The patterns of distribution and morphology of micro aggregates of authigenic late diagenetic kaolinite filling the pore space of rocks of the Bazhenov formation are studied. The analysis of the distribution of its various microstructural types is carried out: 1) in sections with different degrees of late diagenetic transformation of organic matter; 2) in sections of different facies areas; 3) within and outside the oil-saturated intervals for two sections (Zapadno-Kvenzerskaya and Salymskaya areas). For the first time, three types of kaolinite with different microstructures were discovered and described in the Bazhenov formation: collomorphic/transitional (K1); vermicular (K2); "house of cards" (K3). It is assumed that the first type of kaolinite (K1) was formed at the initial stage of late diagenesis (T > 60°C) in single pores, under conditions of weak permeability of the host rocks. The second type of kaolinite (K2) could be formed a) at the later stages of late diagenesis, in rocks in which significant transformations of organic matter occur, but significant oil and gas generation has not yet occurred; b) in late diagenesis in border layers with oil-producing interval with active generation of petroleum hydrocarbons (T >70-100°C). The third type of kaolinite (K3) was formed during a significant thermal transformation of organic matter (T >100-140°C) in the rocks of the Bazhenov formation of the Southeastern region of the West Siberian sedimentary basin (Zapadno-Kvenzerskaya area). To a lesser extent, it was deposited in the pore space of rocks of the oil-producing interval of the Bazhenov formation of the Central District (Salymskaya area), where late diagenetic (partial) dolomitization of rocks was widespread.
