Organic geochemistry of an oil and gas seep in northern Gulf of Mexico sediments

Abstract

During an extensive geochemical and geophysical survey of the outer slope of the northwestern Gulf of Mexico nine piston cores were recovered which had visible liquid organic deposits. In three of the cores deposits were observed concentrated in oblique fracture planes. Other occurrences included large deposits as liquid veins and smaller disseminated pockets in gassy and non-gassy sediments. The benzene soluble material (bitumen) was extracted and chemically and isotopically characterized. Bitumen content ranged as high as 8.6 percent in sediment samples. Gas chromatographic analyses of silica gel fractions showed that both the saturated and aromatic hydrocarbon components are highly biodegraded. The δ¹³C values for the whole oil and fractions were between -26.2 and -26.7 per mil on the PDB scale which closely resembles other Gulf coast oils. The δD values of the oil averaged -104 per mil relative to SMOW. Carbonate nodules found in the oil-rich zones were ¹³C depleted, indicating oxidized organic matter to be the source of the inorganic carbon. Several cores contained natural gas in concentrations high enough to result in large expansion gaps under the reduced ambient pressure at sea level. Hydrocarbon gases from methane through pentanes were sampled in nine cores. Chemical composition and δ¹³C values for methane, ethane, propane, and butanes (-30.5 to -61.9, -28.5, -24.5, -25.7 per mil) indicated that the gas has a major petrogenic component. δD values for methane, ethane, propane and butanes were -172, -101, -104, -101 per mil. Compositional variability of C₂⁺ gases among cores suggests the possible regional influence of gas hydrate formation. Compositional and isotopic variability of methane within and between cores does not conform to a two component mixing model (e.g. biogenic plus petrogenic methane). Instead, highly localized processes, possibly microbial, are implicated