Iran is a part of the Persian Gulf known for its massive hydrocarbon resources. However, it is also home to numerous geothermal resources. This includes ore deposits with ~5,000 ore mining sites extracting over 60 different types of minerals (including copper and gold) and rocks. Another key geothermal resource in Iran is mineral springs (cold/hot). These springs have been utilized for centuries in Iran for therapeutic bathing and swimming. Sareyn, at the foot of the Sabalan Mountains, is renowned for its cold and hot springs and receives ~1.5 million tourists annually (pre-pandemic). Mineral springs and ore deposits often indicate the presence of medium to high geothermal resources that can be utilized for power production. Here we will provide a brief overview of the geothermal resources of Iran.
The Iranian Plateau is a part of the Alpine-Himalayan orogenic belt stretching from the Atlantic Ocean to the western Pacific. It can be divided into three structural (tectonic) units from southwest to northeast: the Zagros Fold and thrust Belt, Central Iran (Iranian Plateau), and North units (Figure 1). These are briefly reviewed below:
Three major fault trends are distinguishable in Iran:
Figure 1: Main structural (tectonic) regions of Iran (Yousefi et al. 2010)
The most important geothermal manifestations and thermal springs are associated with the Alborz Mountains where the most active volcanism (Late Alpine phase) has occurred. The geothermal gradient of the country ranges from 20°C /km (2°C/100 m) in the Zagros belt to 130 °C/km (13°C/100 m) around Damavand volcano in Alborz Mountains.
Geothermal studies in Iran started in 1975. Eighteen promising geothermal areas have been identified with the highest potential in the Sabalan, Damavand, and Taftan-Bazman areas. A review of these areas is provided below.
Figure 2: Geothermal prospects of Iran (From Torbehbar and Liseroud 2015)
The Sabalan area is the most promising geothermal area in northern Iran and is associated with Mt. Sabalan. Its northern slopes host many hot springs with temperatures of 21 – 85 °C. Chemically they fall into different types including neutral, Cl-SO4, and acid SO4. The two most important geothermal fields in the area are NW Sabalan and Meshkin Shahr.
NW Sabalan hosts the only producing geothermal powerplant in Iran. From 2002–2004, three deep exploration wells were drilled evaluating subsurface geological conditions the geothermal reservoir, and response simulation. Two of the wells were successful, with a maximum temperature of 230 °C at 3200 m depth. To date, 11 deep exploration wells have been drilled with depths of 2300 – 3197 m and temperatures of 174.5 – 242 °C.
Geothermometric studies were used to predict reservoir temperatures in the NW Sabalan area. This included chalcedony/silica, quartz, K-MG, Na-K-Ca-Mg, Na-K-Ca, and Na-K geothermometers predicting temperatures of 124 – 244 °C.
A magneto-telluric (MT) study was conducted in 2007 to determine the location of a likely reservoir and possible drilling targets. The results revealed a shallow resistivity anomaly as the likely center of the geothermal system.
The Meshkin Shahr geothermal prospect lies in the northwestern Ardebil province on the western slopes of Mt. Sabalan SE of Meshkin Shahr City. Hot springs in the area show varying composition from neutral-Cl-SO4 to acid-SO4 waters. Popular springs include Gheynarge, Khosraw-su, Malek-su and Ilando, Moil, Moil 2, Aghsu, and Romy springs.
Geothermal exploration in this area started in 1974. Eleven wells have been drilled in Meshkin-Shahr, 7 production and 1 reinjection well with 3 unsuccessful wells. According to the data from well testing, the mass flow of the wells is 30-70 kg/s and the temperature is 130-170 °C. Several geophysical surveys were carried out including D.C. (Schlumberger array), transient electromagnetic (TEM), and MT.
The Damavand geothermal area is associated with the Damavand volcano, located in the central part of the Alborz Mountains is the highest peak in the Middle East reaching 5670 m above sea level. The area hosts numerous hot/cold springs on the eastern part of the Damavand volcano including the Ask e Sar e Pol, Ask e Nadali, Ask e Pashnak, Larijan, and Polor cold springs with temperatures ranging from 25 – 60 °C.
Geophysical studies are focused on thermal, Telluric, and MT surveys. The thermal surveys resulted in twenty-six shallow boreholes (83-120 meters deep). Temperature measurements revealed thermal gradient and heat flow anomalies with SE–NW trend and coincided with volcanic rock outcrops in the region corresponding to volcanic activity. The highest thermal gradient anomaly reached a value of 0.6-0.7°C/10 m, and the highest heat flow reached 4-5 μCal/cm2.s. Remote sensing has been used to Identifying thermal anomaly areas with the aid of thermal infrared images and to identify hydrothermally altered zones.
Geothermometric studies were carried out on waters from numerous hot springs. Reservoir (deep) temperature was calculated using chalcedony ( 59°C to 111°C), quartz (53 – 94 °C) and Na-K (232 – 265 °C) geothermometers and Na/K ratios (125 – 338 °C). In the region of Nunal, an average subsurface/ reservoir temperature of 150°C was determined from these studies.
The Taftan-Bazman Geothermal area is associated with the Taftan Volcano. It is home to some of the most intense geothermal manifestations in Iran including fumaroles, altered ground, and warm/cold springs. The warm springs are at higher elevations (above 3000 m) while the cold springs occur at a lower elevation. Mineral precipitation is common and includes sulfur and salt deposits (gypsum) and silica (sinter and residues). Silica deposition is a reliable indication that the depositing fluids are derived from a hot (>180°C) reservoir.
The geothermal system associated with Taftan appears to be vapor-dominated. After a period of hot water discharge during the dry season, the system produces only steam. Reservoir temperatures for water samples of this system were calculated using geothermometry. Low temperatures are calculated with chalcedony and quartz (79 – 150 °C) and Na-K-Ca, (87-127°C) with higher temperatures calculated with the Na/K geothermometers (63 – 237 °C).
Thank you to our guest author, Jason Fisher for continuing to contribute an exceptional level of in-depth Geothermal Country Overviews.
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