On the origin and evolution of geothermal fluids in the Patuha Geothermal Field, Indonesia based on geochemical and stable isotope data

Journal Pre-proof On the origin and evolution of geothermal fluids in the Patuha Geothermal Field, Indonesia based on geochemical and stable isotope data Yudi Rahayudin, Koki Kashiwaya, Yohei Tada, Irwan Iskandar, Katsuaki Koike, Randy Wijaya Atmaja, Niniek Rina Herdianita PII:

S0883-2927(20)30011-1

DOI:

https://doi.org/10.1016/j.apgeochem.2020.104530

Reference:

AG 104530

To appear in:

Applied Geochemistry

Received Date: 30 August 2019 Revised Date:

28 December 2019

Accepted Date: 20 January 2020

Please cite this article as: Rahayudin, Y., Kashiwaya, K., Tada, Y., Iskandar, I., Koike, K., Atmaja, R.W., Herdianita, N.R., On the origin and evolution of geothermal fluids in the Patuha Geothermal Field, Indonesia based on geochemical and stable isotope data, Applied Geochemistry (2020), doi: https:// doi.org/10.1016/j.apgeochem.2020.104530. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

1

On the origin and evolution of geothermal fluids in the Patuha Geothermal Field, Indonesia

2

based on geochemical and stable isotope data

3 4

Yudi Rahayudina,b, Koki Kashiwayaa, Yohei Tadaa, Irwan Iskandarc, Katsuaki Koikea,

5

Randy Wijaya Atmajad, Niniek Rina Herdianitae

6 7 8 9 10 11 12 13 14 15 16

a. Department of Urban Management, Graduate School of Engineering, Kyoto University, C1-2-215, Kyoto Daigaku Katsura Kyoto 615-8540, Japan. b. PPSDMA, Ministry of Energy and Mineral Resources, Republic of Indonesia. Jalan Cisitu Lama 37, Bandung 40135, Indonesia. c. Faculty of Mining and Petroleum Engineering, Bandung Institute of Technology. Jalan Ganesha 10, Bandung 40132, Indonesia. d. PT. Geo Dipa Energi (Persero), Jalan Warung Jati Barat 75, Jakarta 12740, Indonesia. e. Faculty of Earth Sciences and Technology, Bandung Institute of Technology. Jalan Ganesha 10, Bandung 40132, Indonesia.

17 18

Corresponding author:

19

Katsuaki Koike

20

Department of Urban Management, Graduate School of Engineering, Kyoto

21

University

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C1-2-215, Kyoto Daigaku Katsura Kyoto 615-8540, Japan

23

[email protected]

24 25 1

26

Abstract

27

Volcano-hosted, vapor-dominated geothermal systems have great potential for power

28

generation, although to date, such systems discovered globally remain limited in

29

number. Understanding of the physical and chemical properties of geothermal fluids

30

(water and gas) in vapor-dominated systems is critical for the sustainable development

31

of geothermal resources. This study aims to clarify the origins, water–rock interactions,

32

and chemical evolution of geothermal fluids during migration from a reservoir to the

33

surface by selecting the Patuha geothermal field (PGF) in West Java, Indonesia as a

34

case study. The PGF is characterized by a vapor-dominated system that originated from

35

the subduction of the Indian–Australian plate beneath the Eurasian plate. In total, 26

36

water and 12 gas samples from production wells with 1424 to 2004 m depth, and

37

fumaroles were analyzed for major anions, cations, trace elements, stable isotopes, and

38

gas components to interpret phenomena occurring in deep reservoirs. Ternary diagrams

39

of Cl–SO4–HCO3 ionic compositions suggest that the H2S and CO2 gases are condensed

40

near the surface and changed to sulfate and bicarbonate by mixing with groundwater.

41

Products of water–wall rock interactions appeared in the area with acidic water, which

42

has mainly leached aluminum, accelerated pyrite oxidation, and increased iron

43

concentration in the water. High fluoride concentration at a fumarole site (95.9 mg/L)

44

implies HF gas supply from the deep-seated magmatic plume that is a geothermal

45

source of the PGF system. Oxygen and hydrogen isotopes reveal that meteoric water is

46

the main source of this system, and Na–K–Mg diagrams indicate immaturity of the

47

reservoir water. Through evaporation and mixing with the magmatic waters, the waters

48

have enriched heavy isotopic values, ascend along major faults towards the surface, and

49

partly discharge at hot springs and fumaroles. High temperatures of the reservoir and

2

50

gas-source in the subducted Indian–Australian plate are estimated based on the high

51

CO2 and H2S concentrations and the high N2/Ar ratios, respectively. By integrating the

52

analysis results of the water and gas samples, the well temperature data, and surface

53

geology, the volcanic activity under a crater was estimated as the heat source and to

54

have essential functions with the faults in the formation and fluid system of the vapor-

55

dominated PGF.

56

Keywords: Water–rock interaction, plate subduction, stable isotope, deuterium,

57

volcano-hosted geothermal system.

58 59

1.

Introduction

60

Geothermal heat is a resource of heat energy from the Earth’s interior that is

61

reserved both in rocks and in trapped waters and steams (Ellabban et al., 2014).

62

Geothermal heat is a renewable, infinite energy source (Nasruddin et al., 2016) that can

63

provide constant electricity with comparatively little emission of carbon dioxide or

64

other pollutants (Ferrara et al., 2019). Although Indonesia is ranked the third richest

65

country in the world in terms of geothermal resources, with 311 geothermal fields and

66

28,617 MW of electric potential, only 5.8% of this potential has been exploited as of

67

2017 (MEMR, 2017). Moreover, with demand for electricity expected to grow 8.5% per

68

year until 2025 with minimization of fossil fuel use, geothermal resources have become

69

critically important in Indonesia (ADB and World Bank, 2015; Fan and Sang, 2018).

70

Using a vapor-dominated geothermal system should contribute to the large

71

increase in electricity because its power is more significant than that of a liquid-

72

dominated system; a vapor-dominated system yields superheated or dry steam with little

73

or no liquid in its exploitation. In the liquid dominated systems, the produced two-phase

3

74

geothermal fluid loses a significant amount of heat when separating steam from water,

75

because only the separated steam is used for power generation (Moon and Zarrouk,

76

2012). This system is unique in that a condensate water layer is formed on a steam body

77

(Schubert et al., 1980; Truesdell and White, 1973) and discharge fluids do not yield

78

reservoir water of neutral chloride at low elevations (Ingebritsen and Sorey, 1988;

79

Raharjo et al., 2016; White et al., 1971). The temperature, water/steam ratio, chemical

80

evolution, and flow system of geothermal fluids are essential factors of a reservoir for

81

assessing the potential for power generation and sustainable use of geothermal

82

resources. However, these factors in vapor-dominated geothermal systems are difficult

83

to estimate because of a lack of Cl-rich waters in geothermal manifestations. Selection

84

of alternative geochemical indicators or development of a method that integrates various

85

geological and geochemical data is necessary to estimate the factors that generate vapor-

86

dominated geothermal systems.

