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Carbon isotope studies of ancient and modern natural organic materials have helped reconstruct the geochemical history of petroleum and gas accumulations. Similarity of C13/C12 ratios of petroleums, and of the lipid fraction of plants, suggests that petroleum is derived from lipids, which are the most stable organic constituents of organisms. C13-content of petroleums derived from marine organisms is typically higher than that of petroleums of non-marine origin. This difference is a reasonable consequence of the fact that marine organisms have higher C13/C12 ratios than do non-marine organisms.
Typical gas phase hydrocarbons associated with liquid petroleums have C13/C12 ratios that are 10-22 per mil lower than those of the associated liquid phase components. In contrast, methane produced by bacterial decomposition of more complex organic matter is 40-70 per mil lower in C13/C12 ratio than the organic matter from which it formed. The isotopic composition of narrow distillation fractions of the liquid petroleum phase implies that the low molecular-weight hydrocarbons in the gas and gasoline fractions are formed by decomposition of higher molecular-weight petroleum components.
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