The shelf of the north part of the Caspian Sea is a continuation
of the onshore North Caspian Lowland. Water depths are less
than 5 m in 70 percent of this offshore area. This shelf area is a
zone of junction of the East European craton and the Scythian and
Turan platforms.
The Carboniferous upper Visean - lower Bashkirian carbonate
complex consists of organic and organo-clastic limestone. Its
lithologic composition is persistent regionally. On the paleo-shelf
of the North Caspian depression it consists of massive reservoirs
of the pinnacle reef (Tengiz) and carbonate platform (Astrakhan
and Primore) type.
The upper Visean - lower Bashkirian Astrakhan carbonate massif
is located in the northwest paleo-shelf part of the North Caspian.
Thickness of these carbonates is up to 2 km. The reservoir rock is
biomorphic and organo-clastic limestone. Resting on an erosion
surface at the top of the Bashkirian are deep-water Asselian -
Artinskian clayey limestones and shales. These are the seal for
a massive hydrocarbon pool. Sequence seismic stratigraphy shows
presence of local reef bodies along the east perifery of this massif.
In the northeast coastal area of the Caspian Sea are the Primore
carbonate platform and the Tengiz pinnacle reef. The Primore massif
measures 60 by 20 km - almost half offshore. Biohermal limestones
consist of algal, coral, foraminifera material, and organic detritus. Reef
deposits are widespread in the Tournaisian-Bashkirian part of the
section. These are overlain discordantly by Artinskian carbonate-clay
deposits. Tengiz reef massif (all onshore) is composed of Serpukhovian
limestones and occupies an area of 400 sq km. Thickness of the
carbonates of the Primore and Tengiz features is more than 1 km.
Offshore Shaburbali carbonate massif is west of Tengiz and southeast of
Atarau. It was disclosed by gravity surveys and outlined by seismic
surveys.In the structurally highest part of the massif at depth of 3.8-4.0
km are individual crests, which combine into the northeast-trending
Shaburbali arch. This feature is 80 km long, 20 km wide, and has
closure of 2 km on the surface beneath the Permian Kungurian salt.
Kerogly massif is west of Shaburbali massif and directly south of
Atarau. It consists of a 2-km thickness of Carboniferous-Upper Devonian
carbonate rocks that occur as a series of isolated biohermal buildups.
On the flanks these pass into deep-water facies.
Zhambay carbonate massif extends far offshore from the vicinity of
Astrakhan. It is a clearly expressed structure of biohermal-erosional
origin. Kungurian salt is either not present here or is thin and
flat-lying.
By analogy with adjacent onshore areas the Zhambay, Kerogly,
Shaburbali, and other offshore structures are assessed as highly
favorable for discovery of oil and gas.
Linear downwarps of the south border of the North Caspian depression
are regarded as rift structures, which in late Hercynian time were subjected
to inversion as a consequence of approach and collision of micro-plates.
(Taken from Murzagaliyev, 1995; digested in Petroleum Geology, vol. 30,
no. 1, 1996, one map and one cross section.)
Copyright 2001 James Clarke. You are encouraged to print out this News
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