Drilling on the Novoport structure of Yamal region of northern West Siberia has disclosed clastic-carbonate rocks of Middle Paleozoic age resting on Baykalian and Caledonian metamorphic rocks. These Paleozoics are organo-clastic and pelitic limestones, rarely dolomitized. Siltstone, sandstone, limy shale, and basic volcanics are present in some sectors. Poorly preserved bryozoa, ostracods, crinoids and sponge spicules are adequate for dating these rocks as Silurian and Devonian. Lower-Middle Carboniferous may also be present.
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 indicate presence of reef buildups in Novoport region. A seismic profile 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 from this standpoint and found to indicate additional reef bodies. These had been interpreted previously as anticlinal folds within the Middle Paleozoic complex.
A geophysical program including gravity and magnetic surveys along the seismic profiles indicates folding and faulting in the section from Silurian to Carboniferous. Dome-like structures that measure 2 by 4 to 4 by 8 km with thickness of 0.5-0.7 km are recorded. Some of the gravity anomalies are the "sombrero" type.
Six more or less equant structures of a reef-like nature have been mapped in the area of Novoport structure. They are located on a horseshoe arc along the border of a paleo-depression of 15-16 km width. The most subsided part of this paleo-depression corresponds with a negative gravity anomaly. of northeast trend.
One complication of the model proposed here is that not one well has drilled any reef deposits. This is attributed to the location of these wells having been based on a different concept of the geology
. Another important feature of the study area is an exceptionaly wide distribution of faults. A net of faults imparts a block structure, and overthrusts that dip at 15-30 degrees occur extensively. Duplication of section is possible, and petroleum may be trapped beneath these overthrusts. Along one profile are registered two rectilinear reflectors about 500 m from one another. In the space between them the wave picture is irregular - either chaotic or discontinuous, as is characteristic of intensively sheared zones. This is probably an overthrust feature.
On a basis of significant overthrusting (up to 25 km) over to the west in rocks of the same age in the transition zone between the east flank of the Polar Urals and the West Siberian platform, the same scale of overthrusting may be expected in the study area.
As one conclusion, reef carbonates favorable for petroleum exploration are predicted for the Middle Paleozoics of Novoport area.
Taken from Voronov, Korkunov, and Ivashkeyeva, 1997; digested
in Petroleum Geology, vol. 32, no. 1, 1998, two seismic sections, one map.
Copyright 2002 James Clarke. You are encouraged to print out this
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