Geochemical Data of Hotsprings in Cikahuripan River.docx

Proceedings Indonesia International Geothermal Convention & Exhibition 2015Jakarta Convention Center, Indonesia – August 19th – 21st, 2015

Geochemical Data of Hotsprings in Cikahuripan River: The Origin of the Fluid and the Reservoir’s Characteristics

Syaiful Hilal*, R.A. Julia Satriani*, Agil Gemilang*, Ir. Mega F. Rosana, M.Sc., Ph.D*, Adi Hardiyono S.T., M.T.*

*Faculty of Geological Engineering, Padjadjaran University

[email protected]

Keywords: Cikahuripan River, Geochemistry, Hotspring, Origin and reservoir’s characteristics.

ABSTRACT

One of the objects we observe in geothermal exploration is manifestation. Specifically, this study will focus on the observation of hotspring, as one of many types of geothermal manifestations. We could use the geochemical data of hotspring to determine the fluid’s origin and the characteristics of the associated reservoir. Cikahuripan River is located in Talegong, Garut, West Java and astronomically lies in X 770850-771852 TM and Y -91914920 - -9192998 UM (UTM zone 48). In this area, several hotsprings were found and analyzed.

Methods used in this study are Giggenbach and Fournier to investigate the fluid’s characteristics, also deuterium and O-18 isotope to determine its origin. Based upon the analyzed data, it was found that the hotspring was derived from the local meteoric waters and the reservoir’s characteristics are similar with the Patuha geothermal system.

Introduction

Cikahuripan River is located in southern West Java, flowing from valley between Sanggar Mountain and Tipung Mountain, Talegong Region to Pogor Hill, Cisewu Region. This study was conducted in the meandering part, Talegong, Garut (figure 1). Astronomically, the area lies in X 770850-771852 TM and Y -91914920 - -9192998 UM (UTM zone 48).

Figure 1. Study area (red box) in scale 1:100 km of Java Island.

There were several geothermal manifestations such as altered grounds and hotsprings which occurred in the area. These manifestations indicate that some active geothermal system are working beneath the ground. Due to the absence of the active volcanoes in the nearest mountain (Sanggar Mount. And Tipung Mount.) cross-checked with the nearest geothermal system was done.

To know which geothermal system is the nearest to the field, the calculation to estimate the distance between the upflow of nearest geothermal system and study area was done using the remote sensing method.

Regional Geology

Based upon the divisions of physiography in West Java by Van Bemmelen (1949) the study area was lies in Pegunungan Selatan Jawa Barat. This zone was structural mountains which lied from west to east according to geometry of Java Island (figure 2).

Figure 2. Study area (red box) in the physiography of West Java (Van Bemmelen, 1949)

Therefore, according to geological map part Sindangbarang and Bandarwaru (Koesmono, 1996) regional lithologies that lies underneath are (figure 3),

Koleberes Formation, consists of well bedded tuff sandstones, less consolidated and crystal tuff; with interlayered tuff, pumice-tuff breccia. Black sandstones occurred near Gebeg Mountain and east sides of Citalahab. Mollusk, Gastropods, coral Echinoids and foraminifera were found in upper layers from this unit. Deposition environment were open sea. Thickness of this formation is about 350 meters. This formation were lying conformity to Bentang formation and unconformity lied by Pyroclastic, lahar and lava of Kendeng Mountain.

Unstacked pyroclastic deposits, consists of andesite breccia, tuff breccia and lapilli tuff. In the east sides occurred sheeting pyroclastic and ignimbrite.

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Proceedings Indonesia International Geothermal Convention & Exhibition 2015Jakarta Convention Center, Indonesia – August 19th – 21st, 2015

Figure 3. Regional geology of study area (Koesmono, 1996)

Method

Ternary Diagram Plotting

Elements, ions, and compounds within geothermal fluids could be used in order to comprehend the fluid’s characteristics. Anions such as Chloride (Cl), Sulphate (SO4), and Hidrogen Bicarbonate (HCO3) could infer fluid’s source whether the fluid originated from outflow zone or upflow zone (Giggenbach, 1982). The fluid is said to be mature waters if it is rich with Cl, or peripheral waters (mostly comprised by meteoric water, or has undergone mixing with bicarbonates) if dominated by HCO3, and could be volcanic water (juvenile SO2 rich water mixed with H2S) or steam heated water if contains plentiful amount of SO4.

