Overview of the Wayang Windu Geothermal Field, West Java, Indonesia

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Geothermics 37 (2008) 347–365

Overview of the Wayang Windu geothermal field, West Java, Indonesia Ian Bogie a,∗ , Yudi Indra Kusumah b , Merry C. Wisnandary b a b

Sinclair, Knight Merz Ltd., PO Box 9806, Newmarket, Auckland, New Zealand Magma Nusantara Ltd., Wisma Mulia 50th Floor, Jl. Jend. Gatot Subroto no. 42, Jakarta 12710, Indonesia Received 25 March 2008; accepted 25 March 2008 Available online 16 May 2008

Abstract The Wayang Windu geothermal field, West Java, Indonesia, is interpreted to be transitional between vapour-dominated and liquid-dominated conditions with four coalesced fluid upwelling centres that generally become younger and more liquid-dominated towards the south. Two of these centres are associated with the large Gunung Malabar andesite stratovolcano and the other two with the smaller aligned Gunung Wayang and Gunung Windu andesitic volcanoes to the south. The overall potential resource area is of the order of 40 km2 . Deep wells encounter a deep liquid reservoir whose top, which ranges from 0 to 400 m above sea level (m asl) becomes progressively deeper toward the south. As pressure versus elevation conditions are the same throughout the deep liquid reservoir it is likely to be contiguous. This liquid-dominated reservoir is overlain by three separate vapour-dominated reservoirs. The northernmost is the largest as it is coalesced over two separate fluid upwelling centres. Its low gas content, size, prolonged productivity and isobaric for elevation nature, preclude it from being a parasitic steam zone. Mineralogical relationships demonstrate that this vapour zone was originally liquid-dominated with a deep water level as high as 1700 m asl. Subsequent boil off may reflect low recharge rates due to hydrological isolation at depth. To the south, the vapour-dominated reservoirs decrease in thickness and are characterized by progressively higher pressures, temperatures and gas contents. These changes suggest that the southernmost vapour-dominated zone is the youngest and that these zones become increasing older to the north. © 2008 Elsevier Ltd. All rights reserved. Keywords: Vapour dominated; Liquid dominated; Wayang Windu geothermal field; Java; Indonesia

∗

Corresponding author. Tel.: +64 9 913 8900; fax: +64 9 913 8901. E-mail address: [email protected] (I. Bogie).

0375-6505/$30.00 © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.geothermics.2008.03.004

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I. Bogie et al. / Geothermics 37 (2008) 347–365

1. Introduction—regional geothermal setting The Wayang Windu geothermal field is located approximately 35 km south of Bandung, the provincial capital of West Java, Indonesia (Fig. 1). It is one of a cluster of geothermal fields around Bandung that also includes Darajat (Hadi et al., 2005), Kamojang (Utami, 2000), Karaha-Telaga Bodas (Moore et al., 2002, 2004), Papandayan (Wibowo, 2006), Patuha (Layman and Soemarinda, 2003), Tampomas (Wibowo, 2006) and Tangkuban Perahu (Wibowo, 2006). These fields lie within andesitic, volcanic highlands formed by a concentration of volcanic centres in this part of the Sunda Arc. The city of Bandung is located in a basin (Dam, 1994) near the centre of the highlands. That basin does not appear to be a back-arc basin, as it has arc volcanics on either side, but may owe its origin to flexure from varying rates of subduction roll back along the Sunda Arc. This arc has formed in response to the subduction of the Australian–Indian Plate beneath the Eurasian Plate. It has been active since the Cretaceous (Whittaker et al., 2007), but has undergone changes as increasing amounts of Australian continental crust have become involved in the collision and it is undergoing roll back. The dominant strike directions of the major faults in West Java are 40◦ and 340◦ (Wibowo, 2006; Fig. 1), forming a conjugate pair of strike-slip faults, consistent with compression due to near-perpendicular subduction. 2. Geothermal exploration Initial exploration of Wayang Windu was undertaken by Pertamina (Sudarman et al., 1986). It included sampling and analysis of thermal springs, DC resistivity (Schlumberger array) traversing,

Fig. 1. The distribution of Quaternary volcanic rocks and high-temperature geothermal fields in West Java, Indonesia.


