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

Highlights

• 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.

Abstract

Volcano-hosted, vapor-dominated geothermal systems have great potential for power generation, although to date, such systems discovered globally remain limited in number. Understanding of the physical and chemical properties of geothermal fluids (water and gas) in vapor-dominated systems is critical for the sustainable development of geothermal resources. This study aims to clarify the origins, water–rock interactions, and chemical evolution of geothermal fluids during migration from a reservoir to the surface by selecting the Patuha geothermal field (PGF) in West Java, Indonesia as a case study. The PGF is characterized by a vapor-dominated system that originated from the subduction of the Indian–Australian plate beneath the Eurasian plate. In total, 26 water and 12 gas samples from production wells with 1,424–2,004 m depth, and fumaroles were analyzed for major anions, cations, trace elements, stable isotopes, and gas components to interpret phenomena occurring in deep reservoirs. Ternary diagrams of Cl–SO₄–HCO₃ ionic compositions suggest that the H₂S and CO₂ gases are condensed near the surface and changed to sulfate and bicarbonate by mixing with groundwater. Products of water–wall rock interactions appeared in the area with acidic water, which has mainly leached aluminum, accelerated pyrite oxidation, and increased iron concentration in the water. High fluoride concentration at a fumarole site (95.9 mg/L) implies HF gas supply from the deep-seated magmatic plume that is a geothermal source of the PGF system. Oxygen and hydrogen isotopes reveal that meteoric water is the main source of this system, and Na–K–Mg diagrams indicate immaturity of the reservoir water. Through evaporation and mixing with the magmatic waters, the waters have enriched heavy isotopic values, ascend along major faults towards the surface, and partly discharge at hot springs and fumaroles. High temperatures of the reservoir and gas-source in the subducted Indian–Australian plate are estimated based on the high CO₂ and H₂S concentrations and the high N₂/Ar ratios, respectively. By integrating the analysis results of the water and gas samples, the well temperature data, and surface geology, the volcanic activity under a crater was estimated as the heat source and to have essential functions with the faults in the formation and fluid system of the vapor-dominated PGF.

Introduction

Geothermal heat is a resource of heat energy from the Earth's interior that is reserved both in rocks and in trapped waters and steams (Ellabban et al., 2014). Geothermal heat is a renewable, infinite energy source (Nasruddin et al., 2016) that can provide constant electricity with comparatively little emission of carbon dioxide or other pollutants (Ferrara et al., 2019). Although Indonesia is ranked the third richest country in the world in terms of geothermal resources, with 311 geothermal fields and 28,617 MW of electric potential, only 5.8% of this potential has been exploited as of 2017 (MEMR, 2017). Moreover, with demand for electricity expected to grow 8.5% per year until 2025 with minimization of fossil fuel use, geothermal resources have become critically important in Indonesia (ADB and World Bank, 2015; Fan and Sang, 2018).

Using a vapor-dominated geothermal system should contribute to the large increase in electricity because its power is more significant than that of a liquid-dominated system; a vapor-dominated system yields superheated or dry steam with little or no liquid in its exploitation. In the liquid dominated systems, the produced two-phase geothermal fluid loses a significant amount of heat when separating steam from water, because only the separated steam is used for power generation (Moon and Zarrouk, 2012). This system is unique in that a condensate water layer is formed on a steam body (Schubert et al., 1980; Truesdell and White, 1973) and discharge fluids do not yield reservoir water of neutral chloride at low elevations (Ingebritsen and Sorey, 1988; Raharjo et al., 2016; White et al., 1971). The temperature, water/steam ratio, chemical evolution, and flow system of geothermal fluids are essential factors of a reservoir for assessing the potential for power generation and sustainable use of geothermal resources. However, these factors in vapor-dominated geothermal systems are difficult to estimate because of a lack of Cl-rich waters in geothermal manifestations. Selection of alternative geochemical indicators or development of a method that integrates various geological and geochemical data is necessary to estimate the factors that generate vapor-dominated geothermal systems.

Based on that background, this study aims to clarify the origins, water–rock interactions, and chemical evolution of geothermal fluids during migration from a reservoir to the surface by analyzing water and gas samples from production wells, fumaroles, and hot springs with the initial temperature data of exploration wells. Major and trace elements were first measured to characterize regional water chemistry and flow patterns. Water isotopes (¹⁸O and ²H) were then used to determine the water origin in the geothermal system, following preceding studies that have demonstrated the usefulness of these isotopes (Birkle et al., 2016; Bouchaou et al., 2017; Caron et al., 2008; Dupalová et al., 2012; Marques et al., 2008; Papp and Nitoi, 2006; Purnomo and Pichler, 2014; Xun et al., 2009). The reservoir temperature and boiling and condensation processes were estimated by using well-logging temperature data and the gas compositions of methane, carbon dioxide, and hydrogen as a reference (Giggenbach, 1980; Koike et al., 2014; Lowenstern et al., 2015; Stefánsson, 2017).

The above methods were applied to the Patuha geothermal field (PGF), West Java, Indonesia, because the PGF is associated with a typical vapor-dominated geothermal system that is not yet fully understood (Hanano, 2011). By integrating all the results, a conceptual model of the PGF system was constructed, which will contribute to the exploration and sustainable operation of the reservoir in the vapor-dominated geothermal system and enable better understanding of the system dependence on the local tectonic setting.