In the search for anticlinal traps the flanks of large structures are commonly neglected. This is true for the Kaymysov arch in the south of West Siberia. Exploration drilling on the southwest flank of this feature has disclosed a favorable type of Upper Jurassic section. Fault traps have also been found there.
The Upper Jurassic Vasyugan Formation of the southern part of Kaymysov arch is host to beach and channel sandstones, which are more prospective than high-closure anticlinal traps. The Vasyugan is subdivided into two members. The lower is clay-siltstone of shallow-water origin. The upper is in turn subdivided into two parts by a coal seam. The lower (sub-coal) consists of near-shore marine sandstone and siltstone, and the upper (supra-coal) is shallow-water marine clayey siltstone and sandstone.
The sub-coal part of the Vasyugan Formation was deposited during a regressive stage, and the coal represents maximum regression. The supra-coal deposits collected during a new transgressive stage.
The sandstone bodies of the Vasyugan are characterized by small thickness and weak acoustical contrast; consequently, they do not show up on the seismic sections. Prediction of such bodies is possible by construction of zonal sedimentational models. Such a model was developed using detailed correlation of well sections as follows: The Kaymysov arch was expressed geomorphologically during the Late Jurassic as indicated by differences in thickness and composition of the upper Vasyugan Formation. Eight types of section are recognized; these replace one another from southeast to northwest across the trend of the arch.
Beach sandstone bodies are present in the sub-coal in type I and II sections. A beach origin is indicated by their lithology and morphology as well as by their paleogeomorphological environment. These sand- stone bodies are on the paleo-flanks of Kaymysov arch. They have an accumulational form and are composed of well sorted medium- and fine-grained sandstone, the grain size of which increases from the base upward. These bodies pinch out up the paleo-flank, and down the flank they pass into fine-grained varieties.
The best reservoirs are on the up sides of the paleo-flanks, where well sorted sandstones have porosities up to 22 percent. They exhibit anomolously low values of gamma activity. Beach sandstone bodies have been identified on both the southwest and northeast flanks of this paleo-arch, where thickness of the upper Vasyugan is 30-40 m.
Sandstone bodies of type section VIII appear to be channel deposits. They have a concave base, are incised, have sharp contact with the underlying rock, grade laterally into fine-grained deposits and coal, are coarse-grained, and grade upward into finer-grained deposits. They were deposited during the time of regression when a stream net had developed at the crest of the paleo-arch. These channel sandstones form independent traps.
Beach and channel sandstones should be exploration targets in other parts of Tyumen Region also.
Taken from Slavkin, Shik, Guseynov, and Yermolova, 1995; digested
in Petroleum Geology, vol. 30, no. 3, one stratigraphic chart, one cross
section, one map.
Copyright 2002 James Clarke. You are encouraged to print out this
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