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Fig. 2. Topographic map of the drilled portion of the Wayang Windu geothermal field; contour interval: 100 m; bold contour: 2000 m asl. Also shown are the locations of drill pads, well tracks and of sections A and B described in Figs. 4 and 6.

‘head-on’ resistivity profiling, magnetotelluric (MT) gravity and soil geochemistry surveys, as well as the drilling of temperature gradient holes. The first deep hole (then called WWD-1, now WWA-1ST, after being side-tracked to the west, Fig. 2) was drilled by Pertamina just to the west of the saddle between Gunung Wayang1 and 1

The abbreviation G. will be used for ‘Gunung’ (mountain) from here on.

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G. Windu in 1991 (Budiardjo, 1992; Ganda and Hantono, 1992; Ganda et al., 1992). Well data showed that a perched steam-heated groundwater aquifer overlies a two-phase vapour-dominated zone that in turn overlies a neutral-Cl liquid-dominated reservoir. This was the discovery well for the Wayang Windu field and for transitional liquid–vapour type geothermal systems. A 600-m deep slim hole (MSH-1) drilled by Pertamina in 1993–1994 on the southern slopes of G. Malabar also provided indications of the existence of a shallow two-phase zone, overlain by a steam-heated perched aquifer further to the north. 2.1. Thermal manifestations and surficial hydrothermal alteration The most intense surficial hydrothermal activity occurs adjacent to the small volcanic centres, G. Wayang and G. Windu (Fig. 3). Fumaroles, steaming and altered ground, and acid–sulfate springs occur in the Wayang thermal area. The Wayang thermal area lies within a sector collapse,

Fig. 3. Location of geothermal wells, thermal features, volcanic summits, calderas and sector collapses in the Wayang Windu geothermal field in relation to the base of the conductor.


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with the current peak of G. Wayang representing an eastern remnant of a much larger volcanic centre, originally located to the west along the axis of alignment of other small volcanic centres. The matrix of the debris flow from the sector collapse consists of hydrothermal clay, suggesting slope failure was related to the weakening of the volcanic deposits by hydrothermal alteration or the result of the increase of pore pressures with heating (Reid, 2004). A radiocarbon date of 7450 ± 110 years was obtained from peat from a small swamp that had developed on top of the debris flow deposits at drill pad WWD. This would represent a minimum age for the sector collapse itself. Smaller areas of altered ground with acid–sulfate springs and weak fumarolic activity are found on the southern slopes of G. Malabar, while warm, neutral-bicarbonate-sulfate springs are present south of G. Malabar and to the south, west and east of the smaller volcanic centres (Fig. 3). The springs have temperatures ranging from 25 to 66 ◦ C and are notable for their lack of Cl (Sudarman et al., 1986). The Cibolang spring in the south has an elevated B content (16 ppm). As its other constituents indicate that it discharges steam-heated ground water, the high B content is suggestive of high-temperature boiling at depth, because B is volatile at high temperatures (Ellis and Mahon, 1977). The northernmost area of hydrothermal alteration is exposed on the southwest rim of the Malabar Caldera complex and there is strong alteration in the cirque of the Wayang sector collapse. Small patches of altered ground are scattered around the area, although the overall extent of the hydrothermal alteration is only apparent where deep cuts have been made for roads and drill pads as much of the hydrothermal alteration is covered by a sequence of young unaltered ash beds. A paleosol between the overlying unaltered ash beds and the underlying hydrothermally altered deposits in the vicinity of the WWQ pad has yielded a radiocarbon age of 8700 ± 90 years BP. This represents a minimum age for the underlying alteration. Thus, only the alteration found in the immediate vicinity of thermal features can clearly be regarded as current. The northernmost known thermal activity in the field occurs at a break in slope southeast of Puncak Besar (Figs. 2 and 3). There are no known thermal manifestations on the highest parts and northern slopes of G. Malabar despite the evidence from the MT surveys for the field to extend beneath these areas. As the prevailing weather during the rainy season is from the north, a “rain curtain” may be obscuring thermal activity where precipitation rates are highest. 3. Field development The Wayang Windu field was developed by MNL (Magma Nusantara Ltd.) beginning in 1996 as a fast track development that started in the logistically easier areas in the south and east with a combination of 1500-m deep slim holes (WWC-SH, WWJ-SH, WWL-SH and WWR-SH; Nurruhliati, 1996; Thaysa, 2003) and deeper production drilling. The existence of a large thermal anomaly was established, but productive wells were restricted to sites immediately beneath and southwest of the young volcanic centres. Wells drilled north of G. Bedil encountered a shallower, two-phase, vapour-dominated reservoir than the one found in the first well (WWA-1). These wells confirmed the results from slim hole MSH-1 in the north that indicated a vapour-dominated regime overlies a deep liquid-dominated reservoir. Thus, a combination of deep and shallow wells was drilled; success was greatest with the shallow wells. Three dry-steam wells with depths of up to 1700 m, each produced steam equivalent to over 20 MWe.2 These wells were drilled on the northernmost (at the time) MBD 2 All MWe values given are in terms of the power conversion capacity of the current Wayang Windu power plant (i.e. 1.94 kg/s of steam per MWe).

