The Oligo-Miocene Asmari oil fields of Iron are truly giants, most of them having recoverable reserves greater than 1 billion bbl each, and many having much more. The fields are close together in a region of relatively constant stratigraphy and structure, and have a common genetic history. The individual accumulations occupy very large rock volumes in large-amplitude folds and, although the Asmari reservoir is poor in porosity and matrix permeability, very high production rates are possible because of extensive reservoir fracturing. These rates can be maintained for very long periods because of the great vertical extent of the oil columns. The Asmari fields are prime examples of anticlinal traps and of the effect of fracturing on reservoir performance.

The Asmari reservoir is limestone of Oligo-Miocene age and consists mostly of shallow-water but nonreefal carbonate rocks, with a significant sandstone member in the northwest part of the area. The Asmari is the uppermost wholly marine unit in a shelf-carbonate sequence interspersed with shale which was deposited, with only minor interruptions, from Carboniferous through Oligo-Miocene time. At the end of the time of Asmari deposition, increasing tectonic instability caused more varied sedimentation; this phase of instability terminated in a strong orogeny (Zagros orogeny) which formed the enormous anticlinal traps in this thick sedimentary sequence.