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 fractionatio may be involved and transformation of non-lipid materials, therefore, may occur during petroleum formation.

The C-13/C-12 ratio of the aromatic-enriched fraction of a petroleum does not differ by more than one per mil from the ratio of the paraffinic-naphthenic fraction. This indicates that changes in chemical composition alone are not responsible for the 10-per-mil range of isotopic composition noted for petroleums. Natural gases from petroliferous areas have considerably lower C-13/C-12 ratios than associated petroleums. In view of the narrow range of isotopic composition in petroleums, gas formation does not appear to be an important natural mechanism for depleting oil fields. The isotope ratios of organic carbon in ancient sediments fall within the isotope ratio range of petroleums.