PETROLEUM GEOLOGY OF THE WESTERN PART OF THE CENTRAL IRAN BASIN

Eocene extension and magmatism in Central Iran was followed by late Eocene - early Oligocene uplift, erosion, volcanism and the deposition of the continental and evaporitic sediments of the Lower Red Formation. During the late Oligocene - early Miocene, an extensional (or transtensional) phase occurred with the deposition of the limestones and marls of the Qom Formation, followed by the evaporitic deposits or mudstones of the basal part of the Upper Red Formation. Since the late Miocene, compression has resulted in regional shortening and uplift, with the deposition of the thick, clastic-dominated upper part of the Upper Red Formation and the overlying conglomeratic unit. Between 1951 and 2016, a total of 45 exploration, appraisal and development wells were drilled across the western part of the Central Iran Basin where the Alborz, Sarajeh and Aran fields are hydrocarbon discoveries. Traps at these fields are NW-SE oriented detachment folds formed during the late Miocene - Pliocene. Porous and fractured limestones in the Qom e-member are the principal reservoir units, and are capped by evaporites or mudstones in the basal part of the Upper Red Formation. Organic-rich mudstones in the Qom e- and c-members together with shales in the Jurassic Shemshak Formation are potential source rocks. An overview of 80 years of exploration efforts in the western part of the Central Iran Basin suggests that the main reasons for the general lack of success include drilling-associated problems, poor reservoir characteristics, lack of hydrocarbon charge, and underestimating the thickness of the overburden on top of the Qom reservoir.