87

Based on that background, this study aims to clarify the origins, water–rock

88

interactions, and chemical evolution of geothermal fluids during migration from a

89

reservoir to the surface by analyzing water and gas samples from production wells,

90

fumaroles, and hot springs with the initial temperature data of exploration wells. Major

91

and trace elements were first measured to characterize regional water chemistry and

92

flow patterns. Water isotopes (18O and 2H) were then used to determine the water origin

93

in the geothermal system, following preceding studies that have demonstrated the

94

usefulness of these isotopes (Birkle et al., 2016; Bouchaou et al., 2017; Caron et al.,

95

2008; Dupalová et al., 2012; Marques et al., 2008; Papp and Nitoi, 2006; Purnomo and

96

Pichler, 2014; Xun et al., 2009). The reservoir temperature and boiling and

97

condensation processes were estimated by using well-logging temperature data and the

4

98

gas compositions of methane, carbon dioxide, and hydrogen as a reference

99

(Giggenbach, 1980; Koike et al., 2014; Lowenstern et al., 2015; Stefánsson, 2017).

100

The above methods were applied to the Patuha geothermal field (PGF), West

101

Java, Indonesia, because the PGF is associated with a typical vapor-dominated

102

geothermal system that is not yet fully understood (Hanano, 2011). By integrating all

103

the results, a conceptual model of the PGF system was constructed, which will

104

contribute to the exploration and sustainable operation of the reservoir in the vapor-

105

dominated geothermal system and enable enable better understanding of the system

106

dependence on the local tectonic setting.

107 108

2.

Geological setting

109

The Indonesian archipelago is a geologically complex area located at the

110

southeastern edge of the Sundaland continental core of Southeast Asia. It is surrounded

111

by a tectonically active zone with high seismicity and volcanism caused by plate

112

subduction activity that began at 45 Ma (Hall, 2012; Malod et al., 1995), and is

113

located between three large plates, the Eurasian, Indian–Australian, and Pacific plates.

114

The subduction of the Indian–Australian plate beneath the Eurasian plate has resulted in

115

the formation of a late-Tertiary to Quaternary volcanic belt along the island of Java with

116

many active volcanoes (Sriwana et al., 2001); this area includes the PGF.

117

The PGF is an active geothermal area located 37 km southwest of Bandung city.

118

It is expected to contribute to meeting high energy demand because its power generation

119

capacity is estimated as 120 to 200 MW in the productive 9 km2 area. There are thirteen

120

production wells over a 35 km2 area that includes the productive area, and this

121

contribution is expected to be maintained for over 30 years (Swandaru, 2006). The PGF

5

122

system hosts a vapor-dominated or two-phase reservoir that is penetrated by two

123

volcanic chimneys containing magmatic waters (Raharjo et al., 2016). The formation of

124

a vapor-dominated (natural two-phase) layer about 0.5 km thick at 1 km depth and its

125

high temperature between 200 and 240°C were confirmed by several deep wells with

126

905 to 2351 m depth (Hochstein and Sudarman, 2008; WestJEC, 2007). The PGF is a

127

young volcanic complex surrounded by several older volcanoes such as southern Mt.

128

Patuha (the oldest), Mt. Puncaklawang, Mt. Urug, and Mt. Tikukur (Fig. 1), and is

129

composed mainly of late Pliocene to Quaternary pyroclastic and andesitic lavas. Late-

130

stage volcanic vents are distributed along the west to northwest, which form the

131

volcanic axis and control the geothermal system (Layman and Soemarinda, 2003). Mt.

132

Patuha, an active volcano, is situated in the middle of the PGF, and its degassing makes

133

the crater lake Kawah Putih, about 55,000 m2 in size, acidic and warm (Hochstein and

134

Sudarman, 2008; Pambudi, 2017). This crater was formed on southern Mt. Patuha by

135

the most recent volcanic event. Another crater is Kawah Patuha near the highest point in

136

the PGF.

137

6

138 139

Fig. 1. Geology and sampling site maps of the Patuha geothermal field (PGF) with

140

topographic features after DPE Jabar (2008), showing that the PGF is mainly covered

141

by andesitic and volcanic products. Water and gas sampling sites with locations of

142

exploration and production wells and faults mapped by Suswati et al. (2000). The

143

coordinate system used is UTM zone 48s.

144 145

The PGF is covered by fifteen lithologic types, as shown in Fig. 1 (DPE Jabar,

146

2008) above. The basement is composed of Tertiary volcanic rocks with microdiorite

147

(WestJEC, 2007). An epidote-rich zone formed under the silicification and

148

propylitization zones suggests advanced hydrothermal alteration at 200°C or higher at

7

149

around 1,200 m above the sea level (a.s.l.), and an argillic zone is developed under the

150

near-surface weathered zone (WestJEC, 2007).

151

The PGF’s geologic structure is thought to be controlled by the Bandung

152

depression zone, and Mt. Patuha is located at the zone boundary (Bemmelen, 1949). By

153

the depression, normal faults trending mainly NW–SE and NE–SW are developed (Fig.

154

1) in a regional transverse fault zone (Suswati et al., 2000). Nine major faults are

155

distributed at the feet of steep slopes and eruption points with small waterfalls and

156

geothermal manifestations, such as fumaroles, hot springs, cold gas discharge, mud

157

pools, cold acidic water, and steaming ground. Fumaroles are located at Kawah Cibuni,

158

Kawah Putih, and Kawah Ciwidey (Fig. 2).

159

160 161

Fig. 2. Typical surface manifestations in the PGF: a fumarole in the Cibuni zone (left),

162

the acid lake of Kawah Putih with high Cl concentration (middle), and SO4–HCO3 water

163

at the Punceling site (right).

164 165

Geochemical studies investigating the geothermal system in the PGF have been

166

limited in number (Layman and Soemarinda, 2003; Sriwana et al., 2000; Suryantini et

167

al., 2018). A recent study indicated that the springs at the northern edge of the PGF 8

168

boundary are hydrologically controlled by the topography of the Patuha Volcanic

169

Crown (PVC); the dormant volcano is not related to the active geothermal system, and

170

the cold springs influenced by geothermal fluids have several notable features such as

171

high total dissolved solids (TDS) and low pH (Suryantini et al., 2018). However,

172

previous geochemical studies have not covered the southern part of the PGF nor used

173

gas samples from the manifestations and production wells.

