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.
The 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
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.
Iran has several supergiant and many giant oil and gas fields, all in the southwestern area. Gachsaran in Central Zagros, the largest oil field in Iran, has three oil pay zones with an astonishing 66.7 Bbo in place and 23.7 Bb of recoverable reserves. And the South Pars gas field in the Qatar Arch is the largest gas field in the country with over 509 Tcf of recoverable gas, in combination with Qatar’s North Dome field. More than a half of Iran’s total gas reserves are accumulated in this single supergiant field.
In the last four years alone, 17 fields or reservoirs in existing fields have been found in Iran. These include the offshore Binaloud oilfield, and Sefid-Baghoun and the 11.2 Tcfg Sefid Zakhour fields in Fars, as well as fresh finds in the existing Changouleh and Azar oilfields in western Iran, several additional pay zones in the Azadegan oilfield in the Dezful Embayment, and a new Jurassic gas reservoir in the oldest field of all, Masjid Suleiman. In 2008, the discovery of 525 MMb (in place) of light gravity oil in the Kangan horizon in the East Assaluyeh field in the Fars province came as a surprise, because the field had been producing gas from the Dalan and Faraghan formations. Based on current data, Early Permian and peri Permian sequences, notably the Faraghan and Zakeen formations, may now be targeted as gas reservoirs, particularly in the Persian Gulf, so deeper drilling will be necessary for new discoveries in this area.
So, after over a century of exploration and production in Iran, exciting new discoveries and pay zones continue, even in the well explored Zagros and Persian Gulf Basins. At the same time the north-west and north-east parts of the country remain almost unexplored and a large number of potential structures have been identified in the Iranian Southern Caspian basins.
The exploration potential of this major hydrocarbon-producing country means that there will be many more discoveries before the oil runs out.