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pad. In terms of resource potential, an initial 220 MWe (gross) development was planned with a possible extension to 440 MWe (gross), until the Asian financial crisis of 1997 intervened and the project was scaled back to 110 MWe (gross). The initial 110 MWe (gross) development obtained its main steam supply from the northern two-phase reservoir, with some deep northern production and a combination of shallow and deep production from wells further to the south, on the WWA pad upon which the Pertamina discovery well was located. Two-phase fluid transmission pipelines from these drill pads feed a central separator station with steam passing through a scrubber before entering the dual inlet, 110 MWe Fuji turbine in the power plant. This unit, which was installed in 1999, is one of the world’s largest operating geothermal turbines (Murakami et al., 2000). As a result of this turbine installation, Wayang Windu holds the distinction of being the most rapidly developed geothermal field of its size. Both condensate and separated brine are reinjected by gravity in the southernmost part of the known resource. Unocal Indonesia became a 50% shareholder of the Wayang–Windu project in 2001. The poorly productive deep MBD-1 vertical well was side-tracked to intersect the shallow, vapourdominated zone and came to be the largest producer in the field at the time. That was followed by work-overs of other wells on the MBD pad that had reduced production due to the installation of tie backs (whereby the production casing shoe is cased to the surface reducing the diameter of the cased section of the well). WWQ-3, another deep well, was also side-tracked but with less success than MBD-1. Reservoir pressure drawdown has reduced fluid production with time, but it has now stabilized in wells tapping the vapour-dominated zones, indicating that they are major, sustainable productive reservoirs rather than limited parasitic ones. The field was acquired by Star Energy Holdings Pty Ltd. in 2004. A new drilling program began in August 2006 to supply steam for a second 110 MWe Fuji turbine at the existing power plant. The first well completed under this program (MBD-5) produces the steam equivalent of 40 MWe making it at the time it was drilled the largest dry steam well in the world. The whole eight-well program realized a total of 180 MWe and included make up wells for the existing turbine. The northern extension of the field is currently being explored with the ultimate goal of obtaining steam to generate 440 MWe (gross), which on the results of modeling studies of the field (Asrizal et al., 2006) is eminently achievable. 3.1. Extent of the Wayang Windu geothermal field Only the southern part of the western boundary of the Wayang Windu geothermal field is well defined by drilling (Figs. 2 and 3). Anderson et al. (1999, 2000) found a good relationship between the contoured elevation of the base of the conductive layer (i.e. the “conductor”) produced by hydrothermal smectite, as determined by MT surveys and the temperature distribution in the drilled portion of the field. Thus, the continuation of elevated portions of the conductor outside the drilled area can be considered indicative of the northern extent of the field (Fig. 3). Southwest of G. Windu, productive wells have been drilled on the basis of earlier Schlumberger and MT ressitivity surveys, whereas later re-interpretation of new MT data indicated a deep base of the conductor with no doming or ridging. Since there are indications that the southern area is the youngest part of the system (see below), the position of the conductor is likely to have been dictated by earlier geothermal activity when the deep liquid reservoir was higher. The conductor in the south now appears to be too impermeable for the alteration mineralogy to re-equilibrate and allow formation of a dome or ridge in its base, although the resistivity in the area of productive wells southwest of G. Windu is slightly higher than where the non-productive WWE-1 and WWA-1ST were drilled.


