Upper Oligocene-Lower Miocene Carbonate and Evaporite Depositional Systems of the Northern Arabian Plate
Authors: Grabowski George J. , Tai Po C. , Liu Chengjie , Wilson Augustus O. 
Published: March 2015

The youngest major reservoirs and seals of the northern Arabian Plate occur in the Chattian, Aquitanian, and Burdigalian. They unconformably overlie major reservoirs of the Oligocene Kirkuk Group, shelfal carbonates formed on the northeast margin of the Mesopotamian Basin.

Deep-marine carbonates of the upper-Chattian Ch3 and lower-Aquitanian Aq1 sequences (Serikagni Formation) were deposited within the basin. A thin anhydrite occurs at the base. These pass upwards into shelfal carbonates (Euphrates and Middle Asmari formations), which lie unconformably above older shelfal carbonates around the basin. The basin is completely filled by evaporites and carbonates of the upper-Aquitanian Aq2 sequence and lowstand of the basal-Burdigalian Bur1 sequence (Dhiban Formation and Kalhur Anhydrite). The top of the shelfal carbonates is a subaerial unconformity.

Shelfal carbonates (Jeribe and Upper Asmari formations) were deposited in the transgressive to highstand systems tracts of the Bur1 and Bur2 sequences. Subaerial-exposure surfaces are recognized at the top of each of these sequences. Cyclical marginal-marine to nonmarine evaporites, carbonates and siliciclastics (Transition Beds of the Fat'ha Formation, and lower Gachsaran Formation) lap onto the underlying sequences. In parts of northern Iraq the Basal Fars Conglomerate occurs at the base of the Fat'ha Formation, composed of pebbles of the underlying Oligocene-Miocene carbonates and various lithoclasts of Jurassic-Paleogene age transported from the hinterland to the northeast. Deposition of the evaporite-bearing Fat'ha Formation ended in the late Burdigalian to early Langhian.

Oolitic-skeletal grainstones and skeletal-peloidal packstones and wackestones of the Euphrates and Jeribe formations are partly to completely dolomitized and have 8–20% interparticle, intercrystalline and moldic porosity and 1–10 mD permeability. Basinal wackestones and mudstones of the Dhiban and Serikagni formations are dolomitic and have 10–17% porosity with &lt;1 mD permeability. Thin limestones of the Transition Beds have 4–15% porosity with &lt;1 mD permeability. Evaporites of the Fat'ha and Dhiban formations are the primary seals for these reservoirs.

Stratigraphy and Age Framework

The lithostratigraphic units of Iraq and Iran and the ages of the Lower Miocene formations from the northern Arabian Plate are shown in Figure 1. Major sequence boundaries occur at the top of the Transition Beds of the Fat'ha Formation and the Jeribe and Euphrates/Serikagni formations. The basin-filling evaporites of the Saliferous Beds of the Fat'ha Formation and the Dhiban Formation are lowstand-transgressive deposits formed during one and a half sequences.

Carbonates of the lower Aquitanian and lower and middle Burdigalian are porous and form reservoirs for oil and gas. Evaporites of the upper Aquitanian and upper Burdigalian form seals for these reservoirs.

Biostratigraphic control is lacking or imprecise for these marginal- to shallow-marine carbonates and evaporites. We have relied extensively on strontium stable isotopes of anhydrite for age constraints. The 86Sr/87Sr ratio increases steadily from 0.7083 at the base of the Aquitanian to 0.7089 at the top Langhian. The results are summarized in Figure 2.