MAIN OIL SOURCE FORMATIONS OF THE WEST SIBERIAN BASIN

Two strongly oil-prone source intervals are present in the West Siberi an Basin, the Togur Formation (Early Toarcian, Early Jurassic) and the Bazhenov Formation (Violgian-Berriasian, Late Jurassic to Early Creta ceous). Clay sediments of the Togur Formation were deposited in a syst em of intracontinental sub-basins in the southern half of the West Sib erian Basin. The marine deposits of the Bazhenov Formation accumulated throughout the entire basin. Two types of extractable bitumens and oi ls - marine and non-marine - may be identified from their isotopic car bon composition, sulphur content, bulk composition, and biomarker hydr ocarbons (normal alkanes, pristane/phytane, C27-30 steranes, hopanes a nd homohopanes). Marine oils, accounting for 80% of oil resources, are sourced from the Bazhenov Formation. The genetic relationship between oils in Upper Jurassic and Cretaceous reservoirs and the organic matt er from the Bazhenov Formation is indicated by great similarities in t heir biomarker hydrocarbon compositions and their maturation levels. N on-marine (lacustrine) oils are sourced from the Togur Formation and a ccount for 11% of oil resources.