When exploration began in West Siberia after World War II, expectations were that the productive Paleozoic section of the Volga-Ural oil-gas province would be present there and would be productive. Instead, the Mesozoic was found to be highly productive, and the Paleozoics were regarded as economic basement.
Drilling on the Novoport structure in the south of the Yamal Peninsula has disclosed a predominance of clastic and clastic- carbonate sedimentary rocks of Middle Paleozoic age resting on Baykalian or Caledonian metamorphic rocks. These Paleozoics are organic, organo-clastic, and pelitic limestones, rarely dolomitized. The limestones carry poorly preserved bryozoa, crinoids, sponge spicules, and algae, which are nevertheless adequate for dating these rocks as Silurian and Devonian.
The wells in this region have generally penetrated only the upper 50-150 m of these Paleozoics. Attention has turned to these rocks since limestones at the base of the platform cover have been found to be productive.
Seismic surveys and high-accuracy gravity surveys together indicate the presence of reef buildups in Novoport region. A seismic profile about 6 km long across the main structural feature of the pre-Jurassic base shows clearly a reef body with maximum thickness of 500 m. Earlier seismic data were reinterpreted and were found to indicate additional reef bodies at depths of 3 to 5 km. These measure from 2 by 4 km to 6 by 8 km. They had been interpreted previously as anticlines.
Six more or less equant srtuctures of a reef-like nature have been mapped in the area of Novoport structure. They are located on a horse- shoe arc along the border of a paleo-depression, which has a width of 15-16 km. Inclination of beds along this border is gentle at 15-25 degrees. The most subsided part of this paleo-depression corresponds with a negative gravity anomaly of northeast trend. Within the area of this anomaly are two local depressions, where Middle-Upper Carboniferous clastics are probably present.
To the west in the transition zone between the east flank of the Polar Urals and the West Siberian platform is an area that has much in common with the study area. Devonian bauxite-bearing reef buildups are without doubt analogs of the predicted reef buildups of Novoport region.
One problem with the model proposed here is that not one well has drilled any reef deposits. The reason for this is that location of these wells was based on a different concept of the geology here.
Extensive faulting is present in the Paleozoic section in the study area, imparting a block structure. Overthrusts that dip 15-30 degrees are also common.
In conclusion, reef carbonates and fault traps are favorable for petroleum exploration in the Middle Paleozoic rocks of Novoport region. (Taken largely from Voronov and others, 1997; digested in Petroleum Geology, vol 32, no. 6, 1997; two cross sections and one structure map.)
Copyright 2001 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Earlier News Letters are available at: http://geocities.com/internetgeology/ This News Letter is distributed without charge in the interest of our science of petroleum geology. To be added to the mailing list please send your e-mail address to: jamesclarke@erols.com
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James Clarke