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Hydrogeochemical model of Ciseeng geothermal field, Bogor, West Java

Ciseeng geothermal field is located Northwest of West Java 40 km from Jakarta. Three surface manifestations are in Tirta Sayaga, Panjang Mountain, and Peyek Mountain. This study aims to build a hydrogeochemical model based on water analysis. The secondary regional geological data are retrieved to complete geological information of Ciseeng. The research method is divided into several stages: introduction includes the literature study, data collection includes the measurement of temperature and pH in several surface manifestations, rock sampling and structural geology analysis in Ciseeng. The type of manifestation that can be found in Ciseeng is a warm pool with chloride fluid type. Extensive travertine deposits were found in all three locations. The fluid temperature of three surface manifestations is about 30-38 °C with pH 6. According to the measurement of Na-K-Ca geothermometer, the reservoir temperature in Ciseeng geothermal field is 180 °C. The three manifestations of Ciseeng geothermal field are classified as mature water based on a ternary plot diagram. The isotopic analysis shows shifting in O18 indicating hydrothermal process due to magmatic activities in the subsurface. It is predicted that Ciseeng geothermal field is part of Salak/Pangrango Mountain geothermal system. The presence of surface manifestation confirms that geological structure trending from North to South, play a significant role to circulate the heated fluid in the subsurface and subsequently to the surface. The result is a geothermal conceptual model of Ciseeng field that shows the fluid origin and movement in the subsurface.


Introduction
Geochemical analysis is early indicators to evaluate the potential of geothermal resources [1,2]. Geothermal fluid analysis can demonstrate the variation in both chemical and isotopic compositions that infer their evolution [3]. Moreover, isotopic analysis of geothermal fluids can serve as their genesis and recharging area.
One of the existence indicators of a geothermal system is by the appearance of surface manifestation. Travertine deposits and thermal springs are common features for most geothermal areas [4]. Subsurface hydrogeochemical condition can be interpreted through the characterization of surface manifestations, deposits and alterations, and also from the characteristics of the thermal fluid [5]. Geochemical data in Ciseeng is very limited and outdated. This research aims to update geothermal fluid characteristic of Ciseeng geothermal field and to create a preliminary hydrogeochemical model of Ciseeng geothermal field based on the origin and fluid chemical characteristics.

Geological Overview
Ciseeng geothermal field is in Parung, West Java (60 km south from City of Depok) as shown in figure 1. Three distinguishable travertine deposit mounts are spread throughout the area, Tirta Sanita Bathing Place, Panjang Mountain dan Peyek Mountain, respectively. Figure 2 explains that there are five types of lithology in Ciseeng geothermal field: • Quaternary Alluvium (Qa) is a surface deposit that is located in Jakarta plain as a swamp or stream deposit. • Undak Purba Deposit (Qofc) is the product of precipitation from marine sedimentary rock and weathered volcanic rock • Volcanic Breccia Rock (Tpb) is the volcaniclastic sedimentary deposit with ancient lava deposit mechanism. • Tuff Sandstone (Tnt) is deposited unconformity as disconcordance with sedimentary Miocene rock • Limestone (Tml) could be correlated with Lower Miocene limestone in Klapanunggal, unconformity under tuff sandstone unit.

