenges for planning drilling/production platform placement, and for pipeline routing. Although high resolution seismic and side-scan sonar data have been used to identify areas of carbonate seafloor, their signatures are frequently similar to mud mounds and other soft-bottom fea-Positive identification of carbonate hardgrounds, their relief, and their thicknesses may be difficult using typical remotely sensed hazard survey data. Observations from the submersible, for example, suggest that the topographic variability of carbonate buildups over shallow subsurface salt diapirs is much more dramatic and complex than analysis of seismic and side-scan records has previously indicated.

Publication Source (Journal or Book title)

Proceedings of the Annual Offshore Technology Conference

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