Subsurface Geology and Hydrothermal Alteration of The “X

Journal of Applied Geology, vol. 6(1), 2021, pp. 52–61DOI: http://dx.doi.org/10.22146/jag.63302

Subsurface Geology and Hydrothermal Alteration of The “X” GeothermalField, West Java: A Progress Report

Peter Pratistha Utama1, Pri Utami*1, Gayatri Indah Marliyani1, and Randy Wijaya Atmaja2

1Department of Geological Engineering, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta, Indonesia2PT. Y (Persero), Jakarta, Indonesia

ABSTRACT. “X” geothermal field is one of the geothermal fields in West Java. PT. Y(Persero) developed it in 2014. The geothermal field has produced electricity, with aninstalled capacity amounted to 55 MWe. The “X” geothermal system is vapor-dominated.The geothermal manifestations are located at approximately 2,100 m ASL. The “X” fieldconsists of three main upflow zones: Kawah Putih, Kawah Ciwidey, and Kawah Cibuni.This study analyzed the drill cuttings from 3 wells as the primary data with total depthsranging from 1,581 to 2,166 m with the well’s highest stable temperatures measured of±230°C. The three wells selected for this research—Well A, Well B, and Well C—wereanalyzed to describe the rock properties and estimate the prospect areas of present-daygeothermal exploration in the “X” geothermal field.

The paper aims to understand better the subsurface geology and its correlation to the dy-namic processes (i.e., hydrothermal alteration) in the “X” geothermal field. The hydrother-mal minerals are formed by near-neutral pH fluids and are characterized by quartz, calcite,clays (smectite, illite, chlorite), wairakite, epidote, and actinolite. Acidic fluids are evidentby forming acidic hydrothermal minerals, e.g., anhydrite, at various depths of the stud-ied wells, particularly at Well C, located around the Sugihmukti-Urug area. Moreover,the previous studies by Reyes (1990), Layman and Soemarinda (2003), Rachmawati et al.(2016), Elfina (2017) on hydrothermal minerals, geothermal manifestation characteristics,fluid geochemistry, and conceptual model are adapted to improve the analysis and inter-pretation of this paper.

Keywords: Geology · Subsurface · Hydrothermal alteration · “X” geothermal field.

1 INTRODUCTION

The “X” Geothermal Field, a vapor-dominatedfield (Layman et al., 2003; Hochstein and Sudar-man, 2008), is located at ~50 km southwest ofBandung and ~20 km west of Gunung WayangWindu geothermal field (Figure 1). It is lo-cated at an elevation of ~2,000 m ASL. The “X”geothermal work area currently has 31 drillingwells, which includes 17 slim wells. The in-stalled capacity is measured at 55 MWe and PT.Y (Persero) manages the operation since 2014.

*Corresponding author: P. UTAMI, Departmentof Geological Engineering, Universitas GadjahMada. Jl. Grafika 2 Yogyakarta, Indonesia. E-mail:[email protected]

The three wells are worth being studied due totheir being frequently used as the main produc-tion wells. Thus, this study will enrich the in-formation for future exploration in this area.

Despite the potentials mentioned above, El-fina (2017) reports that the “X” field has prob-lems with declining production from 2014 to2017, including the problem with drilling andproduction facilities. Such problems are pre-dicted due to the existence of swelling clay inthe subsurface. The problems are related to thenatural characteristics of the geothermal sys-tem. This paper aims to analyze the interac-tion between hydrothermal fluid and surround-ing rocks, which is predicted to elucidate the

2502-2822/© 2021 The Authors. Open Access and published under the CC-BY license.

http://dx.doi.org/10.22146/jag.63302

mailto:[email protected]

SUBSURFACE GEOLOGY AND HYDROTHERMAL ALTERATION OF THE "X" GEOTHERMAL FIELD

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Geothermal Working Area

Research area (”X” Geothermal Field)

Source :1. Map : Satellite aerial image from Google Maps, 20192. Geothermal working area : Badan Geologi, 2017

FIGURE 1. Location of the "X" Geothermal Field (red square) with other geothermal working areas in WestJava.

cause of the drilling problems. The analysisis based on examining cuttings obtained fromthree wells, e.g., Well A, Well B, and Well C(1,500 to 2,200 m depth). The cuttings fromthese wells were examined using petrographyand X-ray diffractometry (XRD) methods toidentify the alteration minerals. These data arethen used to understand subsurface geologyand hydrothermal minerals.

