Geology Reference
In-Depth Information
crypto-diapir and diapir folds intensely faulted (normal faults and over-
thrusts with the throw of up to 2,000 m); the faults die-out down the sec-
tion; the folds are often overturned and showing indications of nappe
folding mechanism.
The region is neotectonically active. The manifestations are present-day
horizontal offsets on the surface, a state of stress in mines, great number
and significant magnitude of earthquakes and the association of their epi-
centers with intersections of seismically active faults.
Commercial oil and gas accumulations are present within a broad strati-
graphic range from the Lower Cretaceous (Valanginian) to the Miocene
(Sarmatian). The main reserves are concentrated in the Cretaceous and
Miocene reservoirs. Hydrocarbon fields are mostly associated with the
Tersk, Sunzha and Dagestan Piedmont oil and gas accumulation zones,
which correspond with the same-name anticlinoria. Prevalent in the frac-
tured Mesozoic limestones are massive-type accumulations, whereas the
Neogene granular reservoirs are dominated by sheet-type anticlinal and
fault-trapped accumulations. The number of productive intervals in the
Miocene in some fields reaches 24. A typical feature of the oil and gas
occurrences in the basin is a distinct association between commercial
occurrences and fault tectonics; maximum hydrocarbon concentrations
are found in the intensely deformed local structures positioned along the
regional faults. Another distinction is that if there are accumulations in the
Neogene, they are always found also in the Mesozoic.
The major fields in the region are Malgobek-Voznesenskoye, Karabulak-
Achaluki, Starogroznenskoye, Oktyabrskoye and Bragunskoye.
The general pattern in the basin is the association of over 80% of the
discovered hydrocarbon reserves with the depth interval shallower than
3,000 m, the contracted upper gas zone and the domination of the middle
oil zone. The lower, gas-condensate zone is not penetrated currently due to
great depths (over 7,000 m). However, the entire set of geologic conditions
testifies to its presence (Durmishyan, Muradyan and Rachinsky, 1976;
Stanulis, Khludnev, 1987).
1.2.4
he Zagros Foredeep
The basin is the eastern part of the extensive Mesopotamian Foredeep. Its
northern and eastern boundaries coincide with the Main overthrust of the
Zagros meganticlinorium, the southern boundary formed by the zone of
regional faults in the Hajar folded mountains. The western boundary runs
along the suture fault band in the center of the Persian Gulf (these faults
