Aryskum downwarp is located in the south of the South Torgay depression. It is bounded on the north and northeast by the Mynbulak saddle and Ulytau fold massif, on the west by the Nizhnesyrdar'ya arch, and on the east by the buried continuation of the Ulytau and the Shu-Sarysuy depression. Aryskum downwarp consists of a series of grabens separated by sharply expressed basement highs. Top of basement in axial areas of grabens is at -3.5 to -6.0 km, and on the basement highs it is at -1.5 to -2.0 km. See Internet Geology News Letter No. 16, August 16, 1999.
Two structural stages are present in Aryskum downwarp: The lower is a folded terrane that consists of metamorphosed Precambrian to Ordovician schists and Cambrian to Lower Carboniferous effusive- carbonate-clastic rocks. The upper stage is the sedimentary cover, which consists of two sub-stages. The lower is a rift complex composed of Triassic to Upper Jurassic clastic graben fills. The upper is a Cretaceous-Miocene ortho-platform section that was deposited during a time of general regional subsidence. Thickness of the sediments of the rift sub-stage is 2.5-4.5 km, and that of the ortho-platform deposits is 1.0-1.5 km.
Aryskum downwarp has been studied on a broad scale since 1983. More than 15 oil and gas fields have been discovered. The main exploration targets are the Triassic-Jurassic sub-stage. The Neocomian also is favorable. Potential resources of Aryskum downwarp are assessed at 450 million tons of oil equivalent (3.150 billion barrels), of which 200 million tons (1.4 billion barrels) have already been delineated.
New information was gained in 1991-93 on the pre-Mesozoic section on the eastern border of Aryskum downwarp and also the Aksay horst, which is in the northwest part of the downwarp. By analogy with the Shu-Sarysuy depression to the southeast, reflecting horizon IIb is correlated with the base of the clastic Middle-Upper Carboniferous - Lower Permian section, and reflecting horizon III with the base of the clastic-carbonate Lower Carboniferous Serpukhovian-Visean- Tournaisian and the top of the clastic Lower-Middle Devonian - lower Tournaisian. These latter clastics increase in thickness toward the south. Velocities for the Serpukhovian-Visean-Tournaisian carbonate- clastic complex are high at 6.0-6.5 kmps, whereas those for the Lower- Middle Devonian - Tournaisian and Middle-Upper Carboniferous - Lower Permian are lower at 5.0-5.5 kmps.
On the Aksay horst two wells drilled on the Kenlyk structure yielded flows of oil from carbonates dated as Upper Devonian-Lower Carboni- ferous. Taking into account these oil recoveries, the Paleozoic complexes of Aksay horst and of the eastern border of Aryskum downwarp represent significant interest for oil and gas exploration and should be targets for geological and geophysical studies. (Taken from Kobzarev, 1997; digested in Petroleum Geology, vol. 32, no. 3; one structure map and four seismic 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/ This News Letter is distributed without charge in the interest of our science of petroleum geology. To be added to the mailing please send your e-mail address to jamesclarke@erols.com