Furthermore, anions and cations could be plotted to Piper Diagram (1944) to determine the hydrological facies and to support the result of Cl-SO4-HCO3 Ternary Diagram. Anions used in Piper Diagram are Cl, HCO3, SO4, and the cations used are Ca, Na+K, and Mg.

Ternary diagrams could also be used to investigate reservoir’s temperature, such as the Giggenbach’s Na-K-Mg Ternary Diagram.

Silica Geothermometry

Silica geotermometry depends on certain types of silica dissolved within the hotspring’s fluid (Fournier, 1983).

Quartz

T ° C= 13095.19−logSi O2

−273 .........

(Equation 1)

Chalcedony

T ° C= 10324.69−logSi O2

−273.........

(Equation 2)

α – Cristobalite

T ° C= 10004.78−logSi O2

−273 .........

(Equation 3)

β – Cristobalite

T ° C= 7814.51−logSi O2

−273.........

(Equation 4)

Amorphous silica

T ° C= 7314.52−logSi O2

−273 .........

(Equation 5)

(Fournier and Potter, 1982)

K/Mg Geothemometer (Giggenbach, 1986)

K/Mg could only be used if the reservoir is low enthalpy and the fluid has not reached equilibrium yet. Based on the conditions mentioned above, the geothermometry which represents accurate result is only the K/Mg geothermometer. By calculating the K/Mg in the fluids with Equation 6, subsurface temperature could be obtained.

T ° C=4410¿¿

......... (Equation 6)

O-18 and Deuterium Isotope Plotting

Deuterium (2H or D) and Oxygen 18 (δ O18) isotope analyses are used to determine fluid’s origin, whether the fluid’s origin is from meteoric water andesitic water, or a mixture of both. Samples from Ckh-1 and Ckh-2 are used in this study.

Satellite Imagery

Landsat images of research area is used to estimate the nearest geothermal system which are connected with the geothermal manifestations found within the research area.

Results

Field Tests

This study was held during rainy seasons in August 2014, due to the increasing of water level only several spots of hotsprings that could be analyzed (figure 4).

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Proceedings Indonesia International Geothermal Convention & Exhibition 2015Jakarta Convention Center, Indonesia – August 19th – 21st, 2015

Figure 4. Location of hotsprings in the study area.

- Ckh-3

At this spot, the manifestation was flowing through volcanoclastic and altered the surrounding lithology. The alteration near the manifestation was identified as argilic with clay minerals such as kaolinite and illite. There’s also a neutral pH mineral indicator like Opal found in this place using PIMA analyzed.

Geochemical composition of the fluid in this place were, Temp. air= 220 C; temp. water = 92.20 C; TDS = 1533 ppm; EC = 1.092 mS; and pH = 8.04.

There’s also sinter silica found in the nearby fractured, the thickness was about ± 1 cm (figure 5).

Figure 5. Hotspring manifestation in Ckh-3.

- Ckh-5

The lithology that were flown by fluids in this spot is volcanoclastic. In this spot the fluids were not flowing through the fracture, but using the primary porosity from the

manifested stones (figure 6). The fluids also altered the surrounding and the alteration was identified as sericite prior to the key minerals such as sericite and paragonite were found. As for clay minerals the results of PIMA analyzed shown kaolinite.

Geochemical compositions of the fluids in this spot were, Temp. air= 240 C; temp. water= 60.40 C; TDS = 2450 ppm;EC = 5.48 mS; and pH = 6.29.

Due to the undeposited silica, there were not sinter silica found in this place.

Figure 6. Hotspring manifestation in Ckh-5

- Ckh-7

At this spot, the manifestation was flowing through the lithology that were suspected as andecite (figure 7). The alteration that occurred in this lithology were argilic with clay minerals such as kaolinite and dickite.

