Petroleum Potential of Pre-Jurassic Rocks in Northeast of AmuDar'ya Province
In the northeast of the Amu-Dar'ya gas-oil province (News Letter 22) in
Uzbekistan are two structural steps, the Bukhara on the northeast and the
Chardzhou immediately to its southwest (Clarke, 1988, U.S.G.S. Open File
Report 88-272). The pre-Jurassic section of this region has been studied
over a period of some thirty years. Most of the investigations of the
pre-Jurassic have been confined to only a few tens of meters at the top,
where these rocks exhibit varying extent of metamorphism.
Recent deep seismic sounding and common depth point seismic surveys of the
central and eastern parts of the Chardzhou step have disclosed the Kushab
system of aulacogens, where undeformed or weakly deformed Paleozoic
reflecting interfaces are present at depths of 10-12 km. Total thickness of
the pre-Jurassic deposits here reaches 7-8 km. They are part of the
so-called intermediate complex, the section between basement and platform
sedimentary cover.
Filling of the Kushab system of downwarps appears to have begun in the Early
Paleozoic. Deep-water basin and slope-base seismic facies are noted. The
deep-water basin facies is probably metamorphosed fine-grained clastics.
The slope-base facies is represented by transgressive clinoforms deposited
as low-energy turbidity flows.
Associated with the Kushab system of aulacogens are horsts, which probably
formed in conjunction with intense Late Paleozoic folding. Rift structures
in the Upper Jurassic carbonates are associated strictly with these Upper
Paleozoic horsts. This association connects the Upper Paleozoic and Upper
Jurassic into a single oil-gas-geological system.
On the north of the Kushab system of aulacogens is the Divalkak-Matonat
block, where a Paleozoic flank seismic facies is present in the lower part
of the section and a shelf facies in the upper. Recognized in this section
are Lower Paleozoic, Lower-Middle Devonian, and Middle Carboniferous-Lower
Permian regressive clinoforms. Transgressive cycles are represented by
carbonate and clay deposits; they spread out in a "dart-like" manner. These
clinoforms form the north flank of the Kushab system and adjacent shelf.
The Upper Devonian-Lower Permian rocks of the Kushab system are marked by
more gentle folding, Upper Devonian-Lower Carboniferous carbonates are host
to a large number of anticlines and probably also carbonate buildups.
Overlying shales can serve as seals, Oil-source beds are present in this
section.
Formation of hydrocarbon accumulations in the study area was in two phases.
The first reached climax in pre-Permian time, when Lower Paleozoic source
beds entered the oil window. The Upper Paleozoic source beds had not yet
entered the oil window; they generated oil in Mesozoic and Cenozoic time,
constituting the second phase. It can thus be supposed that the
intermediate complex of the Kushab system of aulacogens is an exploration
target for both oil and gas (Petroleum Geology, vol. 33, no. 2, p. 178-184;
from Babadzhanov and others 1997).
Copyright 1999 James Clarke. You are encouraged to download this News
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