174 175

3.

Sampling and Analysis Methods

176

From the surface geothermal manifestations and deep production wells, 26 water

177

samples were collected in April 2017 and 12 gas samples in October 2016 and April

178

2017 at the locations shown in Fig. 1. During water sampling, electrical conductivity,

179

pH, and temperature were measured in situ using handheld meters (Hanna HI 991301).

180

To prevent algal growth that could remove Mg, NH3, and SO4 from the samples as well

181

as clog the analytical tools, water samples for major ion, water isotope, and trace

182

element analysis were filtered using 0.20 µm syringe filters, then the samples for cation

183

and trace element analysis were acidified using suprapur HNO3 to pH<2. The samples

184

were stored in four polyethylene bottles of 50 and 100 ml and glass bottles of 7 ml used

185

for the water isotope samples. The water samples for measuring silica were diluted

186

using de-ionized pure water to prevent polymerization and precipitation of silica during

187

storage, following preceding studies (Ármannsson and Ólafsson, 2007; Arnorsson et al.,

188

2006; Guo et al., 2017; Kaasalainen et al., 2015; Zhao et al., 2018). Water samples for

189

δ18O- and δD-H2O analyses were collected directly from the manifestations and wells to

190

minimize contact time with the atmosphere, and were then kept out of direct sunlight

9

191

and protected from evaporation by sealing and storing the samples in the dark boxes,

192

which could change the stable isotopic compositions of the samples.

193

Concentrations of major cations/anions and trace elements were measured using

194

ion chromatography (IC, Shimadzu Prominence) and an inductively coupled plasma

195

mass spectrometer (ICP-MS, Agilent 7500cx), respectively. Water alkalinity was

196

measured in a laboratory by the titration method using 0.1 M H2SO4, soon after the

197

sampling, which followed the USGS procedure (USGS, 2012), and silica (SiO2)

198

concentration was determined by the silicomolybdate method using a colorimeter (Hach

199

colorimeter). Measurement of stable isotope oxygen and hydrogen ratios, δ18O- and δD-

200

H2O‰, was performed with an isotope and gas concentration analyzer with cavity ring-

201

down spectroscopy (Picarro L2130-i).

202

In a liquid-dominated reservoir, phase separation between water and steam often

203

begins below the surface, and two-phase fluid appears at the wellhead. Therefore,

204

sampling at the production wells was performed using a centrifugal Webre separator to

205

separate water and steam. Condensate and boiling waters near the manifestation were

206

sampled from the fumaroles, and a titanium tube was used for the gas sampling at the

207

fumaroles to transport geothermal gas from the source to sample bottles (Arnorsson et

208

al., 2006; Giggenbach, W.F., Goguel, 1989; Janik et al., 1992). Great caution was taken

209

in the fumarole gas sampling to prevent air contamination. Commonly, the existence of

210

oxygen in geothermal gas is evidence of atmospheric contamination in the sample

211

(Arnorsson et al., 2006).

212

For gas sampling, evacuated 300 ml cylindrical Giggenbach double-port bottles

213

were used (Giggenbach, W.F., Goguel, 1989). Before sampling, 50 ml of the alkaline

214

solution of 4 N NaOH was added to each bottle following a USGS method by Fahlquist

10

215

and Janik (1992). CO2, H2O, NH3, and H2S gases were dissolved in the solution, and the

216

remaining gases (He, H2, Ar, O2, N2, and hydrocarbons) were concentrated in the

217

headspace above the solution to increase the analysis accuracy for concentration.

218

Concentrations of the dissolved gases and the remaining gases including steam

219

condensate (H2O) were measured by gas chromatography (Shimadzu GC-2014 and GC-

220

8A) and the wet chemical analytical technique (Fahlquist and Janik, 1992), respectively.

221 222

4.

Results

223

Heterogeneous physicochemical features of the water samples were revealed, as

224

shown in Table 1. The temperatures ranged widely from 14.6°C (PTHCS) to 91°C

225

(CWDY15), and the electric conductivity also varied from 42 µS/cm (PTHCS) and

226

20,000 µS/cm (PTH1: Kawah Putih 1). The pH data between 5.9 and 7.8 are categorized

227

as neutral, except for seven acidic waters of samples PTH1, CBN1, CBN2, CWDY15,

228

PTHCS, PTH (Kawah Putih), and TSCS from pH = 0.5 (PTH) to 3.7 (CWDY15).

229

The δ18O and δD values ranged from −9.0‰ to 4.0‰ and −55.8‰ to −16.7‰,

230

respectively (Table 1). The isotopes of the hot springs revealed relatively uniform

231

values, ranging from −9.0‰ to −6.0‰ (δ18O) and −55.8‰ to −47.8‰ (δD), close to the

232

isotope values of local precipitation, whereas the fumarole isotopes showed heavier

233

values from −2.8‰ to 4.0‰ (δ18O) and −34.8‰ to −16.7‰ (δD).

234

For the gas components, CO2 was the predominant component with 91.0% to

235

96.1% (mol %), followed by H2S (0.3% to 6.75%), N2 (1.19% to 3.98%), and H2

236

(0.01% to 1.25%) in descending order (Table 2). The concentration of fumarole CO2

237

was higher than that of the production wells, except for well 1G with 95% CO2, which

238

was slightly higher than CWDY (94.8%). High CO2 concentration over 90% in wells

11

239

and fumaroles is a typical feature of geothermal fields all over the world, including

240

Yellowstone and Reykjanes (Arnórsson, 1986; Lowenstern et al., 2015).

241 242 243 244 245 246 247 No

Table 1 Physical parameters and measured concentrations of major ions, trace elements, and oxygen and hydrogen isotopes of the water samples with analytical errors IC: +5 %, ICP: +10 %, δ18O: +0.02 ‰, and δ2H: +0.2 ‰. “nm” and “nd” mean not measurement and not detected, respectively. Name

Temp (°C)

Type

pH

Ec (µS/cm)

Li+ (mg/L)

Na+ (mg/L)

K+ (mg/L)

Ca2+ (mg/L)

Mg2+ (mg/L)

NH4+ (mg/L)

Cl ¯ (mg/L)

F¯ (mg/L)

SO42(mg/L)

HCO3¯ (mg/L)

1.

