Litologiâ i poleznye iskopaemye

The lithochemical features of clastic rocks of a number of stratigraphic levels of the Riphean stratotype (Ai, Bakal, Zigalga, Zilmerdak and Uk formations), as well as the distribution of detrital zircon populations in the sandstones composing them, are analyzed. Two main trends have been outlined in the formation of clastics that make up the stratotype section of the Riphean – the entry into the sedimentation area of material of the first cycle (petrogenic) and recycled (redeposited, lithogenic) material. It has been suggested that detrital zircon, which came from protosources into the sandstones of the Ai stratigraphic level, was then, apparently, repeatedly subjected to redeposition: crystals with close maxima on the probability density curve are present in sandstones of younger stratons of the Riphean type section, up to the Uk Formation. The most striking renewal of detrital zircon protosources occurs at the beginning of the Late Riphean. The arkosic and subarkosic sandstones of the Biryan Subformation of the Zilmerdak Formation contain both detrital zircon with ages similar to those we see in the sandstones of the Lower and Middle Riphean, and new zircon, characterized by Paleo- and Mesoproterozoic maxima on the probability density curve. A small part of them is repeated in the oolitic limestones of the Uk Formation, but some renewal of the proto-sources is also recorded here. Thus, the boundary of the Middle and Late Riphean was marked probably by a radical change in the age populations of detrital zircon, which can be associated with the formation of a fundamentally new system for transporting clastics from sources to sink.

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.

New data on the lithochemistry of clay-and-carbonate rocks of the Solikamsk Formation (Ufimian stage of the lower Permian) in the southwestern part of the Solikamsk depression of the Pre-Ural foredeep are presented. The cyclicity of sedimentation was determined by the introduction of aluminum-and-silicate materials into the evaporite and then into the marine basin by land waters. In the Solikamsk formation nine cyclites have been identified, in which the content of clay minerals in the rock first gradually increases and then relatively decreases (for carbonates (dolomite + calcite) the reverse distribution is inherent). The presence of sulfate layers is not related to the general direction of Solikamsk sedimentation, but only reflects individual periods of increased evaporitization. Analysis of the chemical composition of the terrigenous component of the Solikamsk rocks and the main paleoclimatic indices (CIA, CIW and ICV) made it possible to show the difference in the incoming material. At the beginning of Solikamsk time, the paleobasin received predominantly petrochemically mature material, probably significantly clayed with a high content of quartz. In the middle of the Solikamsk time, it was replaced by less weathered material (rocks of basic and intermediate composition). At the end of the Solikamsk time, terrigenous material, predominantly of quartz and potassium feldspar composition, was brought into the paleobasin by land waters.