Authigenic Kaolinite in the Upper Jurassic–Lower Cretaceous Bazhenov Formation of Western Siberia

The distribution pattern and morphology of microaggregates of the authigenic catagenetic kaolinite filling the pore space in rocks of the Bazhenov Formation are studied. The distribution of its various microstructural types is analyzed: (1) in sections with different degrees of catagenetic transformation of OM; (2) in sections of different facies areas; and (3) within and outside the oil-saturated intervals for two sections (western Kvenzer and Salym areas). For the first time, three types of kaolinite with different microtextures were discovered and described in the Bazhenov Formation: colloform/transitional (K1), vermicular (K2), and “house of cards” (K3). It is assumed that type 1 kaolinite (K1) was formed during the early catagenesis (T > 60°C) in solitary pores under conditions of weak permeability of host rocks. Type 2 kaolinite (K2) could be formed at: (a) the later stages of early catagenesis in rocks that underwent notable transformations of OM, but lacked significant petroleum generation; (b) during moderate catagenesis in layers at contact with the petroleum-productive interval during active generation of petroleum hydrocarbons (T > 70‒100°C). Type 3 kaolinite (K3) was formed during a notable thermal transformation of OM (T >100‒140°C) in rocks of the Bazhenov Formation in the southeastern region of the West Siberian sedimentary basin (western Kvenzer area). It was deposited to a lesser extent in the pore space of rocks in the petroleum-productive interval of the Bazhenov Formation in the Central region (Salym area) marked by the catagenetic (partial) dolomitization of rocks.