PTH1

Crater Lake

27.5

1.0

20,000

5.5

16.6

18.6

35.2

15.2

0.0

3991.9

95.9

1804.0

0.0

2.

RSRT

Hotspring

37.9

7.8

2540

0.3

150.6

38.3

171.8

107.8

0.0

416.9

0.0

593.3

286.7

3.

PCL1

Hotspring

42.5

5.9

2,560

0.3

151.0

38.5

178.5

108.8

0.0

445.6

0.0

593.5

282.6

4.

PCL2

Hotspring

37.7

5.9

2,280

0.2

132.2

34.1

132.1

97.5

0.0

403.9

0.0

552.2

253.2

5.

NT1

Hotspring

44.1

5.9

2,060

1.2

124.7

48.7

123.6

78.2

1.0

333.4

58.4

277.7

331.4

6.

NT2

Hotspring

38.9

6.1

1,930

1.2

120.6

46.5

113.3

73.6

0.0

289.5

8.2

262.3

309.1

7.

NT3

Hotspring

50.5

6.0

2,140

1.2

127.3

50.2

126.0

79.4

0.7

311.7

8.2

275.9

348.0

8.

WLN1

Hotspring

43.8

6.1

1,300

0.1

53.4

20.8

90.5

57.6

0.0

113.6

8.8

228.0

296.9

9.

WLN2

Hotspring

42.9

6.2

1,350

0.1

52.9

20.8

96.3

61.6

0.0

120.4

8.8

240.3

311.1

10.

WLN4

Hotspring

45.4

6.2

1,180

0.1

51.3

20.4

74.3

45.7

0.0

83.7

8.8

165.4

276.5

11.

CMG

Hotspring

48.5

6.1

810

0.2

65.2

18.1

44.0

14.9

0.0

21.3

8.9

55.8

286.7

12.

UPS

Hotspring

34.8

5.9

730

0.2

37.9

16.4

39.9

25.3

0.0

84.6

0.0

90.2

132.2

13.

WLN5

Hotspring

46

6.1

1270

0.1

75.8

23.9

121.1

61.3

0.3

150.4

4.0

236.1

277.4

14.

CMG2

Hotspring

70.5

6.4

460

0.1

91.0

18.3

55.2

19.7

0.3

42.7

0.0

52.0

344.3

15.

1B

Well

68.9

6.1

133

0.0

6.1

4.0

12.6

3.5

0.0

7.2

0.0

38.0

15.0

16.

1K

Well

27.2

7.67

251

0.0

19.9

3.3

10.3

0.0

0.6

7.3

5.1

26.7

20.0

17.

1C

Well

nm

7.7

nm

0.0

6.1

4.1

13.3

3.8

0.1

0.9

0.2

49.2

21.0

18.

CBN3

Coldspring

20.5

6.1

110

0.0

3.8

1.4

8.7

4.2

0.0

7.3

0.0

7.8

40.7

19.

WLN3

Coldspring

21.2

6.3

330

0.1

14.2

5.0

17.7

9.0

0.3

17.7

0.0

95.7

10.0

20.

PTHCS

Coldspring

14.6

3.6

42

0.0

8.8

1.8

47.5

6.0

0.2

2.6

0.0

157.2

0.0

21.

TSCS

Coldspring

23.6

3.1

1710

0.0

6.6

1.9

7.1

4.9

0.0

2.0

0.0

1564.5

0.0

22.

CBN2

Fumarole

46.3

2.8

780

1.6

0.0

1.6

3.6

1.8

0.1

40.7

0.0

119.9

0.0

23.

CWDY

Fumarole

90.6

6.58

118

0.0

6.9

2.8

8.1

1.3

0.8

7.1

5.1

19.5

12.0

24.

CWDY15

Fumarole

91

3.7

360

0.0

2.0

1.3

52.9

3.5

0.6

0.1

0.0

56.9

0.0

25.

PTH

Fumarole

71

0.5

730

0.5

10.4

0.5

7.6

3.4

0.0

6342.0

0.0

3273.9

0.0

26.

CBN1

Fumarole

89.7

2.5

1,750

1.9

0.0

2.0

6.2

3.3

0.7

40.6

8.1

355.4

0.0

248 249 No

Name

Type

SiO2 (mg/L)

B (mg/L)

As (mg/L)

Rb (mg/L)

Cs (mg/L)

Sr (mg/L)

Ba (mg/L)

Al (mg/L)

Fe (mg/L)

Mn (mg/L)

δ18O (‰)

δ2H (‰)

HCO3¯/ Cl ¯

SO42/Cl ¯

HCO3¯/SO42-

1.

PTH1

Crater Lake

15.0

31.9

0.10

0.05

0.01

0.65

0.24

160.87

22.02

0.67

-1.7

-34.8

0.0

0.5

0.0

2.

RSRT

Hotspring

15.0

10.6

0.01

0.17

0.02

0.33

0.02

0.00

0.05

1.09

-8.4

-55.1

0.7

1.4

0.5

3.

PCL1

Hotspring

15.0

11.0

0.03

0.17

0.02

0.33

0.02

0.00

2.08

1.23

-8.3

-54.8

0.6

1.3

0.5

4.

PCL2

Hotspring

13.0

8.8

0.03

0.15

0.01

0.29

0.02

0.00

2.77

1.12

-8.4

-55.0

0.6

1.4

0.5

5.

NT1

Hotspring

19.0

8.2

0.04

0.28

0.03

0.26

0.06

0.00

0.01

1.44

-7.7

-52.5

1.0

0.8

1.2

6.

NT2

Hotspring

19.0

7.8

0.06

0.26

0.03

0.23

0.07

0.01

0.03

1.45

-7.7

-52.7

1.1

0.9

1.2

7.

NT3

Hotspring

21.0

8.2

0.29

0.29

0.03

0.26

0.06

0.00

4.65

1.51

-7.9

-53.4

1.1

0.9

1.3

8.

WLN1

Hotspring

13.0

3.9

0.13

0.13

0.02

0.19

0.01

0.00

0.02

0.00

-8.7

-55.6

2.6

2.0

1.3

9.

WLN2

Hotspring

12.0

3.4

0.15

0.14

0.02

0.19

0.01

0.01

0.02

0.00

-8.7

-55.6

2.6

2.0

1.3

10.

WLN4

Hotspring

13.0

2.4

0.14

0.13

0.02

0.16

0.00

0.01

0.01

0.00

-8.6

-55.4

3.3

2.0

1.7

11.

