The Upper Jurassic Bazhenov Formation of West Siberia (see News Letter No. 94) is a bituminous cherty rock similar to the Monterey Formation of California. "Anomalous Bazhenov" differs from "Normal Bazhenov" by presence of beds of sandstone with abundant fragments of bituminous shale, clastic dikes, slickensides, and traces of slump structures. This anomalous Bazhenov occurs in north-south-trending zones and has been attributed to action of turbidity and bottom currents or to slump processes that accompanied deposition of normal Bazhenov deposits.
According to Yasovich (1981) and Nesterov and others (1986) the zones of anomalous Bazhenov form several relatively narrow (0.5-3 km) but rather long (up to 200 km) north-south belts, between which the Bazhenov has normal sections. These belts are not continuous but have interruptions.
In many places the zones of anomalous Bazhenov have a patchy character. In some areas these patches are oval or elongate in a nowthwest or northeast direction. The area of these patches ranges from 4 to 200 sq km. Enormous patches of more than 1000 sq km have been mapped in other areas. This patchy distribution of anomalous Bazhenov is not sedimentational in origin but is due rather to post-sedimentational erosion.
In those well sections where anomalous Bazhenov rocks are present, they rest with erosion on the Vasyugan Formation and are overlain discordantly by normal Bazhenov deposits. Significant variations in thickness of anomalous Bazhenov up to complete pinchout indicate its uneven erosion before deposition of normal Bazhenov. Its absence in other places is attributed to erosion that took place after deposition of normal Bazhenov. Zones of erosion of the entire Bazhenov Formation show up clearly on the seismic time sections by loss of reflecting horizon B, which is a regional marker horizon associated with the Bazhenov.
The anomalous Bazhenov differs from the underlying Vasyugan by carrying beds of bituminous clays of various thickness typical of the Bazhenov. With this in mind and on a basis of similarity of fossil remains, the anomalous Bazhenov should be regarded as a more or less sandy facies and be assigned to the lower part of the Bazhenov Formation. Two types of anomalous Bazhenov are recognized: sandstone-shale type and largely shale.
In addition to the prospective sandstone and siltatone bodies of the anomalous Bazhenov, a new exploration play is recognized here - the weathered zone that developed on the upper part of the anomalous Bazhenov at a stage of continental erosion before deposition of normal Bazhenov.
Taken from Onoshchenko, 1995; digested in Petroleum Geology
vol. 30, no. 3, 1996, one map and two well sections.
Copyright 2002 James Clarke. You are encouraged to print out this
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