2 GEOLOGY

The “X” Geothermal Field is located within theBandung Zone, bounded by the Bogor Anti-clinorium Zone in the north and West Java’sSouthern Mountain Range Zone in the south.The main feature of this geothermal field isMt. Patuha, with the peak reaching an eleva-tion of 2,429 m ASL. This mountain is part ofthe NW–SE Quartenary volcanic chain, locatedon the verge of Bandung Zone and West Java’sSouthern Mountain Range Zone (van Bemme-len, 1970). The ongoing tectonism of Java Is-land is characterized by the subduction of theIndian-Australian plates below Eurasian Conti-

nental plates, initiated from the Late Cretaceousuntil Early Tertiary (Hamilton, 1979).

The compressional forces of this period af-fected the structural geology configuration inJava. It then represented in the structural con-figuration as series of structural lineament andvolcanic arc in the NE–SW orientation, namedMeratus trend lineament. The major fault fol-lowing the Meratus trend lineament in the WestJava area is the Cimandiri fault (Purnomo &Pichler, 2014). Along with other lineamentorientation formed by tectonic evolution sinceLate Cretaceous to Quarternary, such as N–SSunda lineament and E-W Java lineament, Mer-atus lineament formed the main structural con-figuration of Java, especially West Java (Pulung-gono and Martodjojo, 1994 and Satyana, 2007 inFauzi et al., 2015).

Volcanoes in the “X” geothermal field aremostly monogenetic volcanoes scattered alongthe NW–SE trend line. Previous studies (An-derson, 1951; Cas and Wright, 2012; Watan-abe et al., 1999; Marliyani et al., 2020) reportedhow the stress orientation is perpendicular to

Journal of Applied Geology 53

UTAMA et al.

the lineament of monogenetic volcanoes. Themagma upwelling through vertical fractures tothe ground is an indication of a monogeneticvolcano forming process.

The “X” field is composed of lava domesand volcanic calderas. According to radiomet-ric dating by Fauzi et al. (1994) in Layman andSoemarinda (2003), the geological age of rocksin the “X” field are ranging from 0.12–1.25 mil-lion years. There are several quartenary mono-genetic volcanoes in the “X” geothermal areasuch as Urug, Sumbul, Puncaklawang, Pun-cakkopsi, Patuha Utara, Patuha Selatan, Tilu,Masigit, Tambakruyung, and Tikukur. Numer-ous volcanic centers or vents are differentiatedby the degree of dissection based on remotesensing analysis of aerial photographs. The de-gree of dissection by erosion might correlate tothe age of volcanic processes.

The oldest volcano in the “X” field isKendeng in the north-northwest. It was ac-tive during the Early Pleistocene. Later, theother younger volcanoes, like Patuha Tua inthe center part of the field and Masigit, Tilu,and Tikukur in the north part, were active.The last phase of volcanic activity is markedby Sugihmukti, Urug, and Patuha Muda fromthe central part to the southeast part of thefield. The volcanic centers, along with NW–SE trending, are associated with geothermalmanifestations. The primary geothermal man-ifestation is fumarolic steam discharges, suchas Kawah Putih, Kawah Ciwidey, and KawahCibuni. The geological map showing the dis-tribution of lithological units and volcanoeswithin the “X” geothermal field and the loca-tion of manifestations and geological structuresis presented in Figure 2.

3 THERMAL MANIFESTATION

Thermal manifestations on the ground can befound at ~2,100 m ASL. Thermal manifesta-tions are associated with the NW–SE trendingvolcanic centers and are perpendicular withregional horizontal principal stress. The fu-maroles and solfataras dominated them with asignificant amount of sulfur deposit, hot andwarm thermal springs with near-neutral andacidic fluid, gas vent, mud pools, mud pots,steaming grounds, and altered grounds. Thesurface alteration minerals include smectite, il-

lite, chlorite, kaolinite, halloysite, and a sparseamount of cristobalite (Pusat Penelitian PanasBumi FT UGM, 2020, unpublished). Near-neutral pH fluid can be found in numerous hotand warm thermal springs, such as Rancaupas,Cimanggu, Cibunggaok, and Punceling in thenorthern and center part of the field. Acidicfluid is mainly found in primary fumaroles andcrater lakes in Kawah Putih, Kawah Cibuni,and Kawah Ciwidey.

