The West Siberian basin (~3.4 × 10⁶ sq km) is one of the largest structural-sedimentary basins of the world. The basin was relatively undisturbed by post-Triassic tectonism and erosion and is little changed from its original form when Early Jurassic deposition began about 180 m.y. ago.

The large Khanty structural high in the central part of the West Siberian basin is nearly 1,000 km long and 400 km wide. The axis bears five large domes, separated by depressions. The Khanty arch was a structural entity throughout the Mesozoic and is quite clearly the locus of greatest oil occurrence.

Although the depositional history of the basin was one of continual incursion and retreat of the sea, three megarhythms are recognized in the sedimentary fill: Triassic-Aptian, Aptian-Oligocene, and Oligocene-Quaternary. Continental sediments predominate at the base of each megarhythm, and largely marine and nearshore sediments are present at the top. Megarhythms are made up of macro-, meso-, and microrhythms, each of which has its transgressive and regressive phases. The search for paleoshorelines and related stratigraphic traps seems to be yet in an early stage.

Three major productive areas are recognized in the basin. In the west, near the Ural Mountains, oil and gas are produced from Upper Jurassic sandstones that pinch out against basement blocks. Along the middle

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course of the Ob River in the central part of the basin, production is largely oil from Lower Cretaceous arkosic sandstones on anticlines. Samotlor, the largest oil field in the USSR, is in this area. In the northern part of the basin, mainly gas is produced from Upper Cretaceous clastic rocks on anticlinal traps. Urengoy, the world's largest gas field, is located in this area.

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