The Amu-Dar'ya gas-oil province extends across central and eastern
Turkmenistan and western Uzbekistan in southern Central Asia. It has an
area of 360,000 sq km. The province lies within the Turan platform, where a
basement that was consolidated during the Hercynian orogeny is overlain by
Mesozoic and Cenozoic marine and continental platformal sedimentary rocks
1-7 km thick. The southeastern boundary of the Amu-Dar'ya province is drawn
artificially at the Afghan border; however, the gas-oil basin extends into
Afghanistan, where it covers an additional area of 45,000 sq km. The
province is gas-prone.
The Dauletabad structure was first indicated by geological surveys on
Neogene sediments during the years 1957-60. Seismic surveys began in 1968
and confirmed the large Dauletabad arch, which extends 60 km east-west.
Three crests are present, the westernmost of which is the Dauletabad high.
The first wildcat was spudded in 1972 on this Dauletabad high and bottomed
at 3542 m in 1973. Testing of Lower Cretaceous Hauterivian sandstone in
1974 yielded a strong flow of gas. Reserves have been placed at 49 tcf.
The field is so large that it extends over the greater part of the 1 degree
quadrangle bounded by latitudes 36-37 degrees N and longitudes 61-62 degrees E.
During the Silurian and Devonian a marine basin was present just north
of the study area, part of a seaway that extended eastward into northern
China, northward as the Uralian miogeocline, and westward through the
Caucasus into Central Europe and thence along eastern North America as the
Appalachian miogeocline. Marine transgression in the study area began in
Early Carboniferous time. Orogenic activity late in the Carboniferous
mildly metamorphosed these rocks. The Amu-Dar'ya region entered a sag stage
in Jurassic time with deposition of clastics, passing upward in the section
to carbonates and finally salt toward the top. The Cretaceous and Cenozoic
are represented largely by clastics.
In Dauletabad-Donmez field Upper Jurassic Callovian carbonates are at
the base of the platform sedimentary cover; they are 300-350 m thick. The
overlying Cretaceous section consists of 1765 m of carbonates and clastics.
Next in the section are 300 m of Paleocene carbonates, 500 m of Eocene
clays, and 500 m of Neogene continental clastics. Near the base of the
Cretaceous in the Hauterivian section is the Shatlyk Horizon, which is the
reservoir of Daulatabad-Donmez field.
The Shatlyk Horizon in the field dips monoclinally to the north across
the area of the field from an elevation of -2400 m on the south to -3450 m
on the north. East-west-trending high-angle normal faults divide the field
into north, central, and south blocks. Two gas pools are present in the
field, one on the north block and one on the south block. The central block
is water-bearing. The reservoir rock is fine- to medium-grained reddish
brown sandstone. In zones where the gas is sour, the sandstone is gray and
pyrite is present. Effective thickness is 17.7 m on the north block and 10
m on the south block. Porosity is 18-21 percent, and permeability is
200-700 md. The seal for the pools is carbonate rock of Barremian age. The
pools are confined along their borders largely by hydrodynamic pressure
(Clarke and Kleshchev, 1992, A.A.P.G. Treatise of Petroleum Geology,
Stratigraphic traps III, p. 285-300).
Golovanova (1995, Geologiya Nefti i Gaza, no. 12, in Russian) used
biomarkers to demonstrate that the gas at Dauletabad-Donmez was generated by
both Lower Cretaceous and Upper Jurassic source beds. Discovery of oil
pools is predicted for the easrten parts of the blocks and gas-condensate
pools in the northwestern parts of the north block and in the western parts
of the south block.
Copyright 1999, James Clarke. You are encouraged to print out copies of
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