Two strongly oil-prone source intervals are present in the West Siberian Basin, the Togur Formation (Early Toarcian, Early Jurassic) and the Bazhenov Formation (Volgian-Berriasian, Late Jurassic to Early Cretaceous). Clay sediments of the Togur Formation were deposited in a system of intracontinental sub-basins in the southern half of the West Siberian Basin. The marine deposits of the Bazhenov Formation accumulated throughout the entire basin. Two types of extractable bitumens and oils - marine and non-marine - may be identified from their isotopic carbon composition, sulphur content, bulk composition, and biomarker hydrocarbons (normal alkanes, pristane/phytane, C (sub 27-30) steranes, hopanes and 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 matter from the Bazhenov Formation is indicated by great similarities in their biomarker hydrocarbon compositions and their maturation levels. Non-marine (lacustrine) oils are sourced from the Togur Formation and account for 11% of oil resources.