Geology Reference
In-Depth Information
A distinctive feature in the region is a drastic decrease in the carbonate
reservoir fracturing with distance from the crests of intensely faulted local
highs and toward the general regional subsidence of the folded system.
The foredeep is asymmetric: the eastern (internal) stronger deformed
geosynclinal flank is narrow and steep; the western (external) platform
flank is broad and low-angle. The axial zone is offset toward the Zagros
Mountains where maximum sediment thicknesses are recorded. The
section is clearly subdivided into two major structural stages. The lower
one includes stratigraphic complexes through Asmari (Oligocene-Lower
Miocene) and is predominantly carbonates. The upper one, the Upper
Miocene-Quaternary (Fars Formation, Bahtiari Series and some others),
is clastics.
A system of lengthwise and crosscutting regional faults (normal faults)
cuts the region into steps/fault-blocks. The throw of some faults reaches
2,500 m.
Specific features of the foredeep's tectonics are diverging structure plans
of the upper and lower structural-formational stages; the nappe fold-
ing type of of the upper clastic complex; and intense faulting of the local
structures weaken with distance from the folded flank and toward the
platform one.
The region is neotectonically-active. This manifests itself in intense
present-day topographic changes, seismic activity of the deep-seated faults,
association with them of the epicenters of strong earthquakes, etc.
The local highs over the geosynclinal flank are large faulted elongated
anticlines, up to 85 km long and up to 10 km wide, with low-angle north-
eastern and steep southwestern flanks. The highs form separate anticlinal
belts (zones) parallel to the Zagros folded system. The closure of many
structures reaches 3,000 m and decreases toward the center of the trough
where they are overlain by the Quaternary. The geometry of the struc-
tures changes in the same direction from linear and high-amplitude near-
the-flank folds to low-angle brachianticlines and sometimes dome-like
features.
The main oil and gas complex in the region is the Upper Oligocene-
Lower Miocene sandstones of the Asmari Fm. The Upper Cretaceous car-
bonates (Bangestan Series) are productive over the crests of local highs
where they form a single reservoir with the Asmari. In a number of fields
the Lower Cretaceous limestones (Hami Series) and the Upper Jurassaic
Arab Fm. also include commercial oil and gas accumulations. The domi-
nating type of accumulations is massive.
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