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Combining new well data with recent MT surveys, which have a greater station density and have yielded better quality data, the bulk of the field is interpreted to lie beneath G. Malabar, the andesite stratovolcano now centred at Puncak Besar (Fig. 3). The potential resource in the north is estimated to be approximately 4 km wide in an E–W direction and to extend approximately 14 km to the south beneath a series of aligned, small volcanic centres, where it narrows down to approximately 2 km across. This gives an overall potential resource area of approximately 40 km2 . However, since slim hole drilling outside of the resource area encountered elevated temperatures at depth, the actual geothermal system could be much larger. The field owes its size to the presence of more than one fluid upwelling centre, as discussed below. This feature is found in other geothermal fields in Java. Layman et al. (2002) recognize three centres at Dieng, multiple centres (referred to as cells) are suggested for Karaha-Telega Bodas (Nemˇcok et al., 2007) and two geothermal centres are recognized at Awibengkok (Hulen and Anderson, 1998). 4. Geology of the field The stratigraphy of Wayang Windu has been discussed by Bogie and Mackenzie (1998) who applied volcanic facies models to subdivide the various volcanic units at depth. These units define a series of overlapping andesitic piles. Their cross section has been extended in Fig. 4, with the trace of the section shown in Fig. 2. Microdiorite, dolerite and diorite porphyry dykes are found, but blind drilling and very limited coring have prevented the clear recognition of any major intrusives. Andesitic lavas, pyroclastic and epiclastic deposits predominate in the volcanic units with dacite only occurring at G. Gambung. Quartz found in rocks of the other smaller volcanic centres is xenocrystic, and geochemically these rocks are andesites. Ash deposits of regional extent occur throughout these volcanic piles. Similar beds are also found at Patuha (Layman and Soemarinda, 2003) and Awibengkok (Hulen and Anderson, 1998), where they are referred to as paleosols. At Wayang Windu there are 14 different beds with thicknesses varying between 5 and 30 m that can be correlated between wells, and numerous thinner ones with solitary occurrences. These beds have a complex distribution suggesting draping over the existing topography with erosion in steeper areas and ponding when deposited in valleys.

Fig. 4. Section across the Wayang Windu geothermal field showing well tracks, geological units (extended north and south from that of Bogie and Mackenzie, 1998) and the top of epidote (section location is given in Fig. 2).