CMG

Hotspring

16.0

1.4

0.07

0.08

0.01

0.14

0.02

0.01

0.16

2.10

-8.9

-55.4

13.5

2.6

5.1

12.

UPS

Hotspring

7.0

1.9

0.02

0.00

0.00

0.09

0.02

0.00

0.14

0.04

-8.7

-54.2

1.6

1.1

1.5

13.

WLN5

Hotspring

16.3

0.0

0.004

nd

nd

nd

nd

nd

1.02

nd

-6.0

-48.4

1.8

1.6

1.2

14.

CMG2

Hotspring

56.1

0.0

0.004

nd

nd

nd

nd

nd

2.73

nd

-8.7

-49.8

8.1

1.2

6.6

12

15.

1B

Well

0.0

0.0

0.00

0.00

0.00

0.00

0.00

nd

0.00

0.00

-6.9*

-50.2*

2.1

5.3

0.4

16.

1K

Well

0.0

0.0

0.00

0.00

0.00

0.00

0.00

nd

0.00

0.00

-3.8

-37.5

2.7

3.7

0.7

17.

1C

Well

0.0

0.5

0.01

nd

nd

nd

nd

nd

nd

nd

-6.6*

-48.0*

23.3

54.7

0.4

18.

CBN3

Coldspring

4.0

0.0

0.00

0.01

0.00

0.03

0.00

0.00

0.01

0.00

-9.0

-55.8

5.6

1.1

5.2

19.

WLN3

Coldspring

4.0

0.6

0.00

0.01

0.00

0.06

0.03

0.28

1.22

0.06

-8.8

-54.9

0.6

5.4

0.1

20.

PTHCS

Coldspring

5.0

0.0

0.004

nd

nd

nd

nd

nd

4.70

nd

-6.6

-51.8

0.0

60.5

0.0

21.

TSCS

Coldspring

41.4

0.0

0.004

nd

nd

nd

nd

nd

44.34

nd

nd

nd

0.0

782.3

0.0

22.

CBN2

Fumarole

4.0

0.1

0.00

0.01

0.00

0.02

0.01

2.59

1.89

0.03

-0.2

-20.1

0.0

2.9

0.0

23.

CWDY

Fumarole

108.0

0.5

0.01

0.00

0.00

0.00

0.00

nd

0.00

0.00

-7.5

-47.9

1.7

2.7

0.6

24.

CWDY15

Fumarole

9.0

0.0

0.004

nd

nd

nd

nd

nd

2.24

nd

-2.8

-29.4

0.0

569.0

0.0

25.

PTH

Fumarole

23.0

0.0

0.004

nd

nd

nd

nd

nd

29.30

nd

4

-16.7

0.0

0.5

0.0

Fumarole

9.0

0.3

0.00

0.01

0.00

0.04

0.03

12.62

10.91

0.16

-7.1

-47.8

0.0

8.8

0.0

26.

CBN1

250

* Data after Amelia (2014)

251 252 253 254 255

Table 2 Concentrations of gas components measured for samples from production wells (1A to 1K) and fumaroles (CBN1 and CWDY) with analytical error +5 %.

1A

CO2 (mol %) 91.2

H2S (mol %) 5.9

NH3 (mol %) 0.08

Ar (mol %) 0.007

N2 (mol %) 1.96

CH4 (mol %) 0.03

H2 (mol %) 0.81

1B

92.7

4.4

0.04

0.007

2.44

0.06

0.36

3.

1C

91.4

5.7

0.02

0.008

2.41

0.07

4.

1D

91.0

6.4

0.06

0.006

1.56

5.

1E

93.6

2.9

0.05

0.003

6.

1F

92.8

4.2

0.11

7.

1G

95.0

2.4

0.04

8.

1H

90.7

6.8

9.

1J

93.3

10.

1K

93.4

No

Name

1. 2.

CO2/H2S

N2/Ar

CH4/Ar

15

280

4

21

349

9

0.44

16

301

9

0.03

0.97

14

260

5

2.91

0.05

0.50

32

970

17

0.007

1.91

0.04

0.85

22

273

6

0.003

1.96

0.04

0.51

40

653

13

0.10

0.004

1.19

0.01

1.25

13

298

3

3.9

0.12

0.004

2.68

0.03

0.03

24

670

8

3.3

0.05

0.004

2.58

0.04

0.58

28

645

10

11.

CBN1

96.1

2.1

0.37

0.002

1.30

0.15

0.01

46

650

75

12.

CWDY

94.8

0.3

0.00

0.086

3.98

0.05

0.73

316

46

1

256

13

257 258 259

Fig. 3. Relative Cl, SO4, and HCO3 concentrations (mg/L) and data classification by the

260

origin and mixing processes. Most samples belong to the sulfate-bicarbonate type,

261

probably resulting from an interaction between gas condensate and surface water.

262 263

5.

Discussions

264

The water geochemical data, isotope data, and gas component data were used to

265

investigate fluid circulation in the geothermal system, the origins of the water and gas,

266

and chemical reaction in water–rock interaction processes. Finally, a conceptual model

267

of the geothermal system in the PGF was constructed by integrating all the data and

268

interpretations. 14

269 270

5.1. Characterization of water geochemical data for the fluid circulation system

271

The ternary diagram of the ionic compositions of HCO3–Cl–SO4 (Fig. 3)

272

indicates that the water samples can be divided into three types: sulfate (SO4), sulfate–

273

chloride (SO4–Cl), and sulfate–bicarbonate (SO4–HCO3) types. The sulfate type

274

appeared in three fumarole samples (CBN1, CBN2, CWDY15) and three coldspring

275

samples (PTHCS, TSCS, and WLN3), with high sulfate concentrations ranging from

276

56.9 to 1,564 mg/L (Fig. 4). The sulfate zone that included the six sampling sites is

277

known as a steam-heated zone where gases dissolved in neutral chloride water at great

278

depth were separated from the depressurized water by boiling (Nicholson, 1993). The

279

gasses are rich in volatiles such as H2S and CO2 (Kaasalainen and Stefánsson, 2012),

280

ascend towards the surface through faults and fractures, and oxidize groundwater.

281

Consequently, H2S gases are condensed in the form of sulfate (Cinti et al., 2017; Li et

282

al., 2019; Nicholson, 1993) .

15

283 284 285

Fig. 4. Maps showing Cl, SO4, and HCO3 concentrations at each sample site. The Cl

286

and SO4 concentrations are extremely high at the Kawah Putih crater and decrease

287

towards the north, but HCO3 is the highest at CMG and NT in the hot springs in the

288

northern part.