4 MATERIALS AND METHODS

Sixty wellbore cutting samples were collectedfrom three wells, selected from the total of31 wells drilled in the “X” geothermal field.The depth of selected wells ranges from 1,500–2,200 meters. First, the samples were inspectedimmediately using a binocular microscope toidentify any alteration products. The sampleswere then processed and examined using pet-rographic and x-ray diffraction analysis. Thepetrographic analysis was performed to deter-mine the lithological composition of the cut-ting samples in detail, including the primaryand secondary mineral assemblage, the texture,and the style of alteration. The x-ray diffrac-tion is used to analyze the mineralogy composi-tion. The analysis was interpreted from the sta-ble well temperature data to build the present-day thermal data model.

5 RESULTS AND DISCUSSION

This study is combined its data from the surfaceand the subsurface geological data from previ-ous studies, i.e., Reyes (1990), Layman and Soe-marinda (2003), Rachmawati et al. (2016), andElfina (2017), to develop a better understandingof the geology of the “X” field. Layman andSoemarinda (2003) suggest that the main vol-canic centers in the “X” field are Kawah Putih,Kawah Ciwidey, and Kawah Cibuni. The mainupflow zone is located on Kawah Putih. Threewells, i.e., Well A, Well B, and Well C, have beenchosen to be studied to understand better thegeological condition of the interest area, the dy-namic of the hydrothermal processes, and theexploration and production’s main problems.

5.1 StratigraphyThe detailed surface geology of the “X” fieldconsists of 25 units of volcanic products. Be-

54 Journal of Applied Geology


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sides the primary data, the interpretation ofthis research is influenced by previous studiesfrom Kusdji (2013) and unpublished researchby Pusat Penelitian Panas Bumi FT UGM (2020).The previous studies divided the stratigraphicdivision of the subsurface rocks into Quar-ternary volcanic products of Puncakkopsi, Pun-caklawang, Kendeng, Tikukur, Patuha, Sugih-mukti, Urug, and Malabar. The subsurfacestratigraphic correlation was interpreted basedon the characteristics and spatial association ofthe rocks with the volcanic products.

Based on the examination results of the pri-mary data and previous studies, the subsurfacestratigraphy of the “X” field, in general, wascomposed of Quarternary andesitic - daciticlava, pyroclastic rocks, and diorite. Based onthe petrographic analysis, the subsurface rocksare categorized into three groups: andesiticlava, dacitic lava, pyroxene andesitic lava, anddiorite. The andesitic lava consists of plagio-clase and pyroxene in a groundmass of micro-crystalline plagioclase and glass. These rockunits are considered to be part of the Puncak-lawang products. The Puncaklawang productsare interpreted as the lowermost volcanic se-quence found in the wells, also found at the bot-tom of Well D and Well E, located at the south-eastern part of the research area.

The Quarternary dacitic lava with pyroclas-tic rocks found from three wells is consideredpart of Patuha Tua and Urug products. ThePatuha Tua products are the most widespreadvolcanic sequence found in the wells. The prod-ucts can be found at the center and the south-ern part of the research area. The dacitic lavaconsists of plagioclase, quartz, and pyroxene asphenocrysts. The phenocrysts are embeddedin a groundmass of microcrystalline crystals ofsimilar mineralogy and minor volcanic glass.The dacitic lava is found in large thickness inWell A, Well B, Well C. The dacitic lava thinsout to the southeast, as observed in Well D andWell E. The pyroxene andesitic lava consists ofplagioclase, pyroxene, hornblende, and sparseamounts of biotite in a groundmass of micro-crystalline plagioclase, pyroxene, and clay min-erals. The pyroxene andesitic lava is consideredto be part of Sugihmukti 1 product. This pyrox-ene andesitic lava can be found mainly in WellB and Well C in the shallow or upperparts.

