South Torgay (Turgay) Oil-Gas Basin, Kazakstan

Internet Geology News Letter No. 6, August 16, 1999

The Torgay regional low connects the West Siberian platform on its north with the Turan platform on its south. It is bounded on the west by the folded Urals and on the east by the Central Kazak Shield. It shows up clearly on any relief map as a topographic low extending northward from the Aral Sea. The region has been known historically as the "Turgay Gates", a lake-filled depression (Columbia Lippincott Gazetteer). Midway along the Torgay regional low is a basement high, the Kustanay saddle, which forms a natural boundary between the West Siberian platform on the north and the Turan platform on the south. To the north of this saddle is the North Torgay depression, which is coal-prone, and to the south is the South Torgay depression, which is oil-prone. The latter is the South Torgay oil-gas basin.

Structurally the South Torgay depression consists of the Zhilanchik downwarp on the north, the Aryskum downwarp on the south, and the Mynbulak saddle between the two. Extending along the western margin of the depression is the Karatau-Talas-Fergana wrench fault. I have seen this fault hundreds of kilometers to the southeast in Kirgizstan, where it has offset modern gulleys by 1-2 meters. You can stand with one foot on one side and one on the other and look down a very long valley that is as straight as an arrow.

Overlying a basement of igneo-metamorphic rocks in the South Torgay depression are Devonian, Carboniferous, and Permian volcanics, clastics, and carbonates. These are generally regarded as an intermediate complex and not part of the sedimentary cover. They have little or no potential for oil and gas.

The Lower and Middle Triassic Turin Series at the base of the sedimentary cover consists of clastics and volcanics up to 800 m thick. The Upper Triassic is composed of clastics and coal and is more than 500 m thick.

The Lower and Middle Jurassic are largely sandstone, siltstone, clays, coal, and oil shale. These are continental deposits - lake beds. They contain source beds, reservoir rocks, and seals. Deposition was confined to grabens.

The Upper Jurassic sediments are similar in composition to the Lower and Middle Jurassic sections; however, they extend beyond the grabens. Their thickness approaches 1000 m.

The Lower Cretaceous rocks are continental, overlain by marine deposits from an early Turonian marine transgression. The overlying Cretaceous and Cenozoic deposits are represented by both continental and marine facies (Petroleum Geology, vol.31, no. 3, p. 240; and no. 4, p. 333, 1997).

Exploration during the Sixties and early Seventies made no commercial discoveries. A series of profile wells was drilled in the Eighties, disclosing direct oil shows. This was the basis for drilling a deep well on the Kumkol structure. The first well was a discovery. Several other fields have now been discovered. Kumkol field went onstream in 1989 (Petroleum Geology, vol. 32, no. 2, p. 107, 1996).

Three oil-gas plays are recognized in the sedimentary cover of the South Torgay oil-gas basin: Middle Jurassic, Upper Jurassic, and Neocomian. The Middle and Upper Jurassic are in the oil window; the Neocomian has not reached the oil window and consequently to oil present there has migrated from Jurassic source beds. Most of the oil was generated beginning with the early Cretaceous, when the reservoirs and seals were in place.

Resources of the Aryskum downwarp are assessed at 480 million tons (3.4 billion bbls) oil equivalent (Petroleum Geology, vol. 31, no. 4, p. 356, 1997).

Copyright James Clarke 1999. You are encouraged to print out copies of this News Letter and to forward it to others. Torgay