289 290

The H2S condensation process can be seen in the hot springs and fumaroles, in

291

which the SO4 concentration is higher than the Cl and HCO3 concentrations. The SO4

292

concentrations, 3273.9 mg/L at PTH and 1804.0 mg/L at PTH1 are much higher than

293

the other manifestations, which can be interpreted by dissolution of sulfide-bearing

294

minerals such as pyrite and condensation of volcanic gases. Pyrite is commonly

295

distributed in PGF (Cahyati et al., 2018). 16

296

The oxidation of H2S generates sulfate ions and increases H+ content, and

297

accordingly, greatly decreases water pH (Nordstrom et al., 2009), as seen in samples

298

CBN1, CBN2, PTHCS, TSCS, CWDY15, PTH, and PTH1. In addition, oxidation of

299

pyrite increases Fe concentration in sulfate water as seen in CBN1 (10.91 mg/L) and

300

TSCS (44.34 mg/L). The pyrite oxidation has occurred through the following equation

301

(Eq.1) (Kaasalainen and Stefánsson, 2012; Bogie and Lawless, 2000; Shvartsev et al.,

302

2018):

303 304

FeS2 + 3.5O2 + H2O = Fe2+ + 2 SO42− + 2H+.

(1)

305 306

This process can be confirmed by a negative correlation of the Fe concentration with pH

307

(Fig. 5). Almost absent Fe concentrations suggest the removal of Fe from the water by

308

secondary hydrothermal mineral formation (Pasvanoǧlu, 2013).

309

PTH (pH = 0.5) and PTH1 (1.0) had extremely low pH, which were categorized

310

as SO4–Cl type with higher concentrations of Cl than SO4. Another cause of that low pH

311

is hydrochloric acid from volcanic gas, and the absence of HCO3 in the low pH samples

312

may have been caused by degassing of CO2 and formation of H2CO3. The low HCO3/Cl

313

ratio in the acid crater lake and fumaroles suggests a long migration of water with deep

314

circulation (Pasvanoğlu and Çelik, 2018), whereas the SO4–Cl water could have

315

originated from several mechanisms, such as (Nicholson, 1993; White, 1957):

316

mixing of chloride and sulfate waters at various depths,

317

near-surface discharge and oxidation of H2S in chloride water,

318

near-surface condensation of volcanic gases into meteoric waters,

319

condensation of magmatic vapor, and

17

320

passage of chloride water through a sulfate-bearing sequence.

321

At PTH1, the occurrences of complex processes such as sulfate enrichment from

322

the sulfide-bearing rock dissolution and condensation of gases from volcanic activity

323

are estimated. The SO4–Cl waters with relatively high temperature and a CO2-rich vapor

324

phase, as seen at PTH, which probably originated from the most common Na–Cl waters

325

in deep parts of volcanic areas (White, 1957), can induce intensive water–rock

326

interaction and dissolve cationic rock components by their highly acidic and oxidizing

327

conditions (Giggenbach, 1987). Interaction between acidic volcanic water and wall

328

rock, mainly andesite, is also indicated by the relatively high Al (160.87 mg/L) and Li

329

(5.5 mg/L) concentrations in the most acidic water and a negative correlation between

330

the Al concentration and water pH (Fig. 5). The direct effect of volcanic activity on

331

PTH1, which is situated on the confluence of two perpendicular faults, can also be

332

interpreted based on the high F concentration (95.9 mg/L). This concentration at PTH

333

may be supplied by HF gas ascending along a conduit, because HF is one of the most

334

abundant volatile gas elements from magma (Rowe, 2000).

18

335

336 337 338

Fig. 5. Correlations of pH with Al (top) and Fe (bottom) concentrations with regression

339

curves using negative logarithm functions and the coefficient of determination (R2). Al

340

and Fe concentrations decrease largely with increasing water pH. 19

341

The SO4–HCO3 type is defined by a lower concentration of Cl than of SO4 and

342

HCO3, relatively high Na concentration, and near-neutrality to alkalinity with primarily

343

calcite precipitation, as shown by travertine manifestation in the Cimanggu area. This

344

type is the most representative in the PGF; 17 out of 26 samples were classified as this

345

type (Fig. 3), and mainly distributed in the northern flank of Mt. Patuha and near the

346

Cibuni fault (Fig. 1). The highest ratios of HCO3/Cl and HCO3/SO4 at the CMG,

347

CMG2, and CBN3 sites are thought to have formed by condensation of a CO2-rich

348

steam into groundwater in a marginal, outflow zone of the geothermal system. This

349

interpretation can be supported by a preceding result that the effect of the PGF system

350

on water geochemistry became weaker towards the north (Suryantini et al., 2018). The

351

enrichment of HCO3 over SO4 may have been caused by the generation of sulfate

352

minerals around the peak of Mt. Patuha, and the selective fluid flows towards the north

353

along the Cimanggu, Punceling, and Cibuni faults, accompanying a mixture of waters in

354

the Walini and Cimanggu regions. This flow direction is supported by the pattern and

355

continuity of low anomalies in a self-potential map of the PGF (Singarimbun et al.,

356

2011) and the temperature profile of the exploration wells (Fig. 6). The general trend of

357

temperature along with depth in the PGF is shown by the E1, E5, and E15 profiles. In

358

contrast, the E16 and E17 profiles near the northern top of Mt. Patuha highlights a

359

sudden peak in the temperature downward at shallow depth starting at 1,600 to 1,900 m

360

a.s.l. (Fig. 6), which demonstrates the northward flow of hot condensed waters from Mt.

361

Patuha. The fluids and gases in the southern part of Mt. Patuha are also influenced by

362

the Cimanggu and Punceling faults. The existence of some faults in PGF is very

363

important in fluid migration mechanism, as also seen in several other volcanic

364

geothermal fields in Indonesia (Brehme et al., 2016a, 2016b; Deon et al., 2015).

20

365 366 367

Fig. 6. Temperature profiles from exploration wells (WestJEC, 2007), showing general

368

pattern of the temperature change in PGF by E1, E5, and E15 and a featured pattern

369

associated with condensate flow near Mt. Patuha by E16 and E17.

370 21

371

Based on the Na–K–Mg ternary diagram (Giggenbach, 1991, 1988), the present

372

water samples are mostly immature far from the equilibrium phase, contain abundant

373

rock dissolution components, and are well mixed with the groundwater, as demonstrated

374

by high Mg concentrations (Fig. 7a). The low Mg in 1K may be due to dilution by the

375

condensate water.

