Exploration in trachyandesitic Mount Sabalan, near the town of Meshkin-Shahr in north western Iran, indicated that this area hosts geothermal reservoir. In this study, we determine the direction of principal stresses using structural geology features exposed on the surface. These features include slickensides and fault steps. Analysis of the recorded slickenside traces in six districts was carried out using multiple inverse method. Results of the analyses indicated that the direction of the three principal stresses in every district is independent of the others. However, in all districts, maximum principal stress is vertical, minimum and intermediate principal stresses are horizontal. One of the applications of stress direction is in the design of hydraulic fracturing. Since the permeability of this reservoir is rather low, this stimulation technique could be a solution for enhancing productivity. Direction of the minimum stress dictates the geometry of created fracture. In this case the fracture would propagate vertically.
In 1974, for assessment of geothermal resources potential in Iran some primarily studies including aeromagnetic and gravimetric operation have been done in area more than 8300 km2. After a long standby, in 1990 previous documents were reviewed and resistivity operations were conducted. According to the resistivity anomalies and geochemical alteration Meskin- Shahr (Northwestern Sabalan) introduced as the first geothermal potential in Iran. So, three exploration (production) wells up to 3200, 3176 and 2260 m and two injection wells up to 650 m have been drilled. Completion investigations such as logging and flow tests indicated that temperature of reservoir is about 240°C and permeability of reservoir is medium to low. Permeability is one of the important factors in using captured heat. Base on some calculations and estimations, it is essential to increase permeability by artificial methods. To increase permeability, we suggest hydraulic fracturing. During the hydraulic fracturing, artificial fractures are created in reservoir. Created fractures extend perpendicular to minimum horizontal stress. So, by knowledge of principal stress directions, we can determine artificially fractured geometry and drill the injection wells in the best direction. In this study, we try to determine direction of the in-situ stresses in Meskin-Shahr geothermal field using multiple inverse method.
Study area is located around the Sabalan Mountain in Ardabil province, northwestern Iran (Fig. 1). Geological setting of this area such as lithology and tectonics is controlled by Sabalan volcano activities. Volcanic structure is spot and stratovolcan like Stromboli volcano in Italy. Central volcano erupted on the main conjugated fracture over the paleohorst with E-W trend. According to published studies, volcanic activity in Sabalan started Eocene and resumed in Pliocene by eruption of trachyandesitic to andesitic lava flow through the main caldera. After caldera collapsing in Early Pleistocene, central caldera partially filled by trachyandesit–trachydacite 1394 lava flows . The study area is located in complex compressional zone between Iran, Arabia and Eurasia plates in Alborz- Azerbaijan structural zone.