Carbon isotopy of individual aromatic compounds of petroleum for their geochemistry understanding

The article analyzes published and original data related to the carbon isotopic composition of individual aromatic compounds of fossil organic matter and oil. It has been shown that there is reliable evidence of the intramolecular isotopic heterogeneity of a number of molecules. For example the isotopically depleted carbon of the methyl group of alkylnaphthalenes and the terminal methyl of n-alkanes. The 13C inheritance from the biochemical precursor during the aromatization is also well documented in the example of diterpenes in the series abietic acid - dehydroabietane - simonellite - retene, as well as in the other terpene and steroid series. At the same time, there is evidence of carbon isotopic fractionation during the formation of several aromatic compounds from a single precursor. The increasing aromatization of the prebuild polycyclic structure does not change the 13C value of the molecule, and the formation of aromatic compounds with different numbers of aromatic rings in competing reactions leads to isotope differentiation in accordance with the thermodynamically determined distribution of carbon isotopes. If the suggestion is correct, it is the key to the understanding of specific petroleum aromatic hydrocarbons formation mechanism. It is possible that a comparison of 13C values for pairs of compounds formed during the transformation of one precursor will also provide information on the temperature conditions for the occurrence of the corresponding reactions.