Composition and formation environments of the Upper Jurassic–Lower Cretaceous black shale Bazhenov Formation (the central part of the West Siberian Basin)

Most researchers consider the black shale Bazhenov Formation as the main source of petroleum in the West Siberian fields. Detailed sedimentological, mineralogical and geochemical investigations reveal that there are three main types of Bazhenov Formation rocks: (i) clayey-siliceous; (ii) argillites; and (iii) anomalous clayey-silt-sandy rocks. The first type is characterized by more frequent benthic and planktonic fossils, extreme reducing conditions and lower rates of sedimentation in comparison with the argillites. The clayey-silt-sandy anomalous rocks are non-fossiliferous, were deposited in oxic conditions, and are characterized by the highest rates of sedimentation. We conclude that the clayey-siliceous rocks were deposited in moderately hemipelagic environments, the argillites were products of distal turbidities, and the anomalous clayey-silt-sandy rocks were deposited in channels.