Stratigraphy and Paleogeography
The West Siberian basin occupies an area of approximately 3.5 million km2 (1.3 million mi2), including the South Kara basin and part of the Khatanga basin, which geologically are a part of the West Siberian basin (Figures 8, 9). The basin is bounded by the Uralian and
Novaya-Zemlya uplifts on the west and northwest, the Kazakh and Altay-Sayan uplifts on the south and southeast, the Siberian craton and Taymyr uplift on the east and northeast, and the North Siberian sill on the north. Thickness of the Phanerozoic sedimentary cover ranges from approximately 3-5 km (10,000-16,000 ft) in the central parts of the basin to 8-12 km (26,000-40,000 ft) or more in the northern part (Figures 10-15). The post-Paleozoic basin (including the South Kara Sea and Khatanga basins) is filled with approximately 16 million km3 (4 million mi3) of Mesozoic-Cenozoic sedimentary rocks ranging in thickness from 3-4 km (10,000-13,000 ft) in the central area to 8-10 km (26,000-33,000 ft) or more in the north (Figures 15-19). The Mesozoic-Cenozoic fill is less than 1 km (3300 ft) thick along the North Siberian sill, which is a basement high of Mesozoic age extending between the northern end of the Novaya-Zemlya and the northwestern part of the Taymyr uplift (Figures 9,16). On the southwest, the basin is connected with the Ust-Urt basin region through a narrow trough between the southern Ural Mountains and the Kazakh uplift. In latest Cretaceous and early Tertiary time, the West Siberian Sea was connected with the Tethys sea through this passageway (Figure 9).
Figures & Tables
Geology and Hydrocarbon Habitat of the West Siberian Basin
The West Siberian oil-gas province comprises the largest flat land area in the world (3.5 million km2, or 1.3 million mi2). Over most of the region, elevations rarely exceed 100 m (330 ft). The basin is bounded on the west by the Uralian and Novaya Zemlya uplifts, on the east by the Siberian craton and Taymyr uplift, on the south by the Kazakh and Altay-Sayan uplifts, and on the north by the North Siberian sill. Structurally, the basm is a broad, relatively gentle downwarp filled with 3-10 km (10,000-33,000 ft) of post-Paleozoic marine, nearshore marine, and continental clastic sedimentary rocks. The basement is composed of Precambrian and Paleozoic fold systems with large areas of partly metamorphosed Paleozoic carbonate and clastic rocks and numerous areas of Paleozoic or older granitic and mafic igneous bodies. In the central part of the basin, the basement is cut by an extensive, northerly oriented Triassic rift system.
Paleostructural and stratigraphic trapping are important aspects of West Siberian petroleum geology. Oil source rocks are mainly marine Jurassic and Lower Cretaceous bituminous shales. Gas source rocks are mainly Upper Cretaceous humic and coaly shales. Petroleum production in the basin occurs in four major areas: (1) Middle Ob: primarily oil from Lower Cretaceous deltaic-marine clastic reservoirs on broad regional uplifts; the Samotlor and other supergiant fields are located in this area; (2) Near-Ural: primarily oil in the south and gas in the north from Upper Jurassic and Lower Cretaceous clastic reservoirs in paleo- structural-stratigraphic traps; (3) Southern Basin: oil and oil-gas from Jurassic clastic reservoirs, mainly on anticlines or arches inherited from basement highs; and (4) Northern Basin: gas primarily from Upper Cretaceous (Cenomanian) and gas-condensate from Lower Cretaceous and Jurassic clastic reservoirs on large anticlinal traps sealed by Cretaceous shales or permafrost. Urengoy, the world's largest gas field, and several other supeigiant gas fields are located in this latter area.
Large parts of the basin are relatively unexplored, particularly the northern offshore segments. The interrelated paleostructural and depo- sitional character of this enormous basin provides excellent prospects for stratigraphic trap accumulations. An estimated 70 billion bbl of oil and 1000 tcf (trillion cubic feet) of gas have been found in the basin. U.S. Geological Survey estimates (1987) of undiscovered, conventionally recoverable petroleum resources are 30 billion bbl of oil and 350 tcf of gas.