An Integrated Survey of the Geochemical Study at the Blawan-Ijen Area, East Java

Riska Laksmita Sari1*, Firman Sauqi Nur Sabila1, Haeruddin2, Eriska Eklezia Dwi Saputri1, Welayaturromadhona1, Hadziqul Abror1, Sartika Dwi Purwandari1, Agus Triono1 1Department of Petroleum Engineering, Faculty of Engineering, University of Jember, Jl. Kalimantan No. 47, Jember, East Java, Indonesia 68121 2Department of Mining Engineering, Faculty of Engineering, University of Jember, Jl. Kalimantan No. 47, Jember, East Java, Indonesia 68121* Corresponding Author: riska.laksmita@unej.ac.id


INTRODUCTION
Geothermal is an environmentally friendly energy source because it uses repeated fluid injection under the earth's surface. Reinjection of geothermal fluid after the fluid is used for electricity generation and the recharge (seepage) of surface water, making geothermal energy sustainable (Nasional, 1998). Emissions from geothermal power plants are very low when compared to oil and coal. Because of its low emissions, geothermal energy has the criteria according to the Kyoto Protocol Clean Development Mechanism (CDM) product. This mechanism stipulates that developed countries must reduce greenhouse gas (GHG) emissions by 5.2% below the 1990 level. It should be done by purchasing clean energy from developing countries whose projects were built above 2000 (Kementrian ESDM, 2005). Clean energy includes geothermal. The development of geothermal energy can be one solution to increase Indonesia's energy reserves aside from fossil fuels. According to existing research, Indonesia is known as the country with enormous geothermal energy potential globally, with around 40% of its geothermal potential. However, in its use, Indonesia is still lagging behind other countries such as New Zealand and the Philippines (Badan Standarisasi Nasional, This research's targets are geochemical studies in the Blawan-Ijen geothermal area, a continuation of previous studies (characterisation of rock parameters and remote sensing analysis). This knowledge is essential for potential investors for geothermal developers to reduce their investment risk. Blawan-Ijen was chosen because it is close to the UNEJ campus, which in the future could be developed as a natural laboratory for the earth, not only in the geothermal field. Several field measurements have been carried out by the Center for Geological Resources (PSDG), the National Geological Agency. This study's results can be expected to improve the quality of potential interpretations that have been done before. Thus, investors will be more confident to participate in the Blawan-Ijen WKP auction because trust in the data and models is getting higher. This condition means that the risk of exploration failure can be reduced. Indirectly, these results are essential to be fostered for collaboration with geothermal development investors, including governments that regulate through regulations, in the form of both laws and government regulations.
This research aims to characterise the geological reservoir of Blawan, Ijen, with geochemical analysis.

Material
The study was conducted in Mount Ijen and its surroundings ( Figure 1). The presence of geothermal potential is characterised by hot springs scattered in the north of the area (Zaennudin et al., 2012). The materials used in this study are secondary data, namely the regional geological map of Mount Ijen and the map of geothermal manifestations on Mount Ijen, East Java.
The tools used in this study include: a. Roll meter, room blade, permanent marker, a geological hammer, plastic, field book, geological compass, and pH meter.
b. GPS (Global Positioning System) The use of GPS to determine the coordinates of sampling points in the field.

Method
Research Stages in this study include: a. Literature Study This study begins with the literature collection from previous research that discusses Mount Ijen, East Java's geological and geographic conditions. Data processing begins with the analysis of hot water content in concentrations of cations and primary element anions. We use F-AAS tools and spectrophotometers. Furthermore, geochemical data is processed and plotted into ternary diagrams (Cl-SO4-HCO3, Cl-Li-B, Na-K-Mg). Furthermore, the process of calculating subsurface temperature estimation is carried out using the geothermometer method.

d. Interpretation of Data
After data processing is complete, interpretation is based on the leading element's anion cation content. Based on these results, an interpretation in geological and geochemical geothermal studies in Blawan, Ijen. The geothermal reservoir is where hydrothermal fluids heated by the heat source, which here means hot rock.

e. Data Validation
The interpretation results are then validated with primary and secondary data (geological maps, geological structure distribution and geothermal manifestations). It aims to get an accurate interpretation and following field conditions in the Blawan-Ijen area.