Geochemical composition of the fluid in this place were, Temp. air = 230 C; Temp. water = 77.80 C; TDS = 2490 ppm; EC = 3.21 – 3.23 mS; and pH = 5.79.

Figure 7. Hotspring manifestation in Ckh-7.

Laboratory Tests

Samples that were taken at the field, laboratory test were held to know further about the anions and cations in the fluids. The results were shown in table 1.

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Proceedings Indonesia International Geothermal Convention & Exhibition 2015Jakarta Convention Center, Indonesia – August 19th – 21st, 2015

Table 1. Anions and Cations of Ckh-3, Ckh-5, and Ckh-7

From the data above further investigation about the fluids and reservoir’s characteristics can be done.

- Anion analysis

Using ternary diagram the type of the fluids could be obtained (figure 8). The ternary diagram plotting were resulted that fluids in the study area were mixing of SO4 and Cl in almost balance proportion. Therefore, the hotsprings were categorized as sulphate-chloride.

The result from the ternary diagram also supported by the piper diagram (1944) which also determined that the fluids in the study area were mixing between Chloride-Sulphate and the main cations were Na/K (figure 9).

Figure 8. Ternary diagram of Cl-SO4-HCO3 in study area.

Figure 9. Piper diagram of hotsprings in study area.

As for the Na-K-Mg diagram (Giggenbach, 1988) the results from plotting in this diagram was shown that fluids in the study area were derived from secondary reservoir due to the

phase from diagram that said partial equilibration (figure 10).

Figure 10. Na-K-Mg diagram (Giggenbach, 1988) in study area

Due to the results were partial equilibration, the Na-K-Mg geothermometer couldn’t be done.

- Geothermometer

SiO2 Geothermometer

SiO2 Geothermometer is based upon the absolute concentration of SiO2 in fluids. The results were derived from the equations above (equations 1 – equations 5). The results from the calculation were shown in table 2.

Table 2. SiO2 Geothermometer in study area

From the table above, the results were inconsistence between one equations with another. Therefore, plotting using graphic K2/Mg to SiO2 were done. The result of this plotting were,

Figure 11. Graphic of K2/Mg to SiO2 in study area

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Proceedings Indonesia International Geothermal Convention & Exhibition 2015Jakarta Convention Center, Indonesia – August 19th – 21st, 2015

From the graphic above, the main SiO2 in each spots were found. So the final results of SiO2 Geothermometer were shown in table 3.

Table 3. Final results of SiO2 Geothermometer

Samples α Cristobalite ChalcedonyConductive

Ckh-3 108 -Ckh-5 - 97CKh-7 - 103

K/Mg Geothermometer

Based upon the calculation from the equation 6, the results of K/Mg geothermometer wer shown in table 4.

Table 4. K/Mg GeothermometerSample K/Mg

Giggenbach (1986)

Ckh-3 100Ckh-5 95Ckh-7 101

- Isotope deuterium

To know about the origin of the fluids, isotope deuterium and oxygen-18 were analyzed. The results from the laboratory then plotted into graphic of Deuterium to Oxygen-18 (figure 12). The results were shown that the fluids in study area were derived from the local meteoric water.

Figure 12. Graphic of isotope deuterium in study area

Discussions

For the anion analysis which were resulting in fluids type, the fluid types were shown that there were mixing between SO4 and Cl. The mixing probably occurred in near surface or when the fluids reacted with the surrounding. The surrounding before the fluids flown were already altered due to the paleo-hydrothermal (ANTAM, 2012).

The reactions also affected the calculation of the SiO2, therefore the first results were inconsistent. Besides all that, the K/Mg geothermometer were still sufficient to use.

To know exactly whether the study area was a part of Patuha geothermal system or not, further study is required.

Conclusions

Based on geochemical analysis, the geothermal system which developed in the research area is a low enthalpy geothermal system with enthalpy ranging from 95 – 123oC (<125oC).According to the nearest geothermal system, the research area is included in Mt. Patuha geothermal system and is the outflow of said geothermal system. This is confirmed with petrographical and geochemical analyses which infer that the research area is classified into an outflow zone of a geothermal system.

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