Biostratigraphy, microfacies and reservoir quality of the Oligocene Qom Formation (Kharzan section, Northwest of Ardestan, Central Iran)

Abstract

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.