376

The origin of PGF waters can also be interpreted from the B–Li–Cl ternary

377

diagram (Fig. 7b). The ratio of B to Cl and Li reaches a maximum at 1C. This suggests

378

that the 1C water originated from a young geothermal system, because B is depleted

379

over time and its concentration becomes high only in a young geothermal system

380

(Pasvanoğlu and Çelik, 2018; Phuong et al., 2012). In contrast, the waters with B

381

concentrations of almost zero and extremely high Cl/B ratios at five sites (PTH, 1B, 1K,

382

PTHCS, and TSCS) are thought to originate from an old geothermal system. The Cl

383

concentrations of the CBN1 and CBN2 waters are extremely low, as waters in granite,

384

diorite, and basalt due to rock leaching. Through the borehole geologic data, deeply-

385

seated diorite and microdiorite were found as the basement rocks in PGF (WestJEC,

386

2007).

387

22

388

389 390 23

391

Fig. 7. (a) Na–K–Mg ternary diagram following Giggenbach (1988) for relative Na–K–

392

Mg concentrations (mg/L) of hot-spring and well water samples, which reveals that

393

immature waters are dominant in PGF (b) Ternary diagram for concentrations (mg/L) of

394

conservative elements, B, Li, and Cl in which waters from young geothermal system are

395

located in the low B zone.

396 397 398

5.2 Oxygen and hydrogen isotopes for specifying the fluids origin A combination of

18

O and 2H isotope compositions is the most effective

399

indicator to interpret water source, mixing processes, and circulation paths (Awaleh et

400

al., 2018; Griffin et al., 2017; Ivanova et al., 2013; Liotta et al., 2013; Xiao et al., 2018;

401

Zhao et al., 2018). The isotopic data, δ18O and δ2H are plotted and overlaid with the

402

global meteoric water line (GMWL: δ2H = 8 δ 18O + 10; Craig, 1961) and the Java’s

403

local meteoric water line (LMWL: δ2H = 7.9 δ 18O + 16; Abidin and Wandowo, 1995)

404

in Fig. 8. Because the general trend of δ18O and δ2H follows well the Java’s LMWL, the

405

meteoric water is the main source in the PGF. Another most remarkable feature in Fig. 8

406

is that the nine samples of PTH, CBN2, PTH1, CWDY15, 1K, WLN5, 1C, PTHCS, and

407

1B increase significantly in their δ18O values towards that of andesitic water from

408

Giggenbach (1992); the PTH water is the closest to it. This shift was caused by the

409

mixing of magmatic water in the reservoir and water-vapor fractionation in the

410

evaporation process. This result is also supported by water facies data that indicate an

411

influence of geothermal activities in the groundwater, and the highest concentrations of

412

Na–K, as seenin 1K and Cl in PTH suggest deep water and magmatic origins (Iskandar

413

et al., 2018; Nicholson, 1993). The magnitude of the shift in δ18O depends on the initial

414

isotopic compositions of the meteoric water and host rock, mineral composition,

24

415

reservoir temperature, water residence time, reservoir permeability and porosity, age of

416

the geothermal system, the rock–water ratio and enrichment by heavy isotope fluids

417

(Giggenbach, 1992; Nicholson, 1993; Ohba et al., 2000).

418

419 420

Fig. 8. Plot of δ18O versus δ2H data for relating waters to evaporation and mixing

421

processes using the global meteoric water line (GMWL; Craig, 1961), local meteoric

422

water line for the island of Java (LMWL; Abidin and Wandowo, 1995), and a range of

423

δ18O and δ2H values of andesitic waters by Giggenbach (1992). Although the meteoric

424

water is the main source of the PGF fluids, several data show the isotope shifting caused

425

by the mixing of magmatic water and water-vapor fractionation in the evaporation

426

process.

427 428

25

429

5.3 Gas chemistry for gas source and reaction with wall rock

430

The high CO2 and H2 concentrations in the PGF as shown in Table 2 are a

431

typical feature of the geothermal area due to high subsurface temperatures. Higher CO2

432

concentrations were measured from the fumarole samples of CBN1 and CWDY than

433

from the well samples. This suggest an effect of the deep fluids that ascend along the

434

major fault of Cibuni and Ciwidey (Fig. 1), similar to a case of the Sipolohon

435

geothermal field in Sumatra, Indonesia (Nukman and Hochstein, 2019). The difference

436

of CO2 concentration between the fumaroles and wells may also be caused by low H2S

437

contents in the steam condensation and transportation of soluble gases to shallow and

438

surface waters.

439

The smaller CO2/H2S ratio of CBN1 (46) compared with that of CWDY (316)

440

suggests a direct gas discharge at the CBN1 fumarole, because the H2S concentration

441

generally decreases towards the surface by reaction with wall rock, dissociation to

442

sulfur, or oxidation (Ping, 1991). The CH4 concentrations are mostly low in the PGF

443

ranging from 0.01 to 0.07 mol %, except the concentration at CBN1 (0.15 mol %). This

444

high concentration was probably caused by the enrichment of organic matter, which is

445

supported by the high ratios of CH4/Ar and N2/Ar, which resulting from the

446

decomposition of organic matter. CH4 is associated with a magmatic, organic, or

447

reductive origin of CO2 by the Fischer–Tropsch reaction (Arnórsson, 1986; Nehring and

448

D’Amore, 1984).

26

449 450 451

Fig. 9. N2–Ar–CO2 ternary diagram to identify the origins and boiling processes of gas

452

samples, which reveals that most samples had high N2/Ar ratios associated with

453

enrichment of organic matter, except for CWDY. ASW stands for air-saturated water.

454

The N2–Ar–CO2 ternary diagram (Fig. 9) is effective for identifying the origins

455

and boiling processes of gas samples; in particular, the N2/Ar ratio is an indicator of the

456

source of nitrogen gas species (Giggenbach, 1987). This ratio depends strongly on N2

457

concentration because Ar, a noble gas, is unlikely to be influenced by chemical

458

reactions that could lower its original concentration. The N2/Ar ratio ranges widely from

459

46 to 970. The lowest ratio (= 46) of CWDY was probably caused by atmospheric

460

contamination (i.e., mixture of the steam with air near the surface), whereas the high 27

461

ratios of CBN1 (= 650) and 1E (= 970) are equivalent to the ratios in typical geothermal

462

fields, such as the Taupo Volcanic Zone in New Zealand (Giggenbach, 1996) and the

463

Cascade Range in United States (Symonds et al., 2003). According to preceding studies,

464

the high N2/Ar ratio was ascribed to the decomposition of subducted sediments with a

465

minor contribution from the mantle (Giggenbach, 1996, 1995; Matsuo et al., 1978;

466

Minissale et al., 1997; Shinohara et al., 1993; Taran, 2005) and abundant organic matter

467

in the subducted Indian–Australian oceanic plate and the continental Eurasian plate. The

468

materials are contained in the Java accretionary wedge composed of fragments of chert,

469

siliceous material, shale, limestone, and pillow basalt (Wakita, 2000). Consequently, the

470

gases in the PGF probably originate from deep regions associated with the subduction

471

mechanism.