Meanwhile, the diorite was identified inwells Well C and Well F, at about 1,000 depth.Based on petrography analysis, diorite consistsof primary minerals, e.g., plagioclase, quartz,and sparse amounts of pyroxene and horn-blende, embedded in microcrystalline crystalsof similar mineralogy. Figure 3 and 4 showa cross-section that shows the distribution oflithological units and isotherms of present-daywell stable temperatures.

5.2 Geological structuresThe evidence of the geological structures onthe surface was difficult to recognize becauseof the high intensity of hydrothermal alterationand diagenesis of the lithology. Most of theregion has been converted into tea plantationsand has reduced the amount of rock exposurein the area. This research relied on its geo-logical structure understanding from previousstudies, in addition to the limited remotely- andfield-based mapping with the IFSAR-DEM asthe base map.

Our mapping indicated the occurrence ofNE-SW and NW–SE trending faults (presentedin Figure 2). These faults were recognizedthrough outcrop observation and were sup-ported by the alignment of volcanic centers andthe distribution of the geothermal manifesta-tion zones related to the regional structures.

Many active geothermal manifestation zonesare associated with volcanic centers, i.e., KawahPutih (associated with Patuha Utara), Sugih-mukti (associated with Urug), Patengan, andKawah Ciwidey. The NE–SW and the NW–SE trending faults were connected to the sub-surface, as indicated by some losses of circu-lation in drilling wells. Some losses of circu-lation in drilling wells were found at 1150 mASL, and 1115 m ASL in Well A, 1055 m ASLand 750 m ASL in Well B, and 1135 m ASL inWell C. These faults were further interpretedas permeability providers in the “X” geother-mal system, proven by present thermal activ-ity in geothermal manifestation areas. On theother hand, the volcanic structures interpretedas crater rims are associated with Mt. PatuhaUtara and Kawah Putih (presented in Figure 2).



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ineralsand

theisotherm

sofpresent-day

temperatures

(°C).



5.3 Subsurface hydrothermal alterationThere are three alteration styles in the subsur-face rocks in the “X” field, namely direct depo-sition, replacement, and leaching. Replacementand leaching indicate interactions between hostrock and hydrothermal fluid. Leaching im-plicates the displacement of primary mineralswithout substituting them. Leaching is iden-tified in Well A, Well B, dan Well C at vary-ing depth, at about 1600 m ASL, 1500 m ASL.,750 m ASL, and 500 m ASL (presented in Fig-ure 5). Replacement implicates mass exchangebetween primary minerals and hydrothermalfluid. Hydrothermal minerals in the vein orveinlets were deposited from the fluid thatflows through open spaces, i.e., fractures andcavities. Subsurface rocks have been alteredwith alteration intensity from 0,1 to 1. Pyroclas-tic rocks are more altered than lavas and dior-ites.

The dynamics of thermal conditions in thesystem are reflected in the distribution of hy-drothermal minerals. The past thermal struc-ture of the “X” geothermal system is con-cluded from geothermometer minerals in thefield. The hydrothermal minerals found instudied wells include carbonate (calcite), sil-ica (quartz), calc-silicate (epidote, actinolite,prehnite), clays (smectite, illite, chlorite), andzeolite (wairakite). These minerals were formedfrom near-neutral pH fluid. Nevertheless, theappearance of anhydrite at the varying depthindicated the presence of acidic fluids. The dis-tribution of hydrothermal minerals and theirrelative depth is presented in Table 1.

The geothermometer minerals found were il-lite, wairakite, epidote, and actinolite. The oc-currence of epidote is the shallowest in WellC, i.e., 1315 m ASL, and deeper in other wells.In Well A and Well B, located in the Sugih-mukti area, epidote occurs about 1200 m ASL.The formation of epidote indicates that the tem-perature is ≥ 250°C (Reyes, 1990). The stabledownhole temperature at this depth is less than200°C. It shows that the systems seem to expe-rience cooling. Actinolite was found at about810 m ASL in Well A and found at 1085 m ASLin Well C. The formation of actinolite indicatesthat the temperature is ≥280°C (Reyes, 1990).

TABLE 1. The relative distribution of subsurface hy-drothermal minerals with depth.

