The north of West Siberia is an extremely important gas-producing region, playing a substantial role in the energy balance of the world. It holds first place in the world in abundance of reserves, and in number of unique, giant, and large fields. Gas resources as of 1993 were assessed at 100 trillion cubic meters (3,500 tcf) and proved reserves of commercial catagories at 50 trillion cubic meters (1,750 tcf).
All these gas pools within the thick (up to 2000 m) Cretaceous Neocomian-Cenomanian oil-gas complex are in its upper part in Cenomanian sediments at depth of 400 to 1200 m directly beneath the Turonian-Paleogene regional clay seal. They occur at crests of highs that have closures of 100-200 m and more. The crest areas of these highs are large at 500 to 4000 sq km.
The continental sand-silt Cenomanian complex is a hydrodynamic unit. The pools have a massive-blanket character and have practically horizontal gas-water contacts. The gas is 99 percent methane and contains up to 0.3 percent ethane, propane, and butane. Traces of condensate are present. Oil rings have been found in some fields; this oil is heavy, viscous, and contains some sulfur.
During the time from the Valanginian to the Cenomanian the north of West Siberia experienced largely continental conditions that were favorable for deposition and preservation of plant matter. There were individual times of maximum coal accumulation that led to formation of coal beds. The number of coal beds in most wells is 10-30, and their aggregate thickness is in tens of meters. Also present are enormous masses of disseminated coaly material in the form of seams and lenses in various lithologic varieties. The Pokur Series of Aptian-Cenomanian age is a typical coal-bearing complex.
A warm humid climate and rapid development of plant life prevailed throughout all of the Early Mesozoic and Cenomanian in the study area. The carbon isotopic composition of the gases in these fields exhibits an elevated content of the "light" isotope C-12. There is an exceptional similarity between the gases of the study area and those of modern swamps, indicating that their formation was under conditions similar to those of modern swamps.
Processes of coalification are accompanied by generation of carbon dioxide along with methane. It is estimated that with increase in degree of coalification from the brown-coal stage to anthracite the amount of methane generated by one ton of coal increases from 68 to 287 cubic meters whereas the amount of carbon dioxide decreases. It is proposed that since at the brown-coal stage of catagenesis of the organic matter of the Pokur Series methane was generated in subordinate quantities in comparison with carbon dioxide, most of thje methane of the Cenomanian gas could have formed as a result of bacterial reduction of carbon dioxide.
Taken from Nemchenko, Rovenskaya, and Schoell, 1999;
digested in Petroleum Geology, vol. 35, no. 3, p. 244-258, 2001;
eight tables of data and one large cross section.
Copyright 2001 James Clarke. You are encouraged to print
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