Most of the gas, condensate, and oil of Novoport field occurs in Cretaceous and Jurassic sandstones and siltstones. Commercial flows of gas and condensate have been recovered also from Paleozoic carbonate rocks of the basement. The Novoport uplifted block of epi-Hercynian basement is composed largely of deformed and weakly metamorphosed carbonates of Middle-Late Paleozoic age. These are host to igneous bodies - largely Permo-Triassic dolerites. Greenstone is present also.
The thickest carbonate sections were penetrated in wells 302 and 300 at 828 m and 1085 m, respectively. These sections consist of metamorphosed and in varying degree dolomitized limestone. Their Devonian age is based on ostracods. Younger Carboniferous carbonates are possibly present also. Crystalline varieties are widespread and commonly carry recrystallized fossil remains.
In Early Mesozoic time before deposition of the platform sedimentary cover, the rocks of the epi-Hercynian basement were subjected to extended erosion leading to formation of a weathered zone at the top of the carbonates and greenstone. The weathered zone on the greenstone is not very thick. It consists of frectured rock. Testing of this zone has not yielded hydrocarbons. In other areas in the Trans-Urals, however, small oil and gas pools have been found in weathered greenstone.
Weathering at the top of basement carbonate rocks on Novoport high in pre-Jurassic time has led to formation of reservoirs of the fracture-cavity-karst type. Permeability and porosity of these zones are high. They are host to all the gas and condensate pools that have been found in the basement of the study area.
Sectors of surface and deep karst are controlled by zones of ancient faulting and the geomorphology of the basement surface. Faults of northeast trend parallel with the Urals are karst generators. A karst plateau and trough formed on the surface of the epi-Hercynian unconformity.
The most favorable conditions for karst formation are zones of flattening out of the flanks of Novoport high and nodes of intersection of northeast and north-south systems of of faults. These conditions are well expressed on the East Novoport basement high, where karst belts developed at several hypsometric levels. Fracture karst may be present for depths of several hundred meters.
Gas and condensate pools have been found in four pools in karsted carbonates of Novoport area. Since they occur at various hypsometric levels, gas-water contacts are at different levels.
High-accuracy gravity and 3D seismic surveys in the area of
East Novoport high indicate several Paleozoic reef bodies.
These are regarded as new exploration targets, and a large
number of exploration wells are recommended. (Taken from
Zhuravlev and Oblekov, 2000; digested in Petroleum Geology,
vol. 35, no. 2, 2001, structure map, cross section, and seismic
section; in press)
Copyright 2001 James Clarke. You are encouraged to print
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