Among the world's petroleum provinces, the West Siberian basin is geologically distinctive in several aspects. The factors of basin size, paleostructural history, continental-marine facies interchange and cyclic depositional history, abundant source rock and reservoir facies, and efficient petroleum generation and preservation conditions combine to make it one of the world's foremost petroleum provinces. This enormous basin, the world's largest intracratonic basin, underwent a remarkably stable, but paleostructurally mildly active, Mesozoic depositional history of slow subsidence and basin filling, all of which combined to provide optimum environmental conditions for petroleum accumulation and preservation. Terrigenous sediment influx, mainly from the southeast, was sufficiently great for coarse clastic materials to spread widely across the eastern half of the basin and intertongue with marine sapropelic facies of the open basin to the west.
At the same time, the basin was sufficiently large and subsidence rates were great enough that terrigenous sediment influx did not fill it entirely. This imbalance allowed the development of partially starved-basin conditions in much of the western half of the basin, where as much as 2500-3000 m (8000-10,000 ft) of dark marine shale, much of it rich in organic matter, was deposited between Middle Jurassic and early Tertiary time. During most of Early Cretaceous time, the site of the deltaic-open basin transition was across the central basin area, the Khanty regional high, a regional paleostructural feature of the basin apparently inherited from late Paleozoic Hercynian tectonic activity. Continuous but mild paleostructural growth of the Khanty regional high and associated smaller structural
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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.