In Late Paleozoic and Early Mesozoic time the southeastern part of the West Siberian platform was relatively uplifted and subjected to weathering and denudation. Thickness of this weathered zone ranges from 5-20 m at the crests of structures to 80-100 m on their flanks. During the last 20 years flows of oil and gas have been recovered from Paleozoic rocks directly beneath this erosion surface as well as from the weathered zone itself. It has been debated as to whether the oil was sourced in the Paleozoic rocks or was generated in Jurassic source beds and migrated downward into the weathered zone and Paleozoic reservoirs.
Two types of erosional-tectonic highs are found on the pre-Jurassic erosion surface: those related to syn-sedimentary uplift of axial zones of anticlinoria, and those caused by block movements within synclinoria. The rocks involved in the first type are generally granites and metamorphic varieties. In the study area they are found in the Mezhov and Dem'yan oil-gas areas of Kaymysov oil-gas region. The second type is generally represented by sedimentary rocks or volcanics and occurs in several fields of Vasyugan oil-gas region.
The reservoirs of these erosional-tectonic highs are local, occupying less than 10 percent of a basin. The seals are also local, consisting of clayey or clayey-silty-carbonaceous rocks of the Lower-Middle Jurassic. A seal may vary in composition within the same field, represented by as many as three different rock units. Clay deposits of the weathered zone at the top of the Paleozoic may also serve as a seal.
Faulting in the Permo-Triassic, Jurassic, Cretaceous, and Recent stages have imparted a small-scale block character to these erosional- tectonic highs. Reservoirs are the fracture-pore, cavity-fracture-pore, and fracture types with abrupt lateral and vertical variations. Some of these reservoirs have yielded oil at more than 700 bbls per day.
Weathering may enhance or deteriorate reservoir properties of the rock. For example, testing of Upper Devonian-Lower Carboniferous limestones in Kalimov well 1 yielded a much greater flow of oil than from the weathered zone of this same well. The situatioin has been the same in several other wells. In many other areas, however, reservoir properties of the weathered zone are considerably better than those of the underlying Paleozoic rocks. As an example, the weathered zone in Verkhtar well 3 yielded oil at 840 bbls per day.
Analyses of cores from the top of the Paleozoics, the weathered zone, and Lower Jurassic sediments of the Middle Ob region indicate that the Paleozoic rocks had no oil-source potential or that it was exhausted by the beginning of Mesozoic deposition. Sub-bituminous rocks of the Lower Jurassic are indicated as the source beds. Absence of a regional seal above these erosional-tectonic highs was favorable for introduction of hydrocarbons into the pre-Jurassic reservoirs from overlying Lower Jurassic source beds. (Taken from Abrosimova and Belova, 2000; digested in Petroleum Geology, vol. 35, no. 1, p. 45-47, 2001, in preparation)
On the other hand, Slavkin and others, 2000 (digested in Petroleum Geology, vol 34, no. 2, p. 375, 2000) describe a lower carbonate reef complex more than 1000 m below the pre-Jurassic erosion surface, and they assess it as favorable for petroleum.
Copyright 2000 James Clarke. You are encouraged to print out this
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