Relativedepths Mineralogy Well A Well B Well C

R S R S R S

Shallow

Calcite v v v v v vQuartz v v vAdularia vPyrite D DSmectite v v vKaolinite v vChlorite v v v v vHematite v v vAnhydrite v

Interme-diate

Calcite v v v v v vQuartz v v v v v vChlorite v v v v vEpidote v v v v vActinolite vAdularia v vAnhydrite v v v vPyrite D D DHematite v vSmectite v vKaolinite vHalloysite vIllite v v v

Deep

Calcite v v v v v vQuartz v v v v v vChlorite v v vEpidote v v v v v vActinolite v v vWairakite v v vPrehnite v v vAnhydrite v v v vAdularia v v vPyrite D D DHematite v v v vCristobalite v vNatrolite vZoisite v

*R = Replacement, S = Space-fill, D = Dispersed


UTAMA et al.

Well

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The occurrence of actinolite in Well C is associ-ated with the existence of a diorite intrusion.

Meanwhile, anhydrite and leached rocksfound in the studied wells at varying depthsindicate that the acidic fluids were circulatingdynamically within the system. The cross-section that shows some hydrothermal miner-als as geothermometers and the occurrence ofanhydrite, sulphur, and leached rocks in thestudied wells are presented in Figure 4 and 5.

6 CONCLUSION

The subsurface stratigraphy of the “X” geother-mal field comprises andesitic – dacitic lava andpyroclastic rocks. At some point, diorite in-trusion occurred beneath Mt. Sugihmukti andMt. Urug, around Well C and Well F well area.The existence of hydrothermal minerals indi-cates that hydrothermal fluid was near-neutralpH, but there has been some acidic fluid circu-lating in the system in some parts of the sys-tem. This acidic fluid formed anhydrite and na-tive sulphur and caused leaching processes inthe system, especially in studied wells.

The present-day stable temperatures (asrepresented by isothermal lines in Figure 4)indicate that the main thermal areas in the“X” geothermal system are located in KawahPutih, Kawah Ciwidey, and Kawah Cibuni.Sugihmukti-1 manifestation area is associatedwith Sugihmukti and Urug Mt and is recog-nized as a minor thermal area. Hydrothermalminerals, which are considered geothermome-ter, reveal that the “X” geothermal systemexperiences cooling, based on the comparisonof present-day stable well temperature and theoccurrence of geothermometer minerals.

ACKNOWLEDGEMENTS

The first author undertook this research to ful-fill the requirements to attain his Master-degreestudy at Universitas Gadjah Mada, under thesupervision of the second and third author. ThePT. Y (Persero) provided data. The authorswant to express their gratitude for the approvaland data assistance by PT. Y (Persero).

REFERENCES

Bird, D.K., Schiffman, P., Elders, W.A., Williams,A.E., McDowell, S.D. (1983) Calc-silicate Mineral-ization in Active Geothermal Systems. California.Stanford University: Departement of Geology.

Elfina (2017) Updated Conceptual Model of ThePatuha Geothermal Field, Indonesia. Iceland:United Nations University–Geothermal TrainingProgramme, p. 89-112.

Fauzi, A., Permana, H., Indarto, S., Gaffar, E.Z.(2015) Regional Structure Control on GeothermalSystems in West Java, Indonesia. ProceedingsWorld Geothermal Congress 2015, Melbourne,Australia, April 2015.

Hochstein, M.P., Sudarman, S. (2015) IndonesianVolcanic Geothermal Systems. ProceedingsWorld Geothermal Congress 2015. Melbourne,Australia, April 2015.

Layman, E.B., Soemarinda, S. (2003) The PatuhaVapor-Dominated Resource West Java, Indone-sia. Proceedings, 28th Workshop on GeothermalReservoir Engineering 2003, Stanford, California.

Reyes, A.G. (1990) Petrology of Philippine geother-mal systems and the application of alteration min-eralogy to their assessment. Journal of Volcanol-ogy and Geothermal Research 43 (1990), p. 279-309, Amsterdam: Elsevier Science Publishers.

Schotanus, M. (2013) The Patuha Panas Bumi Sys-tem: A Numerical Model Of A Vapor-DominatedSystem [unpublished M.Sc. Thesis], UniversiteitUtrecht, 86 p.


Documents

Subsurface Geology and Hydrothermal Alteration of The “X