The backlog of relatively simple anticlinal traps in the Mesozoic section of the southeast of West Siberia has been almost completely exhausted, and exploration has turned to buried hills on the eroded surface of the Paleozoic sub-strat. As of 1995 some twenty pools had been discovered in carbonate rocks at this Paleozoic surface. Pools have also been found that are not associated with these erosional remnants.
This buried-hill play has been downgraded by many because of the small dimemsions of the highs, the complexity of reservoirs that have developed from weathering, and the low oil-source potential of the Lower and Middle Jurassic in this region. In spite of these objections, there is a geological basis for expecting discovery of not only small but also medium-size fields in southeast West Siberia.
Seismic reflection surveys do not always lead to satisfactory results in mapping the surface of the Paleozoic sub-strat. Such surveys pick up differences in impedence. Since the weathered zone at the top of the Paleozoics has essentially the same velocity characteristics as the overlying Mesozoic rocks, no reflections are registered. An interface is picked up, however, at the base of the weathered zone. When flows of hydrocarbons are recovered from below the weathered zone, it is possible that the fluid has migrated downward along fractures from the overlying Jurassic rocks. All the favorable intervals are above the reflecting interface.
Such is the situation in Maloich, Archin, Nizhne-Tabogan, Kalinov, and other fields. The difference between the seismic interface and the actual surface of the weathered sub-strat is generally 20-30 m, but is 70-80 m on the Archin structure. Closure on buried hills from the seismic data is 20-30 m; the actual value, however, may be 15-20 percent more.
The Lower-Middle Jurassic rocks of the southeast of West Siberia that rest directly on the Paleozoic complex are shown by geochemical and lithological studies to be poor source beds. The Mesozoic of this region as a whole is assessed as having very low potential for discoveries. This, of course, limits the possibilities for these buried hills. Nevertheless, there are broad areas in this region where the Togur Formation of the Lower Jurassic Toarcian Stage is very high in disseminated organic matter and rests directly on Paleozoic carbonates or pinches out against buried hills. Such a relationship inhibits dispersal of the hydrocarbons into small deposits but rather favors their migration into the weathered buried hills on the Paleozoic complex. Thickness of the productive interval in these weathered zones is almost 100 m in some wells.
Taken from Abrosimova, 1995; digested in Petroleum Geology,
vol. 30, no. 3, 1996, one illustration showing productive intervals.
Copyright 2002 James Clarke. You are encouraged to print out
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