Title

Carbonates of the Louisiana continental slope

Document Type

Conference Proceeding

Publication Date

1-1-1987

Abstract

The continental slope off central Louisiana has extremely complex surface topography as well as subsurface structures that are primarily inherited from salt tectonics, with other features generated by differential sedimentation, erosion, and mass movement. Dominant geologic features of the area include shallow diapirs, intras10pe basins, sediment deformation features, and various types of faults. Oil seeps, gas seeps, and gas hydrates are also common to the area, and numerous faults in this province provide migration pathways for fluids and gases. Although the sediments are dominantly terrigenous clays, carbonates are significant. Carbonates occur as highly calcareous hemipe1agics, carbonate-cemented clays, shell hashes, hardgrounds, mounds, and bioherms. Seismic reflection profiling has led to the conclusion that most surface topography is related to salt diapirs. However, both field and laboratory data suggest that numerous smaller scale topographic features are calcareous and are commonly linked to the precipitation of carbonate (dominantly aragonite). These zones of lithification appear to range from small localized nodules and cemented burrows to mounds with relief of up to tens of feet. Geochemical analyses of host sediments indicate the presence of crude oil and associated light hydrocarbons generated outside the realm of surface sediments. Microbial oxidation of methane provides an abundant source of CO in interstitial waters and triggers chemical precipitation of carbonates characterized by extreme depletion of the C-13 isotope (8 C-13 values from -30 to -48 permil). In addition, the seeps and associated calcareous hardgrounds provide a setting for carbonate-secreting organisms that require a hard substrate and perhaps a hydrocarbon-related nutrient source, thus encouraging the buildup of carbonates into significant topographic features. Carbonates in slope sediments cause practical problems for platform placement and pipeline routing. The problems range from the occurrence of sensitive soils, through sediments with strengths that vary greatly over short distances, to massive hard- and rough- bottomed regions. 2

Publication Source (Journal or Book title)

Proceedings of the Annual Offshore Technology Conference

First Page

373

Last Page

382

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