Characteristic of the Upper Paleozoic complex of the north of West Siberia are linear downwarps, most of which have horst uplifts in their axial areas. These are rift features. They did not show up well on earlier reflection surveys but were recognized from gravity data. The newer common depth point seismic surveys, however, disclosed their form better and gave information on the time of their development.
Two paleo-rift systems are recognized in the Paleozoic rocks of northeast of West Siberia: the east-west Yenisey-Khatanga (which extends on eastward across the north of East Siberia) and the north-south West Siberian. The common depth point seismic surveys indicate that the Yenisey-Khatanga zone began its formation during the Middle Paleozoic synchronous with similar activity in East Siberia and on the Russian platform. The West Siberian rift system is somewhat younger; subsidence began in the Middle Paleozoic and continued through the Carboniferous, but with less intensity. This was followed in some areas by compression and horst formation that lasted until the Late Triassic and Early Jurassic.
A younger Triassic-Early Jurassic rift system, the Baydaratsko- Yarudey, formed in the northwest of West Siberia. The axial inversion high of this system continues beneath the Pay-Khoy (Northern Urals) fold system, confirming its Kimmerian age.
The seismic data indicate that the northeast part of West Siberia during the Late Paleozoic and Triassic developed as a west continuation of a system of rift downwarps on the margin of the North Asiatic craton. This system extended on to the west as the depressions of the Kara and Barents Seas.
As an isometric subsidence structure of the sineklize type, the Pur-Gyda basin formed in the Late Paleozoic and then continued its development within stable boundaries. The West Siberian sedimentary basin up to the beginning of the Neogene developed centrifugally relative to this feature. This sineklize remained morphollogically well expressed through all stratigraphic levels of the Mesozoic and Paleogene. The M-discontinuity in this depression was a broad high. Depth to basement in the central part of this Pur-Gyda sineklize appears to be 15-20 km.
The Triassic and Upper Paleozoic of the north of West Siberia can be regarded as a new oil-gas play. The main targets are strongly expressed zones of pinch out primarily on the west border of the Pur-Gyda sineklize, where depth to the pre-Jurassic is 4-5 km.
Super-giant gas and gas-condensate accumulations of the Mesozoic of the north of West Siberia are probably present in the pre-Jurassic section as well. This may be the result of an enoumous generating potential of the sediments filling the Pur-Gyda super-deep depression. Also, a high-amplitude carbonate step is present on the top of the Middle Paleozoic complex on the west border of this feature. Organic buildups are possible there.
Taken from Girshgorn, 1988; digested in Petroleum Geology,
vol. 30, no. 3, 1996. one seismic section and one tectonic map.
Copyright 2002 James Clarke. You are encouraged to print out
this News Letter and to forward it to others. Earlier News Letters
are available at: http://geocities.com/internetgeology/
This News Letter is distributed without charge in the interest of our
science. To be added to the mailing list please send your e-mail
address to: jamesclarke@erols.com
For information on the journal Petroleum Geology please
Fax 703 759-3754 or telephone 703 759-4487.