Measurement of Field Data
Measurement and data collection in the field were carried out in three locations: Blawan hot springs, Wurung Crater, and Kalipahit River, as shown in Figure 2. The first location, the Blawan-Ijen hot spring location, is taken with three points. At this location, hot springs were found with 50.9 0 C and pH of 5.8, while air temperatures were 26.6 0 C. In the second point, hot springs were found with T 44.4 °C and pH 6.6, while the air temperature was 26.1 °C, there was dense vegetation surrounded by large trees. The third point is the limestone cave on the riverbank and closes to the Blawan waterfall.
In the Wurung Crater, the second location obtained a surface temperature of 22.6 0 C. The pick-up location is at five points. The sampling points are BL 1, BL 2, BL 3, BL 4, and BL 5, respectively denoted by STA 04, STA 05, STA 06, STA 07, and STA 08 in Figure 2. The vegetation is dominated by grass. In the Kalipahit river, the third location obtained water with a very acidic pH (<0.01) with a temperature of 22.5 0 C. At the same time, the existing rock is volcanic breccia. This river is a stream from the Ijen crater, which crosses the Blawan region. It mixes with neutral pH water in the Blawan waterfall (Suciningtyas et al., 2013). The test aims to know the type of hot water fluid and find out the reservoir temperature using the element geothermometer calculation.

Water Geochemical Test
The test uses various spectrophotometer, AAS (Atomic Absorption Spectroscopy) and acid-base titration. Na, Ca, K, and Mg contents were tested using AAS. The SO4, Cl and B contents were tested using a spectrophotometer. As for HCO3, it is carried out using acid-base titration. Water samples were tested on five samples stabilised from 5 points. The results of sample testing with the AAS method and the spectrophotometer are shown in Table 1. While testing for HCO 3 using the acid-base titration method is being carried out in the lab. MIPA analytical chemistry.

Figure 2. Location of data retrieval in the field
To determine the levels of HCO 3-in hot spring samples, the acid-base titration method was used. The sample is put into the beaker as much as 100 ml. The pH meter is put into the beaker, then titrated with H2SO4 solution up to pH 4.5. The volume of H2SO4 used in the titration process is recorded. The volume of H2SO4 used in the titration process is recorded. The HCO3 level is calculated by equation (1).
A is the volume of the H2SO4 solution used in the titration process. N is the normality of H2SO4, and V is the sample volume.

Type of Hot Water
Chemical data needed in determining the type of reservoir fluid is the relative content of chloride (Cl), bicarbonate (HCO3) and Sulphate (SO4). Data processing is done by calculating the percentage of elements of chloride (Cl), bicarbonate (HCO3) and Sulphate (SO4). Then the data is plotted in the Giggenbach triangle diagram (Figure 3). This diagram is used to indicate the characteristic type of geothermal water and subsurface geothermal conditions. The dissolved ions in the surface ascending thermal fluids originate (Sukhyar et al., 2014). The types of geothermal water fluids found in the Blawan Ijen area vary from the chemical analysis results.
In the BL 1 and BL5 samples included in Sulphate Water's type with the dominant content of Sulphate (SO4). This type of geothermal water, also known as Sulfuric Acid, is a fluid that forms at shallow depths and is formed due to the condensation of geothermal gas goes near the surface. Geothermal gas, with its gas and volatile content, is practically soluble in fluid content, which is located in a deep zone but is separated from chloride water. Sulphate water is usually found at the boundary and not far from the main upflow area. When viewed from the topography, the exact location is far above the water table and around the boiling zone. However, most are often found near the surface (at depths <100 m). Sulphate water can flow through faults to the geothermal system. Sulphate water is heated at this location, then takes part in rock alterations and mixes with chlorine water.
Then for the BL 4 sample included in the type of chloride water. This type of water is a type of geothermal fluid found in most areas with high-temperature systems. Areas with large-scale hot springs flowing with high Cl concentrations originate from deep reservoirs and indicate permeable zones in those areas. However, this area may not be located above the main upflow zone because there are several other possibilities, such as topographic influences, which can significantly impact hydrological control. The eyes of chloride can also identify high zones' permeable areas (e.g., faults, breccia eruptions or conduit).
Meanwhile, BL3 samples are included in the Bicarbonate Water-type. The element HCO3 (bicarbonate) is the most dominant element (primary anion) and contains CO2 gas from the chemical analysis results. HCO3 water is generally formed in marginal and near-surface areas in systems dominated by volcanic rocks, where CO2 gas and condensed water vapour into groundwater. The vapour condensation can either heat the groundwater or be heated by steam (steam heated) to form an HCO3 solution. HCO3 water is formed below the groundwater level. It is generally weak acidic, but with dissolved CO2, this water's degree of acidity can increase to neutral or slightly alkaline (van Hinsberg et al., 2010). The alterations found are generally argillic (kaolinite, montmorillonite) and mordenite. The high levels of HCO3 found in Blawan springs probably resulted from the mixing of surface groundwater with CO2 rich steam, which boiled off the hydrothermal system (Delmelle et al., 2000).

