Distribution of the C/S Ratio in the Bazhenov Formation in Western Siberia and the Possibility of Its Use to Clarify the Genesis of Black Shales

The work is devoted to the study of distribution patterns of the organic carbon/sulfide sulfur ratio (C/S) in Upper Jurassic–Lower Cretaceous Black Shale Bazhenov Formation in Western Siberia. Possibilities of using this indicator to clarify the genesis of black shales are considered. It was revealed for the first time that rocks subjected to silicification in diagenesis are characterized by increased C/S values (5 ± 1), while a significant linear C/S relationship, characteristic of normal marine sediments, is retained. It has been confirmed that the catagenetic pyritization, transformation, and thermal degradation of organic matter reduce this indicator (often <2). Together with the dolomitization and kaolinitization, these processes provoke changes in the initial C and S concentrations and violation of the linear C/S relationship. It has been established that the Bazhenov Formation sections, in which the maturity degree of OM in rocks corresponds to the beginning of the main oil generation zone (Treservoir = 60–70°C, $$R_{{{\text{vt}}}}^{^\circ }$$ = 0.5–0.65) are characterized by a high linear C/S relationship (r = 0.75–0.9). Sections subjected to intense oil generation at the “oil window” stage (Treservoir = 90–120°C, $$R_{{{\text{vt}}}}^{^\circ }$$ = 0.85–1.15) lack the C/S correlation. It is shown that the C/S ratio serves as an effective criterion for identifying groups of black shales subjected to transformations at different stages of lithogenesis, assessing the transformation degree of the material composition of sections during the catagenesis, and rejecting significantly altered rocks from the analysis during paleoreconstructions.