Lithologic and facies characteristics of the Lower Cambrian Usolka Formation and its age analogs in the Cis-Yenisei sedimentary basin, West Siberia

Based on the correlation between Lower Cambrian sections of deep wells on the left bank of the Yenisei River and outcrops in the westernmost Yenisei Ridge, the sedimentologic characteristics of the Usolka Formation and coeval deposits are compared. Three paleogeographic regions (subbasins) are distinguished in the Cis-Yenisei basin in Usolka time. The leading role in the sediment genesis was played by the region of barrier reefs stretching northwestward and northward as a approximately 70 km wide band to their joint with the reefs of the Igarka-Noril'sk facies region in the north. The salt basin east of the barrier reef ridge was a bay of the East Siberian salt basin, separated from the latter by the Yenisei paleoislands (at the place of the Yenisei Ridge). Sedimentation in that basin was controlled by evaporite processes, the supply of terrigenous material from the islands and carbonate debris from the opposite side (barrier reef), and the predominance of storm processes. The subbasin west of the barrier reefs evolved under different scenario. Since the Early Cambrian, a trough has been forming here, which was initially characterized by a regime of starvation and the first appearance of carbonaceous rocks (Lower Churbiga Subformation), regarded as Domanic oil source rocks, in the general Cambrian section. On the estimation of the prospects for the distribution of reservoirs whose origin depends on the facies regularities, the leading role is played by the barrier reef region. Importantly, the Lower Cambrian section of the Cis-Yenisei basin contains two large adjacent paleogeomorphologic structures and the corresponding facies zones of a starved prereef depression with the Domanic oil source rocks of the Lower Churbiga Subformation at the early stage of evolution and a potentially productive barrier reef system with a steep slope toward the basin. The favorable combination of these factors suggests that the Cis-Yenisei basin has a high petroleum potential.