Methodology
We limit the area for this research within a 0.7 x 0.7 km 2 area as shown in figure 3. Several rock samples and geological data were collected from the field. Field sampling includes rock and thermal water samples. Three warm spring water samples were collected from three different manifestations, includes of Tirta Sayaga 1, Tirta Sayaga 2, and Gunung Peyek. Three warm spring water samples were collected from three different manifestations, includes of Tirta Sayaga 1, Tirta Sayaga 2, and Gunung Peyek (figure 4). Thermal fluid sampling follows the procedure in where untreated and treated samples (with addition of HNO3) were retrieved [8]. Water analysis was carried out in Pusat Sumber Daya Geologi, Bandung. The laboratory results were then calculated and plotted using Powell calculation to determine the fluid characteristics, number of reservoirs, the origin of the fluid and geothermometer to predict reservoir temperature.
To help with data interpretation, we use secondary data based on the availability of satellite images and geological map of Parung, West Java. Stratigraphic, arbitrary structure and cross section from such geological map was used as a base for the preliminary hydrogeochemical model. Subsequently, we overlay the geochemical data with geological maps to create a hydrogeochemical model.   The high Cl content also suggests that the water in Ciseeng is very saline (also considering Total Dissolved Solid and conductivity analysis of the water). Present day water analysis shows that the water is partially equilibrated, with the tendency of groundwater dilution based on the concentration of Mg in the fluid (high Mg concentration).
As illustrated in figure 6, Cl/B rasio similarity indicate that the fluids derive from one reservoir and all manifestation has shifted to the right (positive shift). It means there is a surface evaporation occuring in the surface. Table 1 shows the geoindicators and geothermometer each sample in Ciseeng geothermal field. The Na/K ~<15 can be associated with upflow structures or permeable zone which is shown in table 1. Ciseeng water data shows Na/K ~< 15 meaning that the manifestation is located in a permeable zone. The highest Na/K ratio among all data may indicate the main upflow zone of the system in this case, CSN-2.
Normal Ca concentration (Ca < 50 mg/L). Ciseeng water data shows very high concentration of Ca. It is predicted that the loss of CO2 during fluid-rock interaction triggers the formation of travertine deposits. Ca concentration also increase with the salinity of water. Mg concentrations are very low (0.01-0.1 mg/L). Ciseeng water data exhibit very high Mg concentration indicating possible dilution by groundwater. HCO3 content reflects the CO2 content in the deep fluid is influenced by permeability and lateral flow [8].
High HCO3 content shows the direction of lateral flow. CSN-3 shows the highest HCO3 concentration, showing that the fluid flows to the north of the research area High Total Dissolved Solids (TDS) concentration shows that Ciseeng water is very saline. Increasing Boron concentration and increasing HCO3 from CSN-1 to CSN-3 indicate shear faults [9], with possible direction along CSN-1 to CSN-3 (trending North-South). Quartz geothermometer is deemed to be the most suitable considering the temperature of the surface manifestation, water type, and other species concentrations. Subsurface temperature ranges from 91-93 ºC. Present and past data show vast differences, meaning that Ciseeng geothermal field has evolved in the past 25 years.    subsurface. Based on the appearance of surface manifestation, it is possible that is an arbitrary fault along the CSN-1 to CSN-3 however no field observation was found. Presumably, an intra-Miocene fault in the subsurface induce secondary permeability allowing fluid to circulate in the sub-surface. Thermal fluid is able to manifest as hot spring and travertine due to the primary permeability of alluvium (Qa) and the porosity of weathered volcanic rocks and conglomerate (Qofc).

Hydrogeochemical model
Alluvium has good permeability and weathered marine sedimentary and volcanic rocks show good porosity properties as shown in figure 8 [10]. These properties play an important role in fluid circulation in the subsurface. In this model, there are arbitrary fault acts as a fluid pathway where heated thermal water rise up to the surface and exits as surface manifestation. Meteoric water descends to the subsurface with the help of permeability layers making up the host lithologies. Fluid circulation can change the lithologies resulting in the evolution of the system (i.e. changes in water concentration) over the years.

Conclusion
The fluid of Ciseeng geothermal field is very saline water and saturated with HCO3 due to CO2 loss and leading to the formation of CaCO3 deposit (travertine terrace and mounds). The thermal waters are presumably originated from a common reservoir with meteoric water as recharge water. A preliminary hydrogeochemical model of Ciseeng geothermal field was completed. Enough improvement and further geological and geochemical samplings are required to complete a more comprehensive hydrogeochemical model of Ciseeng