Palaeogeography, facies and depositional conditions of the Yanovstan Formation (NE-Western Siberia)

Relevance. The prospectivity of the Yanovstan Formation in terms of the possibility of discovering new hydrocarbon deposits. There are prerequisites for the described above both because of its oil-generating potential and the presence of significant thicknesses of sandy rocks in the composition of the formation, established at the early stages of studying its section. Aim. To create a genetic model of the Kimeridge-Early Berriasian Yanovstan sediments. Objects. Yanovstan Formation within the eastern side of the West Siberian sedimentary basin. Methods. Facies analysis of core and geophysical data in combination with modern ideas about sedimentation parameters of the Bazhenov Sea and adjacent basins. Results and conclusions. The global changes in the transgressive-regressive rhythm and tectonic reconstructions that occurred during the Yanovstan Formation sedimentation period determined the three-dimensional structure of the Formation (its differentiation into three strata) – lower, middle and upper, as well as the variability of facies conditions. The study of sedimentation conditions of the Yanovstan Formation revealed a change in the parameters of the sedimentation basin. Thus, the base of the lower stratum in the form of a basal horizon followed by widespread accumulation of clays reflects transgression at the boundary of the late and early periods of the Georgian time. Subsequent accumulation of sediments of the lower stage occurs with gradual sea regression, decreasing paleotemperatures, and relatively stable sediment supply from the Siberian landmass. Accumulation of the middle stage, characterized by increased natural radioactivity and associated with transgression, decreased velocity (up to 0.5 m/mln) and volumes of terrigenous material input in combination with a decrease in species diversity, which was the result of a drop in paleotemperatures and the impact of cold sea currents. Subsequent tectonic activation led to a gradual global regression of the sea, an increase in the volume and intensity of sediment input (up to 12 m/million) and sedimentation of the upper suite, characterized by a gradual increase in grain size up the section and the formation of significant shelf accumulative bodies, with which the main reservoir rocks of the suite are associated.