Na-K -Mg Geoindicator
The triangular diagram of Na / 1000 -K / 100 shown by Giggenbach (1988) is used to estimate reservoir temperatures and calculate the water that reaches equilibrium in lithology. The plotting is done on the Na / 1000 -K / 100 -Mg1 / 2 triangle diagram ( Figure 4) for each hot spring sample from the data and the percentage calculation of the three elements' contents. Based on the calculation of the relative content of Na / 1000-K / 100-Mg½, the value of the Na hot springs triangle lies in showing that the manifestation temperature that appears on the surface tends to be low. It also influenced by the interaction between hydrothermal fluid and elements in rocks through which silica passes. The condition of immature water is that the reservoir rock is located at a high temperature and pressure condition. Before it reaches the surface, it has also been diluted by surface water (meteoric water).

Na-K-Ca Geothermometer
A good temperature range for use with Na-K-Ca geothermometer is 120 o C to 200 o C, and the rest is not very good. This geothermometer is useful when applied to water that has a high Ca concentration. For calculating the temperature of Na-K-Ca, the following equation is used: °= 1647 ( / ) + ( √ / + 2,06) + 2,47 − 273,15  Table 2). For this reason, a recalculation is performed using β = 1/3 and the results obtained in Table 3.
Based on the calculation results in Table 3, all samples' temperature is obtained, ranging from 63-166 °C. From these results, the Na-K-Ca geothermometer estimation is not very good because it is good to be used in the temperature range of 120-200 o C. The rest is not too good despite the value of water samples analysis with high Ca concentrations.

Na-K Geothermometer
Na-K geothermometer can be applied to chloride water reservoirs with T> 180 o C. However, this geothermometer is not good when applied to T <100 o C (Simmons, 1998). For determining the subsurface temperature using a geothermometer with Na-K content, the following equation is used:  Based on temperature calculations using two equations, high-temperature results are obtained, namely 337-636 °C for the Fournier Equation (Table 4) and 265-636 °C for the Giggenbach Equation (Table 5). This geothermometer is not suitable if the water contains high Ca. From the analysis of water samples with high Ca content, this geothermometer cannot estimate reservoir temperatures. Alternative geothermometers, such as the Na-K, are not suitable for the Blawan hot spring conditions due to their high-Ca content and log ((Ca/Na) +2.06) are positive. The Na-K geothermometer requires a low Ca content and a negative value for log ((Ca 0.5 /Na) +2.06). Additionally, the Na-K geothermometer is applicable for reservoirs with temperature ranges from 180 to 350 °C (Ellis, 1979).

K-Mg Geothermometer
This geothermometer is applied to situations where dissolved Na and Ca have not been balanced between liquid and rock. To determine subsurface temperature using a geothermometer with Na-K content, we can use equation (5): Based on the calculation results in Table 6, the temperature is obtained using the K-Mg temperature estimation (Giggenbach, 1991). All hot springs in the range between 66-135 °C. For geothermometers, this is best used when re-equilibrium takes place quickly at low temperatures. The K-Mg geothermometer is generally used in conjunction with the Na-K geothermometer, using a Na-K-Mg triangle diagram.

CONCLUSION
From the results of the chemical analysis, it is known that the types of geothermal water fluids found in the Blawan Ijen area vary. In samples BL1, BL2 and Kalipahit included in Sulphate Water with the dominant elemental Sulphate (SO4) content. This type of geothermal water is also known as Sulfuric Acid Water (Acid-Sulphate Water). Then for the BL 4 sample included in the type of chloride water. This type of water is a type of geothermal fluid found in most areas with high-temperature systems. Areas with large-scale hot springs flowing with high Cl concentrations originate from deep reservoirs and indicate permeable zones in those areas. However, this area may not be located above the main upflow zone because there are several other possibilities, such as topographic influences, which can significantly impact hydrological control. The eyes of chloride can also identify high zones' permeable areas (e.g., faults, breccia eruptions or conduit). In contrast, BL3 samples are included in the Bicarbonate Water-type. The element HCO3 (bicarbonate) is the most dominant element (main anion) and contains CO2 gas from the results of chemical analysis. HCO3 water is generally formed in marginal and near-surface areas in systems dominated by volcanic rocks, where CO2 gas and condensed water vapour into groundwater. The vapour condensation can either heat the groundwater or be heated by steam (steam heated) to form an HCO3 solution.