Two dimensional geoelectrical structure of Kütahya-Hisarcik geothermal area

Fossil fuels such as coal, petroleum, natural gas and nuclear power are used primarily for energy production around the world. Since the global awareness increased for the detrimental effects of burning fossil fuels, clean and renewable energy sources, ie, hydroelectric, geothermal, sunlight, wind, gain more interest. Geothermal energy which has proven to be clean, renewable and safe, is one of the promising renewable energy source. Turkey is located on the Alpine-Himalayan orogenic belt and locating in region of high tectonic activity. There are Miocene or younger grabens developed in that area as a result of this orogeny. Therefore, Turkey is a country with a significant geothermal energy potential. Electricity generation, direct use (house heating, greenhouse heating) and industrial usage are some of the various geothermal energy utilizations can be done in Turkey. Resource assesment is done many times by the General Directorate of Mineral Research and Exploration (MTA) and today it is discovered more than 1000 hot and mineral water springs which their temperature is up to 100-140 oC and geothermal fields with a temperature range of 40-232 oC. 95% of the discovered 186 geothermal field is low-medium enthalpy source. Although Turkey is the 7th richest country in the word in terms of geothermal potential, most of the development has been achieved at direct-use applications. Exploration of geothermal fields are in great importance since it is a cleaner energy source. For this purpose, direct current geophysical method is a well suited tool for geothermal exploration and for all that, geological structures, lateral discontinuities, sediment thickness and depth, bedrock depth, hydrogeologic studies, groundwater exploration and groundwater contamination and natural resources. In this study, it is aimed to obtain two dimensional subsurface structure of Kütahya-Hisarcık geothermal field. Detailed geophysical survey which consist of 69 vertical electrical soundings (VES) with various AB/2 between 1000 m. and 2000 m. and SP measurements of 4 profile were carried out by MTA. Schlumberger electrode spacing is used for VES surveys and 25 m. electrode spacing is used for SP profiling. Study area is located north of Emet-Gediz graben. Measurements took place at a 69 km2 area near Hamamköy and Sefaköy and can be located at 1/25000 scaled Kütahya J22-c1 map. Survey area is located at 70 km. south-west of Kütahya centrum and 10 km. south of Emet. Fault directions are observed in vicinity are, NW-SE, N-S and NE-SW. While right lateral faults are NW-SE oriented, N-S and NESw oriented ones are dip-slip normal faults. Most important tectonic element is Hisarcık fault. This fault is N-S oriented and observed in Neogene Yeniköy formation. Yeniköy formation consists of sandstone, siltstone, marl and fine grained sandstone-tuff layers. Travertines are observed in a small area north of Hamamköy. Budağan limestones, Arıkaya and Sarıcasu formations underlaying the Yeniköy formation are potential geothermal reservoirs. VES data are inverted by 1D, quasi 2D and 2D inversion algorithms. Occam algorithm is used for 1D inversion which gave a smooth and continuous resistivity variations of the area. It is more satisfactory to allow the model to be as flexible as possible but suppress complexity explicitly. Therefore a rougness value is defined. Goal here is to find the smallest roughness value that agrees with the measurements. In order to convert the VES data by quasi 2D, laterally constrained 1D (LCI) algorithm is used. This algorithm uses similar approach that Occam does and based on applying modified one dimensional inversion with two dimensional smoothness constraints on a vertical electrical sounding data along a line. Therefore this quasi 2D algorithm is compared with 2D inversion to determine 2D structural effects. Lastly, 2D Schlumberger inversion algorithm is used which based on 2D model with fixed block boundaries during inversion and only the resistivity values of each block changes through the iteration. VES data inverted along 5 profiles which are all parallel to each other are named I, J, JK, K and M in respect to north to south. There is a high-low resistivity contrast at northern profiles. The reason for this contrast is thought of Hisarcık fault at vicinity. This boundary is located near I-18 VES station divides the area into two zones with 400 Ω m.- 20 Ω m. resistivity difference. East of I-18, there is a remnant of old depression around I-23. According to geoelectrical structure of profile I, bedrock gets deeper eastwards. Low resistivity zone is seen around 1500 m. along profile and at a depth of 750-800 m. Hisarcık fault is also seen at Profile J with its characteristic high resistivity contrast. Low resistivity (< 10 Ω m.) is 1200-1600 m. along profile and at 400 m. depth. Horst-graben structure is clearly seen at profile J around JK-18 – JK-19 and JK-23 – JK-24. Area between these two faults are between 1300 m. and 1800m. along profil J and between 400 – 600 m. depth. Faults observed at other profiles also seen clearly at profile K around K-18 – K-19 ves stations. Graben structure seen around K-20-1 – K-21 stations and low resistivity zone is located at a depth of 400-675 m. Profile M is the longest profile and located at the south of other profiles. Old depression mentioned at profile I is also shown at this profile between M-20 – M-27 VES stations. East and west of this depression, graben structure is visible clearly. Area is characterize with Pliocene allüvial deposits and Hisarcık formation and Miocene aged Yeniköy formation at the uppermost layer. These formations are underlayed by fructured Arıkaya and Balıkbaşı limestones. It is thought that Simav metamorfics are the bottom layer at vicinity. There are 4 SP profiles consist of 65, 48, 43 and 50 stations are named A,B,C and D. A and B profiles are parallel to each other and perpendiculat to VES profiles. C profile is between I-19 and I-23 VES stations. Profile D is intercepts I, J, JK and K VES profiles. Both profiles B and A are shown similar characteristics as continuous increase or decrease. This indicates that these profiles are along the geological structures and does not intercepts a fault surface. On the other hand, profile C shows negative values between I-21 and I-22. These results concur with the graben structure observed at VES cross sections. In order to find the reservoir's location and depth, fault and fracture structures derived from VES and SP data are used in ANSYS FLUENT 6.3.26 computational fluid dynamics software. This software operates using finite volume method for flow and temperature modelling. Temperature, porosity, permeability, viscosity, heat capacity, thermal expansion coefficient and thermal conductivity are used as model parameters for modelling. These parameters were conbined wth the subsurface geometries derived from SP and VES studies in order to build mesh geometry. Model consist of 40710 finite volume element and 41370 nodes that binds them. Fault zones are defined by permeability contrast within surrounding rocks. While surrounding rocks have a permeability of 10-15, fault zones are defined with 10-13. Model parameters are decided after thorough literature research and concurs with recent studies. Steady state temperature calculations shows that smooth temperature curves. This is in agreement with the expected heat conduction (Rayleigh number