Abstract—: A representative suite of 276 samples was used to study the isotopic and elemental geochemistry of kerogens from the Bazhenov horizon (Bazhenov Formation and its age equivalents) of the western Siberian sedimentary basin and to map the variations in the elemental composition of kerogen in the area. The elemental composition of the kerogen was used to determine the types of organic matter (H and C contents), the initial components of the living matter that were the source of the kerogen (H and N contents), the diagenetic history of organic matter (S content), and the level of catagenetic transformation of the kerogen (C and O contents). Kerogen from the central, western, and southern parts of the western Siberian basin, up to the pinch-out boundary of the Bazhenov horizon, shows strong enrichment in hydrogen (up to 8–9%) and 12С (δ13С‰ from –35 to –29), suggesting its derivation from polymer lipids of aquatic origin. Kerogen from the Bazhenov horizon (catagenetic grade MC1) in the northeast of the basin contains much lower hydrogen concentrations (2–4%). A map of the types of organic matter in the Bazhenov horizon is constructed.