Peculiarities of Oils Content in Pre-Cambrian of the Siberian Platform

Bazhenova, Tatjana¹, Olga K. Bazhenova², N. P. Fadeeva² (1) All-Russia Petroleum Research Exploration Institute (VNIGRI), St. Petersburg, Russia (2) Moscow Lomonosov State University, Moscow, Russia

The objects of investigate are Precambrian oils Ïf the Megabasin of the Siberian Platform. The distinct subdivision of the Siberian Platform’s Megabasin into 3 stages: aulacogenic (R), homogenic (V-D1) and heterogenic (D2-T) is a peculiarity of the basin. Riphean aulacogens experienced the pre-Vendian inversion responsible for the formation of block-folded structures.

The commercial oil-gas deposits are known in Riphean (R) and Vendian (V)reservoirs, main oilbering regions Nepsko-Botuobinsk and Bajkits anticlines.

The Precambrian oils have the following specific features: light isotopic composition (-30-34%o), presence of homological sequence 12-13 methyl-_substituted alkane, high rela­tive concentration of tricyclic HC -cheilantanes C19-C31, distinct domination of ethyl­holestane among steranes -70-80%- typical of the oils of Nepsko-Botuobinsk anticline. However, the former distribution other type-.uniform is also present in some oils (Atovskoe, Kovenskoe fields).

The oils of Kuyumbinskaya area (Bajkitsk anticline) are also characterized by prevalence of ethylholestane, but its domination is not as apparent as in the oils from other regions of the Siberian Platform (C27:C28:C29- 23:22:55). Also oil with uniform distribution of i-ster-anes (34:27:39) is present. Different types of oils according to the composition of biomark­ers characterized by different types of initial organic matter. The initial Precambrian OM was represented by phytoplankton ) and phytobenthos. Vendian OM are characterized by more significant admixture of phytobenthos, these determined the high concentration of ethyl­holestane in oils. It possible to suggest: the oils with sharp prevalence ethylholestane – Nepsko-Botuobinsk anticline generated Vendian OM, oils with uniform distribution - from R source rocks, oils from Kuyumbinskaya area are mixed.