The West Siberian oil-gas province coincides with the West Siberian sedimentary basin. This basin, along with the Ural Mountains on the west, the Yenisey and Taymyr ranges on the east, and the Altay-Sayan and Kazakhstan shields on the south, form the north part of the Ural-Siberian or Ural-Mongolian cratonal mass. The basement of this cratonal mass is composed of Precambrian and Paleozoic fold systems, which are at the surface along the west, south, and east sides of the basin. The basin itself is a broad downwarp within this cratonal mass filled with post-Paleozoic sedimentary rocks (Surkov and Zhero, 1981). Its northern part can be regarded as a segment of the Arctic geodepression along with the Timan-Pechora and Barents Sea, Laptev Sea, East Siberian Sea, and the North America Alaskan North Slope, Mackenzie, and Sverdrup basins (Gramberg et al., 1984).
Structurally, the West Siberian basin is a platformal basin, the largest on the planet, having an area of about 3.5 million km2 (1.3 million mi2). Its closest analog is the Great Artesian basin of Australia (Khain, 1979). Within the West Siberian basin, a thick Mesozoic- Cenozoic section of platform sediments rests on basement that is largely folded and metamorphosed rocks. Several small terranes within this basement consist of unmetamorphosed middle and upper Paleozoic carbonate and clastic sediments and Triassic clastic sediments and volcanic rocks. These unmetamorphosed Paleozoic and Triassic rocks are called the intermediate complex. In the northeastern part of the region, the West Siberian sedimentary basin overlaps onto the western
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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.