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During an extensive coring and geophysical survey of the outer slope of the northwest Gulf of Mexico, a 4-m (13-ft) core recovered from 538 m (1,765 ft) water depth had visible petroleum deposits. The petroleum was extracted and chemically and isotopically characterized. The percent of petroleum soluble in benzene ranged from 4.0 near the surface to 4.5 at 100 cm (39 in.) to 0.1 at 410 cm (161 in.). API gravities ranged between 10.7 and 17.6. GC analysis of the silica gel fractions showed that both the saturated and aromatic hydrocarbon components are highly biodegraded, with the degree of degradation increasing upward in the core. The ^dgr13C values for the whole oil and the fractions were between -26.2 and -26.7^pmil on the PDB scale and showed very little va iation among compound classes. Carbonate nodules contained in the oil-rich core had carbon isotopic compositions depleted in 13C, indicating oxidized organic matter is the source of the inorganic carbon. The core is located in an area with large-scale normal faulting which also shows evidence of gas migration. Based on this and the distribution of oil in the core, it is supposed that the oil has migrated upward.
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