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Analyses were made on 159 sediment samples from Choctawhatchee Bay, Florida, to determine the distribution and significance of bitumen (benzene-soluble organic substances), particularly the hydrocarbons. Column and gas chromatography was used to characterize the bitumen.
In the upper 10-16 cm of estuarine mud, which contains an average of 3.6 percent organic carbon, the bitumen content ranges from 60 to 470 ppm and averages 170 ppm, or about 0.5 percent of the organic carbon. In the relatively clean quartz estuarine sand the bitumen content averages only 20 ppm, or 1.3 percent of the average organic carbon (0.15 percent). The hydrocarbons make up an average of about 50 percent of the bitumen in the mud and an average of about 22 percent of the bitumen in the sand.
The distribution in the mud of normal paraffins C19 to C33 is remarkably uniform among the samples, and shows a marked predominance of odd- over even-carbon-number molecules (average odd over even ratio is about 8), characteristic of a land-plant source of hydrocarbons. In contrast, the n-paraffin distribution in the sand is quite variable, and in many samples the odd over even ratio is less than 2, suggesting brackish-water and marine organisms as a major source of hydrocarbons.
Results of this study and related studies show that recent sands contain bituminous substances, including hydrocarbons, in geologically significant amounts. The inference is drawn that, if the bitumen disseminated in large volumes of interconnected sands is converted, even in part, to crude oil and concentrated in pools, the sands themselves may contribute substantial amounts of the petroleum crude oil in sandstone reservoirs.
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