The Devonian clastic section has long been the most important play in the Volga-Ural oil-gas province of the Russian platform. With new discoveries in Middle Devonian carbonates the range of the favorable section has increased, and in the south of the province the Eifelian carbonates are the principal play.
The Eifelian-lower Frasnian carbonate-clastic sediments are widespread in the study area. Oil and gas pools have been discovered at depths of 3.5-5.5 km. They occur in Biy-Aftonian carbonate and Vorob'yev-Ardatov and Pashiy clastic reservoirs. The carbonates range in thickness from 20 to 95 m and are largely stromatolitic limestones. The clastic reservoirs are sandstones, which range from 10 m thick on the east to several tens of meters farther west. These sandstones are not persistent, and their reservoir properties are variable. The carbonates make the best reservoirs.
The crystalline basement has a block structure. These blocks are bounded by regional faults, which are marked by geomagnetic gradients, lineaments, and flexures in the sedimentary cover. They show up on satellite photographs. Uplifted blocks correspond with gravity maximums.
A characteristic feature of the study area is that the uplifts and paleo-uplifts are related to inversion of local blocks of basement. Among these are the Orenburg, Vostochno-Orenburg, Altatin, Derkul, and Karachaganak highs. For example, on the Rozhkov crest of the Derkul high Famennian carbonate sediments rest directly on Riphean-Vendian clastic deposits.
Downwarps formed in connection with the inversion movements. Thin-bedded carbonate-clayey bituminous sediments were deposited in lows, and shallow-water limestones on the highs. Then on the periphery of the blocks organic buildups formed.
There is consequently a genetic relationship between the lithofacies zonality of the Biy-Aftonian sediments and the block structure of the basement. This relationship can be used to predict zones of development of organic buildups along the margins of raised blocks of the basement and along borders of grabens. Extensive fault-associated highs of the Zaykin and Ol'khov type may be present in these zones. These highs are anticlinal traps for oil and gas in both the Biy-Aftonian organic buildups and in the Givetian-lower Frasnian clastic sediments. Most of the local highs on the top of the Clastic Devonian have closures of 40-60 m. All these highs are favorable for discovery of oil and gas.
Taken from Kiryukhin and others, 1986; digested in Petroleum
Geology, Vol. 23, No. 9/10, one tectonic map and one cross section.
Copyright 2003 James Clarke. You are encouraged to print out
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