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GEOLOGY AND CHARACTERISTIC OF GEOTHERMAL RESERVOIR BASED ON GEOTHERMOMETER ANALYSIS IN MOJOTENGAH AREA, WONOSOBO REGENCY, CENTRAL JAVA

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Abstrak Java Island is one of the rich area in geothermal energy because it is an area that is passed by ring of fire lanes. This shows that Java island has many quarter volcanoes. This research is located in Dieng Complex Mountain and Sundoro
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  GAIA Analysis Project 2019 Yogyakarta, 7 September 2019 ISSN: XXXX-XXX-XX GEOLOGY AND CHARACTERISTIC OF GEOTHERMAL RESERVOIR BASED ON GEOTHERMOMETER ANALYSIS IN MOJOTENGAH AREA  ,  WONOSOBO  REGENCY   ,  CENTRAL JAVA   Laksono Prabowo 1, ,  Raras Prabowo 1 ,  Christian Budi Pradhana 1   1  Geological Engineering, Institute of Technology National Yogyakarta  E-mail  : laksonoprabowo3@gmail.com   Abstrak Java Island is one of the rich area in geothermal energy because it is an area that is passed by ring of fire lanes. This shows that Java island has many quarter volcanoes. This research is located in Dieng Complex Mountain and Sundoro Volcano, precisely located in Mojotengah District, Wonosobo Regency, Central Java Province. The method used in this research is solute geothermometer to determine the equilibrium of geothermal fluid, type of geothermal fluid, and geothermal reservoir temperature. The results of the analysis were obtained from 2 samples of hot springs at the research site. Hot spring samples were taken from Tegalsari, and Wonokromo. Based on regional geology, the research includes Dieng Volcanic Rock (Qd) and Sundoro Volcanic Rock (Qsu). The lithology of the study area consists of sand- boulder deposits, breccia, and andesite. Geothermal area research conducted by the existence of Holocene volcanic activities. Geothermal manifestations in 2 samples in the study area are bicarbonate fluid in dilute zone of Cl-HCO 3  water. Based on the plotting diagram Cl-HCO 3 -SO 4  (Simmons, 1998) Wonokromo and Tegalsari hot springs is included in bicarbonate fluid with contents 36,5 meq/L and 35,11 meq/L and dominated by HCO 3 . In the content of Na, K and Mg plotted on the Na-K-Mg diagram (Simmons, 1998) which shows that the geothermal reservoir is suitable for immature water and meteoric water Meteoric water is very influential on hot springs. In both samples having high Mg content and showing hot springs have regulated contact with metoric water. Based on the calculation of the geothermometer Na-K-Ca, the estimated temperature of Wonokromo and Tegalsari hot springs is 186°C-206°C which can be classified as medium enthalpy, which is estimated to be associated with electrical energy sources such as >200°C, can be developed as a source of electricity. Keywords: Hot Springs, Geothermal Reservoir, Geothermometer, Geology INTRODUCTION  Java Island is one of the rich area in geothermal energy because it is an area that is passed by ring of fire lanes. This shows that Java island has many quarter volcanoes. This research is located in Dieng Complex Mountain and Sundoro Volcano, precisely located in Mojotengah District, Wonosobo Regency, Central Java Province (Figure 1). The island of Java is included in one of the regions that is very prospective of geothermal or geothermal, because basically the island of Java is one of the ring of fire or ring of fire. This can be demonstrated by the number of Quartery volcanic scattered in the area of Java. The research location is in the Quaternary volcanic which is located in the Dieng Volcano and Sundoro Volcano Complex. At the Dieng and Sundoro Volcano Complex, precisely located in the northern part of the North Serayu Mountains. Dieng Volcano Complex is a  plateau. The area is the area of collapse or depression of the highest volcano slope in Dieng due to gravity balance, so that the result of the collapse appears several new volcanic peaks and volcanic craters. This ultimately led to the existence of geothermal manifestations that were utilized by the community. One of the manifestations of geothermal that is utilized by the community that is used as a Geothermal Power Plant, there are also some that are used as hot water baths by local residents. The use of geothermal energy as a renewable energy source that can generate electricity in Indonesia, but currently is still very minimal. Even though Indonesia has the potential of geothermal resources of more than 28,000 MW or 40 percent of the world's potential, only 4.3 percent is utilized. Based on the availability of hot springs in the study area, the authors are interested in conducting research on the manifestations in the study area which is then expected to find  GAIA Analysis Project 2019 Yogyakarta, 7 September 2019 ISSN: XXXX-XXX-XX out how much the geothermal potential is contained in the study area. METHODOLOGY The method used in this research is solute geothermometer to determine the equilibrium of geothermal fluid, type of geothermal fluid, and geothermal reservoir temperature. Field research methods carried out are taking spring samples, measuring  pH, measuring temperature, color, and odor from hot springs. Testing of the chemical elements of hot springs is carried out at the Yogyakarta Center for Environmental