Hydrocarbon Potential of Upper Proterozoic Rocks of Volga-Ural Province
Internet Geology News Letter No. 39, April 3, 2000
The Upper Proterozoic Riphean and Vendian sedimentary rocks of the
Volga-Ural oil-gas province have all the prerequisites necessary for
petroleum generation and accumulation: source beds, reservoir rocks, seals,
traps, and timing. Semi-commercial oil fields and numerous shows of various intensity have been found. However, absence of a satisfactory exploration strategy and inadequate deep drilling and seismic surveys prevent and objective assessment of this part of the section. Deep drilling has been accomplished in Perm and Orenburg Regions, Bashkortostan, Udmurtia, and Tatarstan. Thirty-five wells have reached the Riphean and 235 wells have reached the Vendian - all this in an area of 350,000 sq km. Common depth point seismic surveys have been limited.
The Riphean and Vendian complexes form independent structural stages, separated by angular unconformity. The Riphean is the lowest part of the platform cover and is characterized by graben structures. The overlying Vendian is less deformed.
The Lower Riphean (1650-1350 m. y.) is largely clastic and is thousands of meters thick. The Middle Riphean (1350-1000 m. y.) consists of carbonates and clastics and also is a few thousand meters thick. The Upper Riphean is largely clastic, and thicknesses are in the hundreds of meters. Thicknesses of individual formations have great range due to erosion.
Vendian sediments rest transgressively on various units of the Riphean and on basement and consist entirely of clastics. Thicknesses of individual formations are measured in hundreds of meters.
Although most of the Riphean and Vendian rocks have low porosity and are practically impermeable, some have satisfactory reservoir properties. Some Riphean sandstones have porosity of 10-20 percent, and up to 60 percent of the sand-silt varieties of the Vendian have satisfactory reservoir properties. Seven regionally expressed reservoir strata with porosity up to 20 percent are recognized in the Vendian.
Clay-silt and clay-carbonate rocks 40-140 m thick constitute local and regional seals.
Possible source beds are clastic-carbonate rocks that were deposited under reducing conditions. Content of organic matter ranges from 0.1 to 9.9 percent in these beds. Level of maturation is satisfactory. The oil in these Upper Proterozoic rocks is an independently generated type with high viscosity, low benzine fractions, and low sulfur content. Oil-gas shows have been recorded at various stratigraphic levels in more than 40 wells. Where flows have been recorded, they have been in the 1-3 tons per day range.
Preliminary assessment of the petroleum potential of the Riphean and Vendian of the Volga-Ural province places oil at 700-1500 million tons (4.9-10.5 BBOE) and gas at 308 billion cu m (10.8 tcf). Seismic surveys indicate 57 local highs on the top of the Kaltasin Formation of the Riphean and 277 on the base of the Vendian. (Taken from Golov and Chepikova, 1999; digested in Petroleum Geology, vol. 34, no. 3, p. 239-246, 2000, one map and three cross sections)
Copyright 2000 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/
Inquiries concerning the journal Petroleum Geology can be made by telephone at (703) 759-4487. Please see also our website.
