The stratigraphic section above the Cenomanian of northern West Siberia consists of the Upper Cretaceous Kuznetsov, Berezovo, and Gan'kin Formations, and overlying Paleogene deposits. Although parts of the supra-Cenomanian section have been played in the past, attention is now turning to other targets. The first of these is the lower Berezovo, which occurs 100-150 m above the top of the Cenomanian. The reser- voirs are non-traditional for West Siberia - opoka and opoka-like clays. Opoka is a high-silica sedimentary rock - perhaps best designated as a chert. At the top of the lower Berezovo is a regional dark gray, almost black, dense, brittle siliceous rock up to 20 m thick. This member is designated as electrical logging marker "A" and seismic marker "C". The reservoir is the fracture-pore type, and porosity is 30-43 percent. Overlying the Berezovo is the Gan'kin Formation, in which silty clays are resertvoirs. The reservoirs of the lower Berezovo and Gan'kin have satisfactory seals.
Comparison of electrical and gas logs for Komsomol field shows that methane content of the drilling mud varies directly with resistivity. Total hydrocarbons in muds from the lower Berezovo exceed those from the pool in the underlying Cenomanian. Seismic data for this same area at the level of reflecting horizon "C" show anomalies of the bright- spot and flat-spot types. These correspond with areas of high resistivity. The flat spot indicates a probable massive pool, reflecting a gas-liquid interface. A large part of the lower Berezovo appears to be hydrocarbon saturated.
Pools are indicated in the lower Berezovo in Medvezh, Urengoy, Yamburg, and other areas. Depth to gas-water contact is estimated at 760 m. Small gas pools are present in the Berezovo at the crests of the structures in the Gubkin, Severo-Gubkin, Yetypurov, Tarasov, Zapadno- Tarkosalin and other areas.
Silty-limy clays of the upper Berezovo and Gan'kin may be targets for gas pools. Gas shows have been recorded in several areas. Production potential of these reservoirs is much leas than that of the lower Berezovo; however, their total thickness at 300-500 m suggests that their production may be viable.
A note of caution: The pools in the Berezovo and Gan'kin may be related to artificial flow of fluids related to the drilling.
Gas was discovered in 1990 in the Neogene Tavdin Formation in the Nokhoyakhin area. This in addition to numerous gas shows in the Paleogene -Quaternary section and a persistent clay divider between the Upper Cretaceous and the Paleogene-Quaternary all suggest a Paleogene- Quaternay play in West Siberia.
Considering the shallow depths and number of exploration and production wells that penetrate these predicted pools, the supra-Cenomanian may be regarded as a possible source of production after the gas in the Cenomanian has been depleted. The infrastructure is already in place. These rocks are similar to the Monterey Formation of California. (Taken from Agalakov and Bakuyev, 1992; digested in Petroleum Geology, vol. 28, no. 3/4, 1994, one map, one cross section, and two geophysical logs).
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