Lithofacies variability of the Yarakta horizon as a complex result of geodynamic processes of tectonogenesis and sedimentogenesis

The purpose of the research is to analyze the lithological and facies conditions of formation and the causes of lithological composition variability of the Yarakta horizon deposits as well as formation and distribution features of reservoir rocks in the horizon. The paper uses the data on drilling and geophysical work results and research materials from the public sources. The Yarakta horizon is commercially productive in the deposits of the southeastern slope and the central part of the Nepa-Botuoba anteclise: Dulisminskoe, Yaraktinskoe, Ayanskoe, Danilovskoe deposits, etc. The deposits of the Yarakta horizon were formed during the initial transgressive stage of the Vendian sedimentation cycle and are characterized by extreme lithological irregularity of rock composition and thickness. Investigations of a number of scientists testify that in the early formation of the Yarakta horizon its territory in geomorphological terms was predominantly presented by a coastal plain, which was occasionally flooded by the sea. The rocks in the area are represented by the facies of temporary proluvial-deluvial flows of the coastal plain replaced by coastal shallow sediments. The change in the lithological composition of rocks and thicknesses of the Yarakta horizon is a result of changes in the nature of oscillatory movements, while its structural and textural features are determined mainly by the dynamics of water flows of the sedimentation medium and the paleorelief nature of the underlying surface. The conducted research allows to conclude that lithological heterogeneity of the Yarakta horizon in sections (vertically) and along the strike (laterally) is determined by the frequent alternation of various lithotypes, the formation of which is determined by the complex nature of tectonic movements and irregularity features of sedimentary layer facies.