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Biostratigraphy, microfacies, and reservoir quality of the Oligocene Qom Formation (Kharzan Section, north-west of Ardestan, central Iran)

Arani Asma Aftabi, Ashouri Ali Reza, Daneshian Jahanbakhsh, Ghaderi Abbas, Wood David A.

Geological Quarterly

2023

Vol. 67, No. 3

art. no. 23

The central Iran Basin is a region with unique environmental characteristics in which the Late Paleogene–Early Neogene benthic foraminifera display distinctive distributions and abundances that can assist in identifying the intervals with the best reservoir potential. Lipidocyclinid and miogypsinid zonal marker taxa in this region can be correlated with those in the SBZ23 region (European Basin), indicating an Oligocene (Chattian) age. With sedimentation of continental strata of the Upper Red Formation following the marine succession of the Qom Formation, it seems that the last Tethyan marine transgression in the Ardestan region in central Iran occurred in the Oligocene, and the Tethyan seaway was permanently closed during the Miocene. Seven carbonate microfacies and marl or silty marl facies were identified in the study area based on field investigations, textural analysis and faunal assemblages. These microfacies were deposited on an open-shelf carbonate platform in lagoonal, patch-reef, and open-marine belts that effectively define both inner and middle-shelf environments. Micritization, cementation, mechanical and chemical compaction, dissolution and fracturing are the most important diagenetic processes controlling reservoir quality in the Qom Formation. The investigation of these processes in the facies of the Qom formation in the Kharzan section revealed that intervals associated with shallow lagoonal depositional environments display better reservoir quality than other formation intervals, due to dissolution and fracturing.

Identification of the Oligocene-Miocene boundary in the Central Iran Basin (Qom Formation) : Calcareous nannofossil evidences

Parandavar Mohammad, Hadavi Fatemeh

Geological Quarterly

2019

Vol. 63, No. 2

215--229

A diverse Late Oligocene to Early Miocene calcareous nannofossil assemblage was examined from the Qom Formation in the Central Iran Basin, and the Oligocene-Miocene boundary was identified based on the quantitative analysis of the assemblages in 303 smear slides. Eleven well-established calcareous nannofossil bio-events are delineated in the Upper Oligocene through Lower Miocene. The results clearly show that the Highest Occurrence (HO) of Sphenolithus delphix is the closest bio-event to the boundary as traditionally delineated on the lithostratigraphic criteria, and provides a distinct biohorizon below it. The Lowest Occurrence (LO) of the species Discoaster druggii is the oldest Miocene bio-event that is observed shortly after the HO of S. delphix, showing that calcareous nannofossils are well suited for approximating the Oligocene-Miocene boundary in the Qom Formation. The Oligocene-Miocene boundary is placed in the upper part of Sub-member “c1” in all three sections studied here and it is traceable throughout the Central Iran Basin, which makes a potentially reliable marker horizon for sequence stratigraphic and hydrocarbon studies in the area.