Petroleum Geology of Afghan-Tadzhik depression
Internet Geology News Letter No. 42, April 24, 2000
The Afghan-Tadzhik depression of Central Asia has an area of
some 50,000 sq km. Depth to basement in the structurally high areas
is 6-8 km, and in the most subsided areas it lies at 16-18 km depth.
This depression is the main component of the Surkhan-Vakhsh gas-oil
region (Gabrielyants and others, 1992).
Afghan-Tadzhik depression is a post-orogenic feature in which
there is a great variety of traps characteristic of both the earlier
platform stage and the more recent orogenic stage. Many of the hydrocarbon accumulations of the platform stage were disrupted
and redistributed into traps of the orogenic stage. Some of the
traps of the orogenic stage may be beneath thrust sheets, and
these have not been explored.
Most of the assessed undiscovered oil and gas above the Upper
Jurassic salt is placed in the Paleogene. Maturity of exploration there
is placed at 10-15 percent. Almost all the proved reserves are in
anticlinal traps, and most of these have already been tested.
The most favorable exploration targets are sub-salt Jurassic rocks.
However, these are at depths of 7-9 km and must wait a while before
being played. More immediate targets are Cretaceous-Paleogene
reservoirs, in which more than a score of small fields have been
discovered, most in synclinal zones.
Lower Cretaceous source beds are small in volume and contain
very little organic matter, which is humic. Expulsion of hydrocarbon
fluids from these beds is an average of 9 thousand tons per sq km,
and for gas it is 12 million cu m per sq km. In the Upper Cretaceous
complex there is a greater volume of source rock, content of organic
matter, and sapropelic components. Expulsion of liquid is 13 thousand
tons per sq km, and that for gas is 16 million cu m per sq km. Paleogene
source beds contain yet more organic matter, and the sapropelic
component predominates. Expulsion of liquid hydrocarbons from
the Paleocene-Eocene sediments is placed at 70-90 thousand tons
per sq km, and gas at 69-123 million cu m per sq km. (Kondur and
others, 1992; digested in PETROLEUM GEOLOGY, vol. 27, no.11/12,
1993, with 2 maps and 2 cross sections).
The first discovery in Afghan-Tadzhik depression was Khaudag in
1934 in Paleocene sediments. In the Sixties discoveries were made in
Cretaceous and Jurassic rocks. As of 1989 some 24 fields with 65 pools had been discovered in the depression. The oils of the Paleogene pools
are heavy due to biodegredation in connection with infiltrating water.
(Maksimov and others, 1989).
Seismic surveys are not very effective in the study area because of
steep dip of the reservoir rocks.
Copyright 2000 James Clarke, You are encouraged to print out this
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