Petroleum geology and resources of the North Ustyurt Basin, Kazakhstan and Uzbekistan

The triangular-shaped North Ustyurt basin is located between the Caspian Sea and the Aral Lake in Kazakhstan and Uzbekistan and extends offshore both on the west and east. Along all its sides, the basin is bounded by the late Paleozoic and Triassic foldbelts that are partially overlain by Jurassic and younger rocks. The basin formed on a cratonic microcontinental block that was accreted northward to the Russian craton in Visean or Early Permian time. Continental collision and deformation
along the southern and eastern basin margins occurred in Early Permian time. In Late Triassic time, the basin was subjected
to strong compression that resulted in intrabasinal thrusting
and faulting.
Jurassic-Tertiary, mostly clastic rocks several hundred meters to 5 km thick overlie an older sequence of Devonian?Middle Carboniferous carbonates, Upper Precambrian massifs and deformed Caledonian foldbelts. The
Carboniferous?Lower Permian clastics, carbonates, and volca-basement is at depths from 5.5 km on the highest uplifts to 11
nics, and Upper Permian?Triassic continental clastic rocks, pri-km in the deepest depressions.
marily red beds. Paleogeographic conditions of sedimentation, Three total petroleum systems are identified in the basin.
the distribution of rock types, and the thicknesses of pre-Triassic Combined volumes of discovered hydrocarbons in these sysstratigraphic
units are poorly known because the rocks have been tems are nearly 2.4 billion barrels of oil and 2.4 trillion cubic
penetrated by only a few wells in the western and eastern basin feet of gas. Almost all of the oil reserves are in the Buzachi Arch
areas. The basement probably is heterogeneous; it includes and Surrounding Areas Composite Total Petroleum System in
2 Petroleum Geology, Resources?North Ustyurt Basin, Kazakhstan and Uzbekistan
the western part of the basin. Oil pools are in shallow Jurassic and Neocomian sandstone reservoirs, in structural traps. Source rocks are absent in the total petroleum system area; therefore, the oil could have migrated from the adjacent North Caspian basin.
The North Ustyurt Jurassic Total Petroleum System encompasses
the rest of the basin area and includes Jurassic and younger rocks. Several oil and gas fields have been discovered in this total petroleum system. Oil accumulations are in Jurassic clastic reservoirs, in structural traps at depths of 2.5?3 km. Source rocks for the oil are lacustrine beds and coals in the continental
Jurassic sequence. Gas fields are in shallow Eocene sandstones in the northern part of the total petroleum system. The origin of the gas is unknown.
The North Ustyurt Paleozoic Total Petroleum System stratigraphically underlies the North Ustyurt Jurassic system and occupies the same geographic area. The total petroleum system is almost unexplored. Two commercial flows of gas and several oil and gas shows have been tested in Carboniferous shelf carbonates
in the eastern part of the total petroleum system. Source rocks probably are adjacent Carboniferous deep-water facies interpreted from seismic data. The western extent of the total petroleum system is conjectural.
Almost all exploration drilling in the North Ustyurt basin has been limited to Jurassic and younger targets. The underlying Paleozoic-Triassic sequence is poorly known and completely unexplored. No wells have been drilled in offshore parts of the basin.
Each of three total petroleum systems was assessed as a single assessment unit. Undiscovered resources of the basin are small to moderate. Most of the undiscovered oil probably will be discovered in Jurassic and Neocomian stratigraphic and structural
traps on the Buzachi arch, especially on its undrilled off-shore extension. Most of the gas discoveries are expected to be in Paleozoic carbonate reservoirs in the eastern part of the basin.