Petroleum Potential of Lower-Middle Jurassic of West Siberia

Internet Geology News Letter No. 10, September 13, 1999

The Upper Jurassic and Cretaceous of West Siberia have been studied quite thoroughly and produced intensively. In order to stabilize production in the region attention is turning to the Lower-Middle Jurassic section, which is assessed to contain not less than 30 percent of the petroleum resources of the West Siberian basin. More than 50 fields have now been discovered in this part of the section, and in-place resources are estimated at 80 billion barrels of oil equivalent. Up until recently the Lower-Middle Jurassic of West Siberia had been designated as the Tyumen Formation. Within it are now recognized three facies regions: Yamal-Gyda marine on the north, Ob-Tazov transitional in the middle, and Ob-Irtysh continental on the south. The thickness of this part of the section ranges from 50-100 m in some parts of the south to more than 2000 m in marine sections in the north. These facies regions are subdivided further into facies zones, each with common facies characteristics. Eustatic sea-level changes in the Early and Middle Jurassic led to cyclic alternation of transgressive clayey deposits and regressive sand-silt units in the sedimentary pile. There are five such cycles, each consisting of reservoir rock overlain by seal and source-bed material. These pairs are Zimniy reservoir and Levin seal, Sharapov reservoir and Kitterbyut seal, Nadoyakh reservoir and Laydin seal, Vym reservoir and Leont'yev seal, and Malyshev reservoir and Nizhnevasyugansk seal, During Zimniy time a shallow-water marine basin occupied the north of the West Siberian basin. In the south sandstones and siltstones collected in river valleys and lakes. Sea level rose during Levin time, depositing clay and silt and sealing the Zimniy reservoirs. The area of deposition of the Sharapov reservoir rock was much greater than that of Zimniy time due to denudation of adjacent land areas where elevations were 200-400 m. In the following Kitterbyut transgression clay deposition extended over very broad areas sealing the Sharapov reservoirs. In Nadoyakh time a shallow sea continued to be present in the north of the basin, whereas in the south the river valleys were filled with channel and flood-plain deposits. This marked the end of the Early Jurassic. In Laydin time at the beginning of the Middle Jurassic the sea penetrated far to the south, particularly along the Koltogor-Urengoy rift. Clays and fine silt were deposited to form a seal. Deposition of the Vym reservoir rock was accompanied by extensive coal accumulation. The remaining islands in the sea were greatly reduced. Leony'yev transgression reached far to the south to the latitude of Omsk, sealing the Vym reservoirs. Drop in sea level in Malyshev time did not reduce the size of the basin; however, water depth decreased. The mountainous border of the basin had by now been reduced significantly. In Nizhnevasyugansk time a marine basin with normal salinity extended over most of the basin. Reservoirs, source beds, and seals are thus present in all five cycles of deposition of the Lower and Middle Jurassic of West Siberia, thereby favoring discovery of large oil and gas fields (Petroleum Geology, vol. 33, no. 4, p. 229, 1999; vol. 34, no. 1, p. 10, 2000; also Peterson and Clarke, 1991, A.A.P.G. Studies in Geology, no. 32). Copyright 1999 James Clarke. You are encouraged to print out these New Letters and to forward them to others. Jurassic