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Scanning electron microscopy and XRD analyses indicates that they originally consisted of very fine-grained glass shards and titanomagnetite, but the glass has altered to calcium smectite and in places interlayered smectite-illite. As these clays are usually found at temperatures of less than 200 ◦ C (Anderson et al., 2000), we suggest the beds must have very low permeability, since measured temperatures at the corresponding depths (>300 ◦ C in some instances) are much greater than the typical stability limit of the clay. G. Malabar sits on the boundary fault of the Bandung basin (Figs. 1 and 3; Dam, 1994) There is a multiphase summit caldera complex on G. Malabar. Rocks from the summit of G. Malabar to the east of the calderas, Puncak Besar – a prominent peak south of the caldera complex directly above the Bandung Basin boundary fault – and G. Gambung – a parasitic dacite dome to the southeast – all have K-Ar dates of 0.23 Ma and have bulk and trace element chemistries of a differentiated series (Bogie and Mackenzie, 1998). G. Bedil, the next volcanic centre to the south, was dated at 0.19 Ma and G. Windu, the southernmost volcanic centre at 0.10 Ma. The 0.49 Ma date for G. Wayang breaks the trend of having younger centres towards the south. While the other young volcanic centres are well preserved and samples for dating were obtained from the youngest part of the volcano, G. Wayang has undergone sector collapse, and it is likely that the dated sample from that eruptive centre was taken from a much older part of the volcanic pile. The geochemistry of the younger volcanic centres is variable, although G. Windu has some trace element similarities to G. Malabar. As these rocks contain quartz xenocrysts and diorite xenoliths, the chemical variation may be reflecting the degree of crustal assimilation by an original magma similar to that producing the Malabar rocks. A more extensive, but less intensive geochemical study of samples of older units collected in the wells concluded that they are geochemically similar to the Malabar rocks (Asrizal et al., 2006). Structurally the field conforms best to regional patterns in the south, with faulting exhibiting steep dips (>80◦ ) and strikes of 30–40◦ and 330–340◦ . In the north, along the southern boundary of the Bandung Basin, further deformation results from movement along a boundary fault. G. Malabar is actively subsiding into the basin and is deforming the basin fill, as can be seen by the presence of upthrust Tertiary sediments (Alzwar et al., 1992) as northern foothills to G. Malabar. The wells have highly localized structural permeability, with the most permeable geologic structures following the regional trend of 40◦ . As these structures have trends similar to regional faults, then it is likely they are strike-slip faults. Strike-slip faults tend to have lower permeabilities than normal faults because of shearing and consequent rock comminution. Structures that were reactivated as the volcanic sequence was deposited will display less shearing at their upper ends than their extensions into the basement. Thus, the same faults may have lower permeabilities at depth due to prolonged shearing and comminution of the rock compared to the younger overlying volcanic pile; at least in competent lithologies. Bandyopadhay et al. (2006), however, calculated the direction of least principal stress in the Wayang Windu area as far north as the WWQ pad, utilizing borehole breakouts (the tendency for drill hole cross sections to become elongated in relationship to the local stress field) as determined from the caliper measurements obtained from micro-resistivity formation imaging logging. They found that the calculated stress field did not correspond to the regional orientation, but had the least principal stress striking at 310◦ , with an overall normal faulting regime. The NE-striking faults are thus likely to have been regional strike-slip faults reactivated as more permeable normal faults due to a change from a regional compressive to a local extensional regime. Further to the north, extension may be even stronger at the boundary and inside of the Bandung Basin.


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Table 1 Alteration mineralogy (possible relict phases shown in italics) Location

Initial alteration

Above conductor

Opal, cristobalite, kaolinite, alunite, natro-alunite and sulfur Smectite, illite-smectite, quartz, chlorite, albite, calcite, pyrite, heulandite, mordenite, clinoptlolite, stilbite, analcime, laumonite Quartz, chlorite, calcite, albite, pyrite, illite-smectite, corrensite, epidote, illite, chalcedony, wairakite Amphibole Pyrophyllite, diaspore, quartz, anhydrite

Conductor

Vapour-dominated reservoirs

Deep liquid reservoir

Over print

Kaolinite, anhydrite, calcite, quartz Anhydrite, calcite, pyrite

Quartz, chlorite, illite, pyrite, wairakite, epidote, prehnite, adularia, albite, tourmaline Dickite, pyrophyllite, quartz, woodhouseite, pyrite Amphibole, orthoclase, magnetite Diopside, oligoclase, magnetite

