The North of West Siberia is characterized by the presence of unique gas accumulations such as the Urengoy, Yamburg, Medvezhie, Zapolyarnoe, Bovanenkov and other fields. Discovery of gas fields is primarely related to elevated zones of the corresponding megaswells. At the same time there are a number of large negative forms - intraplatformal troughs gas content of which has been poorly studied (Yuzhno-Karskaya, Ust-Obskaya, Antipayutinskaya, Bolshekhetskaya and other troughs) within a northern part of West Siberia.

The Nerrutinskaya trough is of particular interest from the viewpoint of new gas fields discovery.

This trough, 15 000 sq. km in area, is limited with large high- amplitude rises such as Medvezhie, Kharvutinskoe, Yamburg, Pestsov and so forth. In Mesozoic section of the obove rises there are unique Cenomanian gas pools and Lower Cretaceous multilayer accumulation of gas, condensate and oil. The thichness of the Nerrutinskaya trough's sediments is 6-7 km with a large number of positive structures including Yuzhno-Pestsov, Tabjakha etc. Within each of these dome-like rises there are a number of reservoirs with different stratigraphic occurrence of which the Achimov sands of Berriasov age being found in the Nerrutinskaya trough is of the most promising for gas, oil and condensate. The sands are occurred at a depth between 3500 to 4000 m. At a moment the knowledge about this trough is required to be improved.

To reconstruct the paleoevolution of the Achimov thicknesses, to correct a complicated lens-like occurrence of Lower Cretaceous deposits and to define the location of reservoirs a number of geological and paleostructural profiles were constructed based on deep drilling data and the results of seismic survey.

This investigation allowed to reveal the nature of spatial distribution of high- resistance Lower Crateceous rocks. Besides this also allowed to differentiate the trough's structure with revealing some elevated zones. The Nerrutinskaya trough is one of the promising structures for prospecting works which as one can expect will allow to protect gas production decline in West Siberia after 2000-2010. Similar structures are expected to be found in the Bolshekhetskaya trough. Thus the intraplatformal troughs of the West Siberia platform are an important resource of the raw material base of the region.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah