Hydrocarbons in Sediments of the Gulf of Mexico1
Complex hydrocarbon-asphaltic mixtures appear to be common constituents of the organic debris buried in soils and Recent marine sediments, although generally in very small concentrations. Chromatographic, infrared, and mass spectra analyses show that the composition of these mixtures differs significantly from those comprising crude oils.
In a large and varied group of crude oils, the hydrocarbon-asphaltic portion fractionated by chromatography was found to comprise at least 65 per cent hydrocarbons, whereas in extracts of soils and Recent muds from the Gulf, hydrocarbons usually comprise less than 20 per cent. Infrared spectra of the benzene fraction of the mud extracts show a relatively simple mixture of aromatic compounds in contrast to complex mixtures found in crude oils. Mass spectra data reveal that the high molecular weight normal paraffin hydrocarbons extracted from Recent muds and soils have a strong preference for molecules containing an odd number of carbon atoms. Molecules containing 29 or 31 carbon atoms were the most abundant. No odd carbon preference was observed for crude oils or a postulated source bed, the Woodford shale.
Radiocarbon age determinations show that the organic matter in the Recent muds of the Gulf, used in this study, ranged in age from 3,120 ± 220 to 9,360 ± 600 years. Mixtures of generally similar composition were extracted from soils less than 500 years in age. Hydrocarbon-asphaltic mixtures from the older Beaumont clay were found to be more variable in composition, in several instances having some properties associated with the Woodford (Mississippian) shale, a postulated source bed.