Petroleum geology and resources of the middle Caspian Basin, Former Soviet Union

The Middle Caspian basin occupies a large area between
the Great Caucasus foldbelt and the southern edge of the Precambrian
Russian craton. The basin also includes the central part
of the Caspian Sea and the South Mangyshlak subbasin east of
the sea. The basin was formed on the Hercynian accreted terrane
during Late Permian?Triassic through Quaternary time. Structurally,
the basin consists of the fold-and-thrust zone of the
northern Caucasus foothills, the foredeep and foreland slope, the
Stavropol-Prikumsk uplift and East Manych trough to the north
of the slope, and the South Mangyshlak subbasin and slope of
the Karabogaz arch east of the Caspian Sea. All these major
structures extend offshore.
Four total petroleum systems (TPS) have been identified in
the basin. The South Mangyshlak TPS contains more than 40
discovered fields. The principal reserves are in Lower?Middle
Jurassic sandstone reservoirs in structural traps. Source rocks are
poorly known, but geologic data indicate that they are in the Triassic
taphrogenic sequence. Migration of oil and gas significantly
postdated maturation of source rocks and was related to
faulting and fracturing during middle Miocene to present time. A
single assessment unit covers the entire TPS. Largest undiscovered
resources of this assessment unit are expected in the largely
undrilled offshore portion of the TPS, especially on the western
plunge of the Mangyshlak meganticline. The Terek-Caspian TPS occupies the fold-and-thrust belt,
foredeep, and adjoining foreland slope. About 50 hydrocarbon
fields, primarily oil, have been discovered in the TPS. Almost all
hydrocarbon reserves are in faulted structural traps related to
thrusting of the foldbelt, and most traps are in frontal edges of
the thrust sheets. The traps are further complicated by plastic
deformation of Upper Jurassic salt and Maykop series (Oligocene?
lower Miocene) shale. Principal reservoirs are fractured
Upper Cretaceous carbonates and middle Miocene sandstones.
Principal source rocks are organic-rich shales in the lower part of
the Maykop series. Source rocks may also be present in the
Eocene, Upper Jurassic, and Middle Jurassic sections, but their
contribution to discovered reserves is probably small. Three
assessment units are delineated in the TPS. One of them encompasses
the thrust-and-fold belt of northern Caucasus foothills.
This assessment unit contains most of the undiscovered oil
resources. The second assessment unit occupies the foredeep and
largely undeformed foreland slope. Undiscovered resources of
this unit are relatively small and primarily related to stratigraphic
traps. The third unit is identified in almost untested subsalt Jurassic
rocks occurring at great depths and is speculative. The unit
may contain significant amounts of gas under the Upper Jurassic
salt seal.
The Stavropol-Prikumsk TPS lies north of the Terek-Caspian
TPS and extends offshore into the central Caspian Sea
where geologic data are scarce. More than one hundred oil and
gas fields have been found onshore. Offshore, only one well was
recently drilled, and this well discovered a large oil and gas field.
Almost the entire sedimentary section of the TPS is productive;
however, the principal oil reserves are in Lower Cretaceous clastic
reservoirs in structural traps of the Prikumsk uplift. Most
original gas reserves are in Paleogene reservoirs of the Stavropol
arch and these reservoirs are largely depleted. At least three
source rock formations, in the Lower Triassic, Middle Jurassic,
and Oligocene?lower Miocene (Maykop series), are present in
the TPS. Geochemical data are inadequate to correlate oils and
gases in most reservoirs with particular source rocks, and widespread
mixing of hydrocarbons apparently took place. Three
assessment units encompassing the onshore area of the TPS, the
offshore continuation of the Prikumsk uplift, and the central Caspian
area, are identified. The