Lithological and Geochemical Evidence of Catagenetic Transformations in Black Shales Based on the Example of the Western Siberian Bazhenov Formation

The regularities were determined for changes in the lithological and geochemical composition of rocks of the Bazhenov Formation (BF) characterized by various degrees of transformation of organic matter (OM). From the M K₂^1 to MK2 stages (140–170–195°C), trends are seen toward (a) an increase of the quantity of limestone–dolomitic interlayers along with the dolomite content; (b) a decrease of the OM content over the entire section; (c) a decrease of the OM and sulfide sulfur concentrations in the pyrite–kerogen band of the roof of the BF; (d) a decrease of the C/S ratio; and (e) an increase of the thickness of the pyrite layer in the transition zone between the BF and overlying sediments along with the growth of the pyrite content. It was concluded that the interval between the M K₁^2 and MK2 stages experienced OM transformation, the migration of transformation products, dolomitization of the rocks, and pyritization of low-carbon rocks in the BF-overlying Podachimovo strata.