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In this world-class setting, the potential for major gas discoveries is high.
In a global ranking of geologic provinces in terms of petroleum resources, the Zagros and Persian Gulf basins are among the richest provinces. According to the “BP Statistical Review of World Energy June 2011,” these areas hold 2,777 Tcf of natural gas reserves, roughly 40.5% of the world’s total. Several supergiant nonassociated gas reservoirs have been found in this region.
The Permo-Triassic successions (the Dehram group in Iran and its lateral equivalent, the Khuff formation), are major gas-producing intervals in these basins. The supergiant North Dome/South Pars field alone is estimated to hold about 19% of the world’s total gas reserves, producing gas and condensate from these intervals.
Current data indicate 38 petroleum fields in the Dehram intervals of Iran. The majority of the Dehram reservoirs are clustered in the Eastern Zagros or Fars and offshore Fars provinces. In addition, these successions offer potential petroleum exploration opportunities in the Western Zagros.
Moreover, 14 giant/supergiant Dehram gas reservoirs (about half of the total gas reserves of Iran) are in the Persian Gulf, and several Dehram giant/supergiant gas reservoirs are not developed yet, according to the 2009 “Iran Energy Balance Sheet.”
These Dehram reservoir distributions and their absence in the Dezful Embayment and Izeh Zone indicate main controls of Qatar-Kazerun and Balarud faults for these reservoir configurations. No information is available for the Dezful Embayment and the Izeh Zone because no wells have penetrated the Triassic and deeper intervals. The Dehram sediments in the Dezful Embayment are too deep (deeper than 6,000 m, or 19,685 ft) to have been reached by the drill bit.
Petroleum system components
The Paleozoic petroleum system is the main petroleum system in the Persian Gulf basin and surrounding areas, particularly in Qatar/Fars High. The Lower Silurian Sarchahan formation (or Qusaiba Hot shales) is believed to be the main source rock for this petroleum system. Thermal evolution studies denote inception of oil generation in the Middle Jurassic in areas of greatest subsidence, while the gas window was reached locally as early as the Middle Cretaceous. Then, gas was accumulated in the pre-Zagros trap structures. Major reservoir rocks of this system are the oldest reservoir rocks in Iran and consist of three formations, Faraghan, Dalan, and Kangan. The Dashtak formation, containing shale, claystone, anhydrite, and limestone, is the regional and efficient cap rock of the Dehram reservoirs.
Geologic history and depositional setting
The Permian rifting along the Zagros by the opening of the Neotethys Ocean coincided with transgression and deposition of the cyclic, dominantly shallow marine carbonates and evaporites of the Early Permian to Early Triassic Dehram group. The lithology of this group ranges from siliciclastics to limestones and dolomites, anhydrites, and shales.
The Faraghan formation is composed of clastic deposits. In fact, this formation represents post-Hercinian continental deposition in fluvial to deltaic systems. It is a time equivalent to the Unayzah formation in the Arabian countries. The Unayzah formation hosts enormous oil and gas potential in the fields of Saudi Arabia. Results of deep drilling in the Persian Gulf indicate that this formation has good reservoir properties. As expected, with deeper drilling, Faraghan reservoirs could be new discoveries in the Persian Gulf area in the future.
The Middle-Late Permian Dalan formation is the middle interval of the Dehram group and is divided into three units, Lower Dalan, Nar, and Upper Dalan members. The lower and upper units consist of carbonates with approximately 160 m and 180 m (525 ft and 590 ft) thickness, respectively. Based on reservoir properties, Dalan reservoirs are divided into four reservoir units from base to the top, including K5 (Lower Dalan), Nar member, and K4 and K3 (Upper Dalan). Most of the Dalan gas reservoirs are hosted by the formation’s upper part (K4 and K3) in the Persian Gulf area. Dalan reservoirs are associated with shallow carbonates (particularly grainy facies of oolitic shoals) of paleoplatform. In this platform, mud content of the facies (grain/mud ratio) increases basinward, so reservoir characteristics decrease accordingly; for example, mean porosity decreases from 9% in the Persian Gulf reservoirs to 3% in the Interior Fars reservoirs.
Therefore, on a regional scale, reservoir characteristics are mainly a function of the depositional environ- ment in both the Dalan and Kangan reservoirs, but in some fields fracturing and dolomitization are the main impact on reservoir quality.
The Early Triassic Kangan formation is a major gas and condensate reservoir in many fields in the Iran territories. Inhouse data indicate that 28 gas reservoirs have been discovered from the Kangan formation in the Zagros and Persian Gulf basins. The depositional system of the Kangan formation is similar to the Permian platform, but seemingly there were no major paleohighs and reefal bodies as depicted for the Dalan formation. Besides, from Permian to Triassic, there is a decreasing trend in the volume of anhydrite facies and an increase of shale facies. Based on reservoir properties, Kangan reservoirs are separated into two main reservoirs units, K2 and K1. Development of clean oomoldic shoal facies bodies after the Permo-Triassic mass extinction led to high reservoir quality in the lower part of Kangan Formation (K2), which is known as an excellent reservoir unit.
Iran has several supergiant and many giant gas fields in the Permo-Triassic Dehram group, all in the southwestern area. In the last 10 years alone, seven fields have been found in Iran. Based on current data, Early Permian sequences, particularly the Faraghan and Zakeen formations, now can be considered as good targets for gas reserves, particularly in the Persian Gulf, so deeper drilling will be necessary for new discoveries in this area.