Carbon Isotopic Compositions of Petroleums and Other Sedimentary Organic Materials

Abstract

Carbon isotope analyses of petroleums, modern plant constituents, and other sedimentary organic materials indicate that the range of C-13/C-12 ratios in organic carbon is about 4.5 per cent. The range of variation in petroleums alone is about one per cent. Ratios of marine organisms average about 10 per mil higher than non-marine organisms; petroleums of marine origin are correspondingly higher in C-13 content than non-marine petroleums. Petroleums have lower C-13/C-12 ratios than their biologic sources have. The lipid fractions of plants, however, are isotopically lighter than the whole plants, so that isotope fractionation is not necessary if petroleum is derived from lipid materials. Analyses of soluble organic matter in Recent sediments imply that isotope fractionation may be involved and transformation of non-lipid materials, therefore, may occur during petroleum formation.

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