Yanovstan formation of Western Siberia: Lithology, structure, and correlation of deposits

Research subject. Yanovstan f ormation ( J3-K1jnv) of Kimmeridgian-Early Beriasian age. Aim. To identify structural features of the Yanovstan formation, to carry out a detailed correlation of its deposits, including the development of criteria for determining the boundaries of the suite. Materials and methods. The studies were carried out on the basis of borehole data, including core material from 5 boreholes and borehole logging data from 209 boreholes. The borehole logging complex comprised the following methods: gamma ray logging, neutron gamma ray logging, acoustic logging, caliper logging, as well as apparent resistivity and self-polarization logging. The borehole sections were correlated by tracing individual sections of the geological section, which are characterized by relatively stable geophysical parameters. Additionally, a sequence stratigraphic approach was used. Results. The boundaries between the Sigov and Yanovstan formations were identified based on the description of core material and its comparison with the borehole logging data. The Yanovstan formation was subdivided into three units proposed by the authors: lower (ЯНв), middle (ЯНб), and upper (ЯНa). A borehole correlation of the units of the formation was carried out, on the basis of which maps of the total thickness and net-to-gross ratio and the structural plan of each of the identified stratigraphic units were constructed. It was determined that the total thickness of the lower unit reaches 80 m and is characterized by maxima within the eastern part of the study area. The thickness of the upper unit reaches 240 m and is identified within the northeastern part of the area. This part is represented by a predominantly clay composition. The total thickness of the formation as a whole varies from 40 m in the west to 320 m in the northeast. The thicknesses of the middle unit are relatively consistent throughout the study area. The sandiness of the lower unit varies from 0 to 0.27 units; the upper ranges within 0–0.45 units. The middle unit is not characterized by the presence of sandstones. Conclusions. The theory about the supply of sedimentary material during the sedimentation of the suite from paleomounts within the eastern edge of the West Siberian Jurassic paleobasin was confirmed. An assumption was made that the increased thickness of the predominantly clayey suite within the northeastern part of the study area is associated with the redistribution and transfer of terrigenous material carried from the Siberian land by currents moving unidirectionally along the coast (counterclockwise). These currents thus seized the lightest (clayey) material and transported it to the northern part of the basin.