Lithology, geochemistry, and aspects of separation of the Kogalym Member within the Lower Vasyugan Subhorizon (Upper Bathonian-Lowermost Oxfordian) of West Siberia

It is proposed to separate a regionally persistent member of fine-dispersed mudstones as the individual Kogalym Member within the Lower Vasyugan Subhorizon (Uppermost Bathonian-Lowermost Oxfordian). We present data on the lithologic composition, geophysical characteristics, and geochemical properties of the organic matter and depositional environments of the member. The Tyumenskaya SG-6 well is proposed as a type section of the Kogalym Member, where a biostratigraphic approach has yielded the Middle-Upper Callovian volume of the member. We also discuss the aspects of specification and detalization of the Callovain-Oxfordian part of the Callovian and Upper Jurassic regional scheme of West Siberia.