4.1. Hydrothermal alteration at depth Hydrothermal alteration (Table 1) at depth is most strongly developed in the pyroclastic deposits with more structurally limited alteration zones in the lava flows. Shallow alteration (above and locally within the conductor), is marked by the presence of kaolinite, alunite, natroalunite and rare native sulfur. This alteration is associated with perched steam-heated groundwater aquifers mainly in the vicinity of updoming in the base of the conductor, pressure profiles from slim holes drilled to depths of up to 1500 m that did not penetrate into the deep reservoir (Fig. 5) indicate perched aquifers are widespread and probably continuous. As the warm springs discharge at elevations similar to those of perched steam-heated aquifers, it is likely that they are fed by them. Possible condensate aquifers, not connected to the regional ground waters, are indicated by high porosities measured by using a down hole magnetic resonance imaging logging tool and these are characterized by strong alteration to kaolinite, calcite, quartz and anhydrite. The conductor itself is made up mainly of an argillic assemblage dominated by smectite along with near ubiquitous quartz, chlorite, calcite and pyrite with zeolites, including heulandite, mordenite, clinoptilolite, stilbite, analcime and laumontite. Kaolinite, calcite, anhydrite and quartz are found within parts of the conductor occurring as an overprint. With increasing depth, interlayered illite-smectite rather than smectite is found, until there is a transition to a propylitic assemblage with its top marked by the presence of corrensite and epidote. At greater depths, illite becomes the main sheet silicate. Secondary amphibole, orthoclase and magnetite making up a high-temperature potassic assemblage are encountered at still greater depths. The formation of secondary amphibole appears to be related to dike emplacement. A contact metamorphic assemblage of diopside, oligoclase and magnetite has been observed in WWA-4. In the deep liquid reservoir, wairakite and prehnite, along with epidote, are common as alteration and vein minerals, with less common adularia. Other than a generally prograde transition, with illite-smectite and corrensite detected shallow in the vapour-dominated reservoirs, and prehnite in the lower parts of the deep liquid reservoir,

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Fig. 5. Graph of pre-production pressures in main feed zones of Wayang Windu geothermal wells versus elevation. Note that the casing in well WWE-1 has a hole allowing the inflow of water from a shallow perched aquifer.

there is no clear difference in the hydrothermal alteration of the liquid- and vapour-dominated reservoirs. Wairakite, however, is common in the vapour-dominated reservoir, as has also been noted at Karaha-Telega Bodas (Moore et al., 2002). Rare occurrences of advanced argillic alteration are found below the conductive cap, both in the vapour-dominated and the deep liquid reservoirs. Minerals of this assemblage include pyrophyllite, diaspore, woodhouseite and dickite, with accompanying quartz, anhydrite and pyrite. As these hydrothermal minerals form under high-temperature, acid conditions and considering that the


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Fig. 6. Section across the Wayang Windu geothermal field showing well tracks, isotherms and the known location of the tops of the vapour-dominated and deep liquid reservoirs (section location is given in Fig. 2).

