The composition of saturated and aromatic hydrocarbons (HC) of oils in the Central part of Eastern Siberia was studied by gas-chromatography and chromato-mass-spectrometry. It is shown that the source organic matter of all studied oils were formed in marine conditions, and the level of its thermal conversion maximum in oils from Riphean deposits of the Baikit petroleum region (PR). According to the thermal maturity of organic matter, as well as by the nature of the molecular mass distribution of saturated and aromatic HC containing long alkyl chain substituent, Riphean oil form a independent group. The second group consists of oils from Vendian-Cambrian sediments of the Katanga, and Nepa-Botuobian PR.