Significance of Anoxic Slope Basins to Occurrence of Hydrocarbons Along Flexure Trend, Gulf of Mexico: A Reappraisal

Menno G. Dinkelman, David J. Curry

Recently, Tertiary anoxic slope basins have been proposed as the sources for much of the oil occurring along the Flexure Trend in the Gulf of Mexico. The "intraslope basins" are thought to have been formed in response to salt diapirism and concomitant salt withdrawal resulting from differential sediment loading between the basins and the diapirs, as well as due to associated faulting. Of the modern intraslope basins, the black, organic-rich muds accumulating in the Orca basin have especially attracted attention and are suggested to be modern analogs to late Tertiary source rocks accumulated and buried across the continental slope.

Although the organic carbon content of the anoxic sediments in the Orca basin is generally high (2 to 3%), the concentration of preserved oil-generative organic matter in these sediments is low. Rock-Eval P2 yields are usually in the range of 340 to 1620 ppm, and hydrogen indices are generally less than 100. Pyrolysis-GC and 13C-NMR data show that up to 30 + % of the organic carbon is contained in carboxyl and other oxygenated groups, which are lost during diagenesis and early catagenesis of the sediments, and that much of the remainder is aromatized and degraded. The degradation was probably by oxidation during settling through the oxic water column. The geochemical data indicate, therefore, that the bulk of the organic carbon in the Orca basin is not capable of forming oil during catagenesis.

Published regional cross sections across the Texas-Louisiana continental margin commonly show a thick (0.5-4 km), continuous salt sequence, sourcing salt diapirs and ridges, to underlie the Oligocene(?)/Miocene to Pleistocene sedimentary section of the outer continental shelf and slope. However, interpretation of regional multichannel seismic data indicates the salt sequence, in addition to local diapirism, to be locally thickened, stretched, thinned, and perforated as a result of basinward flow or creep of broad and extensive salt tongues and sills, thereby allowing for stratigraphic contact between the upper Tertiary sediments above the salt and the older ones below. Analysis of the seismic data also has failed to reveal the existence of Miocene/Pliocene basins similar to the Orca b sin. Geochemical analyses of Cretaceous basinal marine sequences drilled and recovered in Deep Sea Drilling holes and comparison with organic geochemical data of oils and sediments from wells on the shelf and upper slope give strong support that Cretaceous sediments underlying the Texas-Louisiana Continental Margin are a significant source to the hydrocarbon volume along the Flexure Trend.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.