Carbon isotopic evidence for the role of lipids in petroleum formation

Abstract

Data available on stable carbon isotope ratios (C¹³/C¹²) are reviewed. It is concluded that data for modern and ancient natural carbonaceous materials reveal consistently low C¹³/C¹² ratios for substances of biologic origin as opposed to the ratios for inorganic carbonate minerals deposited in the oceans. Ratios, of modern marine organisms are about 1% higher than those of terrestrial organisms. The average C¹³/C¹² ratio of modern marine organisms is also about 1% higher than that of ancient organic matter in marine sediments. A similar relation exists between the ratios of terrestrial organisms and those of petroleums and other noncoaly organic matter genetically related to nonmarine sediments. Coals of all ranks have the same range of isotope ratios as those noted for terrestrial plants.

Lipid fractions of organisms have consistently lower C¹³/C¹² ratios than do the whole organisms. The average difference between nonlipid and lipid materials for all organisms studied is about 0.5% and ranges in individual species from as little as several hundredths to more than 1.5%. This suggests that petroleums and other noncoaly organic matter in ancient sediments are derived from lipids, or at least from certain components of the lipid fraction. In contrast, coal deposits apparently are derived from whole plants or from the cellulosic fraction of land plants, which is the major nonlipid constituent, of plant tissues.

The isotopic compositions of narrow distillation fractions of petroleums suggest that the lowmolecular-weight hydrocarbon components of the gas and gasoline fractions are formed by decomposition of higher-molecular-weight compounds during the post-depositional history of petroleum. Concurrently with the formation of low-boiling compounds, these transformation processes initiate the polymerization reactions responsible for the formation of some of the most complex petroleum components, such as the asphaltenes. New data obtained from distillates of Colombian crude oil are reported in this paper. The fraction with an average boiling point of 450C and average molecular weight of 420 had the lowest C¹³/C¹² ratio. This suggests that the fraction, which includes the boiling range of typical triterpenoids, contains a major proportion of the primary components from which the light and heavy ends of petroleum are derived. This implication is supported by other geochemical information, including the distribution of optical activity in petroleum fractions and the identification of specific petroleum components which are structurally related to lipid components of modern organisms.