The question as to the oil-gas prospects of the south of the Cis-Ural downwarp and adjacent parts of the North Caspian depression in the southeast of Orenburg Region and Kazakstan has been examined in connection with predictions of a huge reef system of Early Devonian age. In these regions the Lower Devonian rests on rocks of Vendian, Ordovician, and Silurian age. Four types of section (facies zones) are recognized from east to west: reefs, stratified carbonate, carbonate-clastic, and clastic.
The reef type of section is found in the zone of foreland folds of the Urals. It consists of algal bioherms and detrital and fragment limestones. Thickness of these Lower Devonian reef deposits measures 1087 in one drill hole. They have been studied also in outcrops, where in addition barrier reefs are found.
The stratified carbonate type of section is represented by the Vyazov Horizon of the upper Emsian Stage. It consists largely of dolomite and limestone and rare beds of marl and argillite. The dolomites are gray, dark gray to almost black, microcrystalline, fossiliferous (ostracods and algae), and irregularly fractured. Some have very high fracture permeability along with low porosity. The limestones are gray, dark gray, organic carrying corals, ostracods stromatoporoids, and trilobites, unevenly recrystallized, dolomitized, and carry seams of clayery and bituminous matter.
The carbonate-clastic type of section is found in the zone of junction of the Cis-Ural downwarp and the East Orenburg arch.
The clastic type of Lower Devonian section has been mapped in the Bashkortostan part of the Cis-Ural downwarp, where it is called the Khlebodarov Formation. There it consists of argillite, siltstone, and sandstone. Thickness is up to 200 m.
A facies zonality is predicted for the Lower Devonian rocks. The barrier reefs are associated with the outer margin of the paleo-shelf of the East European continent. To its east was a bathyl zone. Then to the west of the reefs there is a systematic variation in lithologic composition of the Lower Devonian rocks: Stratified carbonates pass westward into carbonate-clastics and then into largely clasics. All this is accompanied by a decrease in thickness to final pinchoit.
Prediction of the position of the Lower Devonian reef system is based on drilling and working hypotheses. The latter propose several large highs along the top of the Middle Carboniferous Bashkirian Stage that are drapes on Lower Devonian reefs. It should be noted that the north-northeast trend of the reef system is in good agreement with the position of the shore line of the East European continent in Silurian and Early Devonian time.
At least two two types of favorable target are then recognized in the study area: Lower Devonian reef buildups and drape structures above them.
Taken from Politykina and Tyurin, 2001; digested in Petroleum
Geology, vol 36, no. 3, 2002, in press, one map.
Copyright 2002 James Clarke. You are encouraged to print out
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