The Soviet Union now produces more than 20% of the world’s oil, and the West Siberian oil-gas province accounts for more than half of this production. Soviet planning calls for maintaining this high rate of production through 1990 and beyond; therefore, the importance of West Siberian oil in the world economy is very significant.
Study of the petroleum geology of the West Siberian basin also contributes much to our understanding of the geological processes by which these huge quantities of oil and gas were generated and then trapped in giant and supergiant fields. Present in the basin are all the geological circumstances favorable for petroleum: highquality source beds, excellent reservoirs, extensive seals, just the right temperatures for maturation, and absence of later significant faulting or erosion to adversely disturb earlier accumulations of oil and gas.
The West Siberian oil-gas province has an area of 3.5 million km2 (1.3 million mi2) (Figure 1) and is the largest flat area on earth. More than 1000 km upstream on the Ob River, elevations are still less than 100 m above sea level. Although the winters are unrelentingly cold, it is during this time that work is best accomplished in many areas because in the summer, the upper part of the permafrost melts and the ground becomes very soggy. Furthermore, the rivers flow north, and spring melting takes place first in the upstream parts, causing downstream flooding over vast areas. This results in mosquito populations that are at best an extreme nuisance.
There were very few roads
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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.