The study explored the taxonomic composition of foraminiferal assemblages from cores sampled in the Sil’ga and Yamal–Tyumen’ facies regions—the southern and southwestern West Siberian sedimentary basin. A complete sequence of biostratons was established for the Callovian–Kimmeridgian sediments of the southwestern West Siberian Lowland and was correlated with the units of the General Stratigraphic Scale. A number of biostratons have been revealed for the south of West Siberia. The biofacies analysis of the foraminiferal assemblages revealed that the Yamal–Tyumen’ facies region belonged to a relatively deep marine environment, far away from the Jurassic Sea littoral zone, in the Callovian through the Kimmeridgian and that the Sil’ga facies region was a relatively shallow marine environment in the Oxfordian through the Middle Volgian.

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