Sequence-stratigraphic interpretation of the Callovian and Upper Jurassic (Vasyugan horizon) of the southeast of West Siberia

The main oil-bearing horizon (U-1) is widespread in the southeast of West S iberia. The horizon consists of several sand beds, which are sometimes spli t by clay interbeds or merge into a uniform bed, or sometimes partially thi n out from sections. A sequence of transgressive-regressive (T-R) events wa s fixed in the interbedding. Traces of regional and local breaks in sedimen tation (erosion) are well identified in the majority of sections. The struc ture of the Bathonian-Kimmeridgian of the southeast of West Siberia was int erpreted by a sequence-stratigraphic analysis. Four sedimentation sequences were identified and traced in sections of the Vasyugan Formation in the no rthern and southern areas of the Kaimysovsky arch. Both Original (stratigra phic, paleontological, and lithological) and revised known data were used. An advanced regional stratigraphic scheme is given, and the stratigraphic r anges of sand beds of the horizon U-1 are established. Typical system tract s (TST, HST, LST), complexes of incised-valley fill connected with river ch annels, and formation of forced regression are determined. The first type b oundary between sequences (SB1) was traced in the subface of the Mezhugol'n aya Subformation. A transgressive-regressive (T-R) curve for the Upper Jura ssic of the south of West Siberia was constructed by data of a sequence-str atigraphic analysis. Comparison of the T-R curve with a quantitative eustat ic curve for the Upper Jurassic of the Russian Platform shows their good co rrelation. Hence, eustatic fluctuations can be considered a primary factor governing simultaneous changes in sea level in different basins from late B athonian to early Kimmeridgian. Eustatic curves can be used as an independe nt method for interregional correlation.