The bituminoids extracted from Vendian and Cambrian oil-saturated deposits of the southeastern slope of the Nepa-Botuobian Anteclise, adjacent to the cis-Patomian trough, have been studied in detail. Compositionally, the naphthides saturating the deposits are referred to oil and sometimes to maltha. There were variations noted in naphthide hydrocarbon composition through the sections of individual wells and in the direction from the marginal part of the anteclise towards its center, the Nepa arch. The character of the naphthide composition variations support the idea of HC migration during their accumulation according to the well-known Maksimov-Gussow scheme. The set of relict methane-naphthenic hydrocarbon naphthides in the Vendian deposits and in the Lower Cambrian Osa horizon are genetically close and related to the organic matter in the Upper Precambrian. The naphthides of the upper Bulai Formation (Cambrian) are characterized by peculiarities in composition of methane-naphthenic hydrocarbons. As opposed to the rest, these naphthides reveal no 12- and 13-methylalkanes but abundant diasteranes. These naphthides show biodegradation evidence also. Black shales enriched in organic matter, which are locally distributed within the Bulai deposits, most probably may be a source of the naphthides.

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