GEOCHEMICAL INDEXES OF THE HYDROCARBON AC CUMULATION OF THE NEPA-BOTUOBA ANTECLISE (SIBERIAN PLATFORM)

The bitumoids extracted from Vendian and Cambrian oil-saturated deposi ts on the southeastern slope of the Nepa-Botuoba Anteclise adjacent to the cis-Patomian trough were studied in detail. Compositionally, the naphthides saturating the deposits are referred to oil and sometimes t o maltha. There were variations noted in naphthide hydrocarbon composi tion through the sections of individual wells and in the direction fro m the marginal part of the anteclise towards its center, the Nepa arch . The character of the naphthide composition variations support the id ea of HC migration during their accumulation according to the well-kno wn Maksimov-Gassow scheme. In the set of relict methane-naphthenic hyd rocarbon 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 (C ambrian) are characterized by peculiarities in composition of methane - naphthenic hydrocarbons. As opposed to other these reveal the absenc e of 12- and 13-methylalkanes and the presence of a great amount of di asteranes. These naphthides show biodegradation evidence also. Black s hales, enriched in organic matter, locally distributed within the Bula i deposits most probably may be the source of the naphthides.