Rifting controls not only the break up of the lithosphere into plates but also subsidence of large regions during the time of attenuation of the rifting and cooling of the upper mantle. This downward tectonic movement lasts 200-250 million years with formation of huge sedimentary basins. Such basins retain a high residual tectonic activity, a high heat flow, and contain a large concentration of organic matter in the sediments. All this determines the uniqueness of such basins as sources of petroleum.
In the initial stage of rift basins, continental and transitional types of sediments - largely fluvial and lacustrine - are deposited. For example, in West Siberia zones of grabens in the Triassic and topographic lows in the Early and Middle Jurassic were river valleys where great thicknesses of sediment, largely sands accumulated. Smaller thicknesses of silt and clay collected in lakes and swamps on the higher areas between the rifts.
In the structure of the West Siberian basin and apparently in other basins of this type the rifting predetermined development of two distinctly different structural stages: lower platform and upper platform (platform proper). The platform proper complex of West Siberia began to form in the Late Jurassic. It comprises marine deposits of the Upper Jurassic and Lower Cretaceous, and marine and transitional deposits of the Upper Cretaceous and Paleogene. These deposits are also thicker in the lows and thinner on the highs.
At the beginning of the Jurassic West Siberia was a strongly dissected region with mountainous and hilly relief in the south and a marine basin in the north. Products of erosion in the south were transported along deep river valleys to the marine basin on the north. There were times of transgression as during the Toarcian when the sea penetrated far to the south along the rift valleys with formation of large fresh-water and saline lakes. This was the time of deposition of clays of the Togur Formation, which have a high content of disseminated organic matter. These clays are very similar to the Bazhenov Formation in oil-generating capacity.
The lower platform complex extends over an area of more than 2 million sq km. Its thickness ranges from tens to hundreds of meters in the south to 2-4 km in the north. Depth to this complex is 1-1.5 km in the south to 4-4.5 km in the north. This complex is assessed to contain 20 percent of the initial oil resources, 34 percent of the condensate, and 16.5 percent of the non-associated gas of West Siberia.
Some 130 hydrocarbon pools have now been discovered in the lower platform complex of West Siberia. About 120 are in the Middle Jurassic, and 10 in the Lower. Discovery of large and high-producing fields is expected both in the zone of junction of marine and continental deposition and on the flanks of basement arches. In contrast to the upper structural stage, traps in the lower stage as a rule are non-anticlinal.
Taken from Surkov (1993); digested in Petroleum Geology, Vol. 30,
No. 2, 1996, two maps and one cross section.
Copyright 2001 James Clarke. You are encouraged to print out this News
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