reservoir pH is now near neutral, and has temperatures below those at which these minerals are formed (Reyes et al., 1993), we consider this deep advanced argillic alteration to be relict. It may possibly reflect the earlier presence of acidic condensed magmatic volatiles, particularly since woodhouseite (CaAl3 PO4 SO4 (OH)6 ; Stoffregren and Alpers, 1987) has an exclusively magmatic association (Bogie and Lawless, 2000). In the northern part of the Wayang Windu geothermal field, hydrothermal epidote is found at elevations of up to 1330 m above sea level (m asl). The shallowest appearance of this mineral is above the vapour-dominated reservoir, and all its first occurrences in well samples are above the deep liquid reservoir (Figs. 4 and 6). As epidote generally forms in geothermal fields at temperatures above 240 ◦ C under near neutral pH conditions in a liquid reservoir (Browne, 1978), we infer that the water level in the system was previously higher. If the geothermal system is related to recent volcanism, then it is no older than 0.23 Ma. Thus lowering of the water level could not be caused by tectonic uplift, which occurred much earlier, during the Pliocene (Alzwar et al., 1992). The epidote occurs 400 m below the top of the smectite-bearing argillically altered rocks, which marks the top of the conductor. If it is assumed temperatures followed boiling point-todepth conditions, the original water level would have been at an elevation of 1730 m asl. To the south beneath the younger volcanic centres, the top of the conductor is found at an elevation of 1400 m asl, and there is a concurrent deepening to the first appearance of epidote (Fig. 4). Thus, the top of the conductor to the south can be considered to correspond to the original water level. The top of the epidote zone is very close to the top of the Waringin Unit (Fig. 4) and the top of the vapour-dominated reservoir (Fig. 6) reflecting, perhaps, a porosity variation. In the northern part of the field, the Malabar Unit consists mainly of lavas whose average porosity is ∼1% (Asrizal et al., 2006). The underlying Waringin Unit, consisting mainly of lapilli tuffs, these have an average porosity of ∼8% (Asrizal et al., 2006). It is possible that the original porosity of the pyroclastic deposits was higher prior to alteration, but this would not be the case for the lava flows. Therefore, the thick sequence of lava flows could have acted as the initial caprock of the system. The permeability associated with vertical faults that cut the flows may have been restricted by the presence of the regional ash deposits, which because of their high clay content would tend to deform plastically rather than act as hosts for vertical conduits. In the south, the Waringin Unit contains more lava flows in its upper parts. The overlying Pangalengan Unit, which contains many ash deposits and fine-grained sediments, could have

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formed the original cap in this part the field, and thus might have influenced the distribution of epidote. However, the plot of depth of the first appearance of epidote follows a similar shape to the 300 ◦ C isotherm, suggesting instead that the initial temperature distribution may outweigh the influence of porosity variations in the south. In some places the original conductor has been overprinted by hydrothermal alteration caused by perched steam-heated aquifers that, where topography allows, extend to higher elevations and above topographic highs in the conductive layer. However, these aquifers have higher resistivities (∼5 m) than the main conductor (∼2 m), due to the presence of kaolinite, which is more resistive than smectite, as the predominant clay mineral. The areas of the conductor that have slightly higher resistivity more clearly define the four fluid upwelling centres at the Wayang Windu Geothermal Field (i.e. where productive wells have been drilled; Fig. 3) than the elevation of the base of the conductive layer. Presumably this is because the areas of higher resistivity in the conductor reflect near present conditions, whereas its base was defined during an early stage in the development of the system. Similar higher conductivities in the conductor have been reported at Karaha-Telaga Bodas (Raharjo et al., 2002), and may generally serve, in combination with the geometry of the base of this conductive layer, as a pre-drilling indicator of the potential presence of vapour-dominated reservoirs. North of the wells drilled at Wayang Windu the main conductor is found at up to 1800 m asl. Here it is possible that the water level may have reached this elevation during the history of the field. Moreover, the higher resistivities indicative of perched steam-heated aquifers are found at even higher elevations. Since the current deep liquid level is at much greater depth (∼400 m asl in the north) and the alteration mineralogy is that of a near pH-neutral fluid, most of the alteration in and above the vapour-dominated zones is now above the water level and must be relict. That is, the alteration, which includes the electrically conductive argillic zone, must have formed early in the development of the system. This zone, which is characterized by a conductive temperature profile indicative of low permeability, now constitutes the caprock of the geothermal reservoir. On the margins of the field, where the base of the conductor deepens, these clay-rich altered rocks form the lateral hydrological boundaries of the vapour-dominated resource, at least in its upper parts. This may explain why the vapour-dominated zone is best developed in the north where there is the steepest drop off in depth of the margin of the conductor. Hydrothermal alteration in the two-phase, vapour-dominated reservoirs that have formed above the deep water is only weakly developed. Epidote is partially replaced by calcite, white clay (possibly kaolinite), pyrite and anhydrite, a further indication of its relict nature. The pervasive calcite veining present in the rocks may have formed as the system boiled off; but platy textures typical of boiling (Browne, 1978) are not commonly observed. Alternatively, the calcite could have formed by