Health Engineering and Disease Control. The chemical test parameters used against two pools of hot springs are the same ie Boron (B), Sodium (Na), Calcium (Ca), Potassium (K), Magnesium (Mg), Chloride (Cl), Fluoride (F), Bicarbonate (HCO 3 ), Lithium (Li), Sulfate (SO 4 ), ph.. The results of the analysis were obtained from 2 samples of hot springs at the research site. Hot spring samples were taken from Tegalsari and Wonokromo. GEOLOGY IN RESEARCH AREA Regionally, the study area was included in the Banjarnegara and Pekalongan Sheets Geology Map (Condon, et al., 1996). Based on regional geology, the research includes Dieng Volcanic Rock (Qd) and Sundoro Volcanic Rock (Qsu) (Figure 2). The lithology of the study area consists of sand- boulder deposits, breccia, and andesite. Dieng Volcanic Rock (Qd) terdiri dari andesitic and quartz – andesitic lavas and volcaniclastics. Silica contents increasing downward. Sundoro Volcanic Rock (Qsu) terdiri dari augite – hypersthene andesitic and augite – olivine basaltic lavas, flow  breccia, pyroclastic breccia, and lahar. The study area consists of sand-boulder deposits and breccia. Sand-boulder deposits are river deposits that range in size from sand to lumps, form rounded corners to rounded, graded structures (Figure 3a). Breccia is shown by fresh breccias as  brownish gray color, weathered reddish  brown color, texture consisting of grain size ± 3 cm, closed packaging, good sorting, angular shape, angular structure, massive structure, fragment composition in the form of andesite, matrix in the form of tuff. Megascopically, andesite fragments are  brownish gray, aphanitic texture, mineral composition composed of plagioclase, hornblend, quartz, and pyroxene (Figure 3b). MANIFESTATION GEOTHERMAL In the area of research geothermal manifestations that can be found in the form of hot springs spread at two points (Figure 4). The first point is on the banks of the Serayu river which is an alluvial sedimentary unit (Tegalsari hot spring) and the second point of the hot springs is on andesite breccia rocks (Wonokromo hot spring). 1.   Tegalsari Hot Spring This hot spring is located in Tegalsari Village, Mojotengah District, Wonosobo Regency. The breccia unit is located in the  paddy field area (Figure 5). The hot springs appear on the topography of the hills  precisely located on the slopes of Mount Bisma. The hot spring has no color, smells of sulfur, has a temperature of 50 o C, at an air temperature of 25 o C and a pH of 6 (measurement using a pH meter), while the  pH of a chemical analysis is 6,3. This hot spring has a bubble gas in the rice field area right next to the hot spring (Figure 6). Currently the emergence of hot springs is used as a hot spring by the local residents. 2.   Wonokromo Hot Spring This hot spring is located in Wonokromo Village, Mojotengah District, Wonosobo Regency. There are alluvial sedimentary units on the banks of the Serayu river (Figure 7). The hot springs are colorless, have a slight sulfur odor, have a temperature of 47 o C, at an air temperature of 25 o C and a  pH of 5.8 (measurement using a pH meter),  GAIA Analysis Project 2019 Yogyakarta, 7 September 2019 ISSN: XXXX-XXX-XX while the pH of a chemical analysis is 6.2. Currently the emergence of hot springs is used as a hot spring by the local residents. GEOCHEMICAL ANALYSIS The analysis was conducted on 2 samples of hot spring manifestation, namely Tegalsari and Wonokromo (Table 1). Observations were made to see the chemical content in the manifestation which was then analyzed. The analysis uses the Cl - SO 4  - HCO 3 trilinear diagram, Na - K - Mg triliniear diagram, and Cl - Li – B trilinear diagram. From the diagram can be detected the characteristics of each manifestation, if related can also determine the character of the reservoir. In addition, the entire value of the fluid geochemistry concentration in the analysis of each concentration value, although not through plotting on the Triliniear Diagram, through the amount of concentration of certain compounds and also the ratio of the elemental ratio value of some particular compounds. Type of Hot Spring Fluid Determination of the type of hot spring fluid  based on geochemical analysis of hot springs in the research area using HCO 3 -SO 4 -Cl trilinear classification diagram (Simmons, 1998), Fluid type analysis using elements and compounds from HCO 3 , SO 4  and Cl on the manifestation of hot water of research area got result (Chart 1). The percentage of  bicarbonate concentration (HCO 3 ) is the most dominant, so the type of hot water fluid in the study area is bicarbonate water. Precipitated travertin deposits are also formed by water that is rich in CO 2  or water  bicarbonate (HCO 3 ), this water reacts with Ca dissolved in rocks, thus forming the travertin deposits. Equilibrium of Geothermal Fluid Surface temperature calculation of the research area can be done by using the Na-K-Mg geothermometer method (Simmons 1998). The analysis of temperature subsurface using sodium (Na), potassium (K), magnesium (Mg) element parameters and calculation of ion content for springs of research area. The results of the plotting show that the fluid balance of the geothermal reservoir is in the immature water zone (Chart 2). Because the result of equilibrium of geothermal reservoir