Abstract

As part of a program to evaluate the influence of environment on the composition of organic matter in marine sediments and the changes in composition that occur in the upper zones of sediment, cores from several basins representing different environments off southern California were examined for extractable organic matter, hydrocarbon content, and certain other properties. The data are of particular interest in considerations of the early stages of petroleum formation.

Hydrocarbon fractions were separated from extractable organic matter by chromatography on alumina. This technique permitted semiquantitative separations of a paraffin-naphthene hydrocarbon fraction and an aromatic fraction. The aromatic fraction, in the present work, contained large amounts of nonhydrocarbon aromatic compounds, and a more refined procedure seems necessary for a detailed study of aromatic hydrocarbon distribution.

While the number of samples is too small to allow definite geologic conclusions, the following are indicated:

(1) The sediment in the shallowest basin with the fastest sedimentation rate contains the highest proportion of material soluble in organic solvents and the highest hydrocarbon content even though this sediment is lowest in total organic matter.

(2) Hydrocarbons comprise 2.3–18.6 per cent of the extractable organic matter and 0.05–0.64 per cent of the total organic matter in the three basins examined. The hydrocarbon content does not differ greatly in the different basins, nor with depth (to 4 meters) in a given basin. Variations with depth appear to be random.

(3) Pyrolysis of sediments at 500° C (modified Fischer retort assay) shows only random variations with depth to 4 meters in a given basin, and the average yield of oil indicates about 7–9 per cent conversion of organic matter to oil. Compared to typical oil-shales, which give 50–75 per cent conversion, this low conversion indicates that basin sediments and oil-shales differ significantly either in the nature of the organic matter or in catalytic activity of the mineral components.

(4) The average potential oil yield, computed from hydrocarbons extracted from the sediments, is about 10 times the eventual recovery from oil fields of the Los Angeles Basin, but is only one-tenth the yield of oil obtained by pyrolysis.

(5) The proportions of hydrocarbons and more complex materials separated from the sediments differ from their proportions in petroleum. These differences may result from later changes and/or fractionation during migration from source beds.

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