Iran is one of the richest regions in the world in terms of hydrocarbon resources. It has the world’s third biggest reserves of oil, after Saudi Arabia and Canada, and also the world’s second largest gas reserves, behind Russia. According to the Oil and Gas Journal, as of January 2010, Iran has an estimated 137.6 Bb of proven oil reserves, roughly 10% of the world’s total reserves, and 1,045 Tcf of proven natural gas reserves, around 16% of the world’s total. Moreover, at 2.58 MMbopd, Iran is OPEC’s second largest exporter of crude oil after Saudi Arabia, according to that organization.
Iran has more than a century of history in exploration and production activities; the first successful exploration well was Masjid Suleiman-1 on May 26, 1908. Since then, based on the latest oil and gas reports, 145 hydrocarbon fields and 297 oil and gas reservoirs have been discovered in Iran, with many fields having multiple pay zones. A total of 102 fields are oil and the remaining 43 are gas, and there are 205 oil reservoirs and 92 natural gas reservoirs. According to Iran Energy Balance Sheet (2009, in Persian), 78 of these fields are currently active, with 62 onshore and 16 offshore, leaving 67 fields inactive at present. Some 23 hydrocarbon fields lie in border areas and are shared between Iran and adjacent countries, including Kuwait, Iraq, Qatar, Bahrain, UAE, Saudi Arabia and Turkmenistan.
Fields have been found in North Central Iran, in the Southern Caspian Basin and most recently in the Kopet-Dagh Basin in North East Iran. However, 140 fields, the vast majority of those discovered to date, are in the Zagros and Persian Gulf Basins in the south-west, and we will concentrate on those in this article.
Prolific Basins
The majority of gas fields in the Zagros Basin are found in Permo-Triassic carbonates, while Cretaceous and Tertiary carbonates host most of the oil.
An aerial view of a double plunged anticline in the Zagros Mountains. About a third of these anticlines contain hydrocarbons. Source: M. MirkazemianThe Zagros Basin can be divided into three sub-basins – Eastern, Central and Western – based on stratigraphic framework, structural elements and petroleum habitat. Similarly, the Persian Gulf Basin is split into the Eastern, Qatar Arch and Western sub-basins. Enormous gas/condensate reservoirs like the Dehram or Khuff formations are situated in the Qatar Arch and the Fars province of the Eastern Zagros; the Qatar Arch alone is estimated to contain about 19% of the world’s non-associated gas reserves, including the super-giant North Dome/South Pars fields. The Central Zagros, particularly the Dezful Embayment, is famed for its supergiant and giant oil fields based on the Asmari and Sarvak reservoirs. There are 16 exploration blocks in these sub-basins, four offshore and the remaining 11 onshore.
In nearly 90% of Zagros fields the hydrocarbons are found in structural traps at depths of less than 4,500m. The Iranian part of the Zagros Basin has more than 300 anticlines with north-west to south-east dominant trends, and roughly one third of these contain petroleum resources. There are approximately 200 salt domes in Eastern Zagros and the Persian Gulf, and many studies show that salt tectonics in the Hormuz salt horizons in this area are related to petroleum accumulations. In fact, some workers believe that as many as 60% of traps in the Persian Gulf are configured by evaporite diapirism, including North Pars, Iran’s second largest gas field, as well as the Abuzar, Froozan, Nowruz, Esfandiar and Soroush fields.
Important Carbonate Reservoirs
Stratigraphic position and geological distributions of five major reservoir rocks in the Zagros and Persian Gulf Basins. Generally, the oldest, predominantly gas, reservoir rocks (Dehram reservoirs) are clustered in East and West Zagros in the Fars, offshore Fars and Lurestan sub-basins, while the younger four oil reservoir rocks are concentrated in Central Zagros in the Dezful Embayment and Persian Gulf.Hydrocarbons are found in the Zagros and Persian Gulf Basins in reservoirs which range in age from the Guri limestone member of the Neogene Mishan formation to the Lower Permian Faraghan Sandstone formation.
In the Zagros and Persian Gulf Basins the highly porous Cretaceous and Tertiary carbonate rocks make very important oil reservoirs, while Permo-Triassic carbonates, particularly the Dalan and Kangan formations, are the main gas and condensate reser voirs. In fact, it is reported that a total of 38 gas/con dens ate pools have been discovered in the Dalan and Kangan formations in these basins alone. The mid Cretaceous Sarvak formation is significant for the volume of recoverable oil it holds, while the Oligo-Miocene Asmari formation is the best cur rent producer.
Our estimates indicate that in the Zagros and Persian Gulf Basins nearly 85% of oil reserves and more than 90% of gas/condensate reserves are hosted in carbonate rocks, including dolomitic, karstic, fractured and reefal limestones. For example, more than 60 fractured and karstic carbonate reservoirs are found in Asmari and Sarvak sequences in the Dezful Embayment. Not all the hydrocarbons are found in carbonates, however, and there are a number of major sandstone reservoirs in south-west Iran. The Ahwaz, Feridoun, Nowruz, Foroozan, B-structure and Soroush fields all have oil reservoired in sandstone rock units of the Ahwaz, Ghar, Zubair and Burgan sandstone members.
At a regional scale, the Dalan and Kangan formation gas reservoirs (Khuff equivalents) are sealed by Dashtak formation anhydriticshaly rocks, while the Gachsaran formation and the form ations of the Bangestan groups are the main cap rocks to the Cretaceous and Tertiary oil fields respectively.
The sapropelic shaly rocks of Silurian age, such as the Sarchahanformation, and the Jurassic and Cretaceous Khami and Bangestan groups, particularly the Pabdeh and Gurpi formations, are the main source rocks for the Zagros/Persian Gulf hydrocarbon fields. Iran’s crude oil generally has medium sulfur content and is in the 28°–35° API range.
Based on their stratigraphic position, petroleum system types, reservoir rock nature and properties, significance and geological distributions, the oil and gas reservoir rocks in the Zagros and Persian Gulf Basins have been classified into five carbonate sequences, ranging from the Oligo-Miocene Asmari to the Permo-Triassic Dehram. In some cases they also contain important sandstone reservoirs.
Discoveries Continue
Iran’s first drilling rig at the Masjid-Suleiman-1 well (1908) being prepared by local Bakhtiari workers. This photo was published by NIOC in the honor of the one hundredth anniversary of the founding of Iran’s petroleum industry. Source: NIOC
Classification of reservoir rocks of Zagros/Persian Gulf Basins based on stratigraphic position, petroleum system types, reservoir rock nature and properties, significance and geological distributions (modified after Motiei, H., 1995).
Farkhondeh Kiani Harchegani received her B.Sc in geology from the University of Isfahan in 2000, and her Master’s degree in sedimentology and sedimentary petrology from Azad University (Khorasgan Branch), Iran in 2010. Her area of interest is carbonate reservoirs, sedimentology and diagenesis.
Behrooz Esrafili-Dizaji undertook his undergraduate studies at the University of Zanjan (2001–2005) and received a Master’s degree from the University of Tehran in 2008. He is at present a Ph.D student in sedimentology and sedimentary petrology at the University of Tehran. His research projects focus on the characterization of Zagros/Persian Gulf carbonate reservoirs.