fluid has a larger  percentage of Mg element. GEOTHERMOMETER ANALYSIS Geothermometer is a method for calculating fluid temperature in a reservoir. The geothermometer used is adjusted to the nature of the geothermal system. Reservoir fluid temperatures have a fault tolerance of up to 100 o C. The application of the concept of a geothermometer is based on the assumptions of previous researchers that if the fluid moves rapidly to the surface, the fluid will maintain its chemical composition during the course of the reservoir to the surface due to slight mixing. There are 3 types of geotermometer methods for geotermometers to determine subsurface temperatures, namely silica geotermometers,  Na-K geotermometers, and Na-K-Ca geotermometers. In the silica geotermometer method, the resulting temperature has a temperature that is too low at <100 o C, so that the proper geotermometer method to be applied in the study area is Na-K-Ca. Na-K-Ca geothermometer calculation formula as follows: t o C = 1217/[log(Na/K) + 1.483] – 273 (Fournier, 1979 within Nicholson, 1993) t o C = 1390/[log(Na/K) + 1.750] – 273 (Fournier, 1988 within Nicholson, 1993) The Na-K geothermometer formula usage has limits: 1.   Used for water estimated to have a temperature reservoir of ≥180 o C. 2.   Used when water contains a low Ca  based on the calculation (Log (√Ca /  Na) + 2.06)) results in a negative value.  GAIA Analysis Project 2019 Yogyakarta, 7 September 2019 ISSN: XXXX-XXX-XX A good temperature for the Na-K-Ca geothermometer is 120-250 o C. From the calculation of the Na-K-Ca geothermometer, it can be seen the range at the reservoir temperature in the study area is 186-206°C. The geothermal area of the study is influenced by the existence of Quaternary volcanism activity. Geothermal heat starts from the presence of meteoric water that descends to the earth's surface, wherein some of the infiltration down to the earth's surface through fractures formed by faults or through cavities between rock grains. Meteoric water will enter the subsurface from accumulating in reservoir rocks that are Early Quaternary. The reservoir rock will be heated by a heat source. If the temperature of the water increases, the volume increases and the pressure rises. Causing hot gas to find a breakthrough to release the pressure. The geothermal system in the study area is interpreted to have a low-moderate temperature, so the pressure is not too high. Fluid in the form of hot water that flows laterally is in the outflow zone looking for gaps to come out to the surface. This causes the formation of hot springs manifestations. CONCLUSION Based on geothermometer calculations in the study area, a geothermal reservoir of 186-206° C was obtained. The geothermal system is triggered by volcanic activity associated with Quaternary volcanism. At a temperature of 186-206° C, it can be classified as a geothermal reservoir enthalpy, which is medium enthalpy. The medium enthalpy has a temperature limit <125-225 o C, with surface temperatures in the study area at 47-50 o C. According to the geological agency, geothermal potential can  be categorized into three, namely the  potential for small energy, medium energy, and large energy. Geothermal with a temperature of <200 o C has a small energy  potential of around 50 MW, geothermal with a temperature of 200-250 o C has a medium energy potential of around 50-100 MW, and geothermal with a temperature of >250 o C has a large energy potential of about >100 MW. Based on the temperature calculated using the geothermometer equation that is 186-206°C, the study area including the geothermal area has a medium energy  potential of 50-100 MW. Which is estimated to be associated with electrical energy sources such as >200°C, can be developed as a source of electricity. At present the Panasa Springs research area has become a hot spring bath, but the place has not been explored widely. If properly developed by the government, it can be integrated as a tourist location that can attract the attention of tourists and can be developed as a source of electricity. ACKNOWLEDGEMENT The support by lectures of Institute of Technology National Yogyakarta in supporting the writing of this paper is gratefully acknowledged. REFERENCES Condon, et al. 1996. Banjarnegara and Pekalongan Sheets Geology Map. Giggenbach and Goguel. (1989). "Chemical Techniques in Geothermal Exploration". Chemistry Division,   DSIR, Private Bag .  New Zealand.   Goff, F. Dan Janik, C. J, (2002), Geothermal Systems.  Encyclopedia of Volcanoes  : Academic Press, New York, 817-834. Hochstein, M.P dan Browne, P.R.L., (2000), "Surface Manifestation Of Geothermal with Volcanic Heat Source"  , In Encyclopedy of Volcanoes , H. Siguardson, B.F. Houghton, S.R. Mc Nutt, H. Rymerdan J. Stix (eds). Academic Press.  Nicholson, K. (1993). Geothermal Fluids: Chemistry and Exploration Techniques, Springer-Verleg, Berlin Heldederberg. Simmons, S. F. (1998). Geochemistry Lectures Notes. Geothermal Institute University of Auckland.  GAIA Analysis Project 2019 Yogyakarta, 7 September 2019 ISSN: XXXX-XXX-XX Figure 1.  Location of research area Sumber www.google.com Figure 2.  Geology map in research area based on Banjarnegara and Pekalongan Sheets Geology Map (Condon, et al., 1996).
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