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

Abstract

The cen tral Iran Ba sin is a re gion with unique en vi ron men tal char ac ter is tics in which the Late Paleogene-Early Neo gene ben thic foraminifera dis play dis tinc tive dis tri bu tions and abun dances that can as sist in iden ti fy ing the in ter vals with the best res er voir po ten tial. Lipidocyclinid and miogypsinid zonal marker taxa in this re gion can be cor re lated with those in the SBZ23 re gion (Eu ro pean Ba sin), in di cat ing an Oligocene (Chattian) age. With sed i men ta tion of con ti nen tal strata of the Up per Red For ma tion fol low ing the ma rine suc ces sion of the Qom For ma tion, it seems that the last Tethyan ma rine trans gres sion in the Ardestan re gion in cen tral Iran oc curred in the Oligocene, and the Tethyan sea way was per ma nently closed dur ing the Miocene. Seven car bon ate microfacies and marl or silty marl fa cies were iden ti fied in the study area based on field in ves ti gations, tex tural anal y sis and fau nal as sem blages. These microfacies were de pos ited on an open-shelf car bon ate plat form in la goonal, patch-reef, and open-ma rine belts that ef fec tively de fine both in ner and mid dle-shelf en vi ron ments. Micritization, ce men ta tion, me chan i cal and chem i cal com pac tion, dis so lu tion and frac tur ing are the most im por tant diagenetic pro cesses con trol ling res er voir qual ity in the Qom For ma tion. The in ves ti ga tion of these pro cesses in the fa cies of the Qom for ma tion in the Kharzan sec tion re vealed that in ter vals as so ci ated with shal low la goonal depositional en vi ron ments dis play better res ervoir qual ity than other for ma tion in ter vals, due to dis so lu tion and frac tur ing.