472

Moreover, CO2/H2S ratio is influenced by condensation and boiling processes

473

(Nicholson, 1993). the CO2/H2S ratios of 1G (40) and CWDY (316) are much higher

474

than those of the other samples, which suggests the condensation and loss of H2S gas in

475

the steam ascending process around well 1B. In contrast, the low CO2/H2S ratios of 1H

476

(13) and 1D (14) imply an intensive boiling process that increased the H2S

477

concentration.

478 479

5.4 Conceptual model of the Patuha geothermal system

480

By integrating the analysis results of the water and gas samples and the well

481

temperature data, a conceptual model of geothermal fluid flow in the PGF is

482

summarized in Fig. 10. Generally, the fluids in the reservoir were derived from meteoric

483

waters that infiltrated through permeable zones such as faults and fractures. Then, the

484

waters were boiled by the heat source, which is thought to be below the Kawah Putih

28

485

and to cause the volcanic activity of Mt. Patuha. With the decrease in pressure, the

486

fluids in the reservoir underwent a boiling process and yielded hot waters and vapors

487

that ascended through faults and fractures. In the flowing process, the waters and vapors

488

interacted with the wall rocks and changed their chemical compositions and pH. H2S-

489

rich vapors partially discharge at fumaroles such as at the CBN site, and the remaining

490

vapors decrease in temperature and generate sulfate condensates near the surface. A

491

portion of the condensates are mixed with the groundwater and yield SO4- and HCO3-

492

rich waters, as observed in the northern part of the Kawah Putih.

493

The formation process of the liquid and vapor zones in the southern part of the

494

PGF is the same as that in the northern part. Based on the well temperature and steam

495

distribution data, the steams originate from the location of well 1B, extend toward the

496

north to east (Amelia, 2014), and ascend to the surface as fumarole manifestations.

29

497 498 499

Fig. 10. A conceptual model of the geothermal fluid flow system in the PGF along SE–

500

NW. Kawah Putih is estimated as a center of volcanic activity. The boiling of

501

geothermal fluids yields H2S-rich vapors that directly ascend towards the surface,

502

discharge at fumaroles, and produce sulfate condensates near the surface by cooling.

503

The angles of faults and flows in this figure are not to scale, and depths and dimensions

504

of geologic structures are for illustrative purposes only. Isotherm lines and locations of

505

the liquid and vapor zones are based on our interpretation of the drilling data of

506

WestJEC (2007). “>” means high concentration or ratio.

507 508

6.

Conclusion

30

509

This study aimed to clarify the origins, water–rock interactions, and chemical

510

evolution of geothermal fluids along the flowing process from the reservoir to the

511

surface in the Patuha Geothermal Field (PGF), West Java, Indonesia. For this purpose,

512

26 water samples and 12 gas samples from production wells, hot springs, and fumaroles

513

were analyzed for major ions, trace elements, oxygen and hydrogen isotopes, and gas

514

components. The main results obtained are summarized as follows:

515

(1) Water samples from the production wells and manifestations were classified into

516

three types, mostly sulfate-bicarbonate type with minor sulfate and sulfate-chloride

517

types. The sulfate-bicarbonate type with high Mg and H2CO3 concentrations was

518

distributed in the northern part of PGF probably formed by mixing of geothermal

519

water with shallow groundwater. Boiling of chloride-rich water from the deep

520

reservoir and condensation of gas near the surface are causes of the sulfate and

521

sulfate-chloride types at Kawah Putih.

522

concentration suggested mixture with HF gas that originated from a magmatic

523

plume under the Kawah Putih crater.

524 525

In addition, the water with a high F

(2) Sulfate condensate water was estimated to form near the surface of the Kawah Putih crater, and its northward flow induced mixing with the surface water.

526

(3) Strong interaction between deep acidic waters and wall rocks probably increased

527

the concentrations of trace elements represented by Al and Fe in the upflow zones

528

and produce surface manifestations with low pH.

529

(4) The PGF water is predominantly meteoric in origin, but an upward δ18O shift

530

(excessive δ18O) was observed at nine sites of sulfate and sulfate-chloride water

531

types. This shift was probably caused by mixing of deep magmatic water with the

532

surface water and water-vapor fractionation in the evaporation process.

31

533

(5) The origin of high N2/Ar was interpreted as the result of organic matter from the

534

subducted Indian–Australian oceanic plate and the continental Eurasian plate. High

535

CO2/H2S ratios could be ascribed to the reaction of H2S gas with the wall rock,

536

whereas low values of this ratio could be attributed to fast steam upflows from the

537

reservoir without occurrence of that reaction.

538

(6) By integrating the analysis results of the water and gas samples, the well

539

temperature data, and surface geology, the volcanic activity under the Kawah Putih

540

was estimated as the heat source in the PGF. These heat source and faults (with NE–

541

SW and NW–SE trends) have essential functions in the formation and evolution of

542

the geothermal system.

543 544

Acknowledgments

545

This study was supported by the Ministry of Energy and Mineral Resources of

546

Indonesia (Grant No. 5960 K/69/MEM/2016) and by Japan Science and Technology

547

(JST) and the Japan International Cooperation Agency (JICA) through the Science and

548

Technology Research Partnership for Sustainable Development (SATREPS, Grant No.

549

JPMJSA1401). Dr. S.S. Rita Susilawati at the Geological Agency of Indonesia is

550

acknowledged for cooperation in this study and helpful discussion, and PT. Geo Dipa

551

Energi (Persero), Indonesia is acknowledged for permission to sample and survey the

552

Patuha geothermal field. Sincere thanks are extended to the three anonymous reviewers

553

for their valuable comments and suggestions that helped us to improve the clarity of the

554

manuscript.

555 556

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Waters from production wells and manifestations were classified into three types. The gasses rich in volatile H2S and CO2 ascend through faults and fractures. Interaction between volcanic water and wall rock is indicated by high Al and Li. Heat source and faults have essential functions in the formation of fluid system. The gases originate from deep regions associated with the subduction mechanism.

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Conflict of Interest:

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The authors declare that they have no competing interests.

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