Micro-continents were accreted onto the East European continent during the Late Paleozoic. The Ural and Ustyurt micro-continents joined on the east and south, respectively, during middle Visean time of the Early Carboniferous. Somewhat later in Middle Carboniferous- Permian time the North Caucasus approached on the south and southwest. Collision of the East European and Kazak continents took place on the east in the Middle Carboniferous-Permian-Early Triassic, leading to deposition of orogenic sediments, new pulses of overthrusting, and final formation of the Zilair-Aktyubinsk branch of the folded frame of the depression on the east and southeast. The southern semi-ring of the mountainous frame of the depression was completed by collisions by the beginning of the Kungurian Stage of the Early Permian. The North Caspian basin was thereby closed off by the East European continent on the north and west and by the orogenic belts on the east and south, providing conditions for deposition of the thick evaporites of Kungurian age over the entire basin. Oceanic crust is present in an east-west zone in the central part of the basin; this feature is interpreted as a spreading center.
The post-Kungurian evolution of the North Caspian depression was a time of isostatic adjustment with a predominance of vertical movements, breaking the depression into blocks, salt tectonics, and redistribution of oil and gas pools.
The North Caspian province is not a single zone of oil-gas formation; rather, each of the plate tectonic features is an independent sub-basin with its own source beds, types of traps, and types of pools.
Pre-Kungurian source beds were deposited on passive continental margins as well as in intra-continental rift zones. The carbonates have low contents of organic matter. Liquid hydrocarbons expelled from these rocks have been estimated at less than 30,000 tons per sq km. Clastic source beds of the passive continental margins are largely fore-delta facies, debris cones, and turbidites. An example is a thick (700 m) alternating sand-clay unit of Early Carbonifeous age deposited on the Emba passive margin. These source beds contain largely humic organic matter. A rapid rate of deposition and close proximity to the land source-area protected this organic matter from oxidation. The ratio of liquid to gas from these sediments is 1:4. Total expulsion from these rocks reached 900,000 tons per sq km.
In Late Paleozoic time shelf carbonates predominated along the margins of the basin and uncompensated clayey-carbonate-siliceous sediments in its interior. The latter are equivalents of the Domanik Formation of the Volga-Ural oil-gas province to the north. Largely liquid hydrocarbons collected in pre-Kungurian time in the basin. Then after deposition of the Kungurian evaporites the Paleozoic source beds entered the thermal gas window, and this gas then expelled oil from the traps in a large part of the region. The oil in the southeast of the basin may have been protected from this expulsion by a structural high on its north.
Taken from Shein, Payrazyan, Pazmyshlyayev, and Sokolov,
1989; digested in Petroleum Geology, vol. 28, no. 5/6, 1994, one
plate-tectonic map, one cross section, and one oil-gas map.
Copyright 2001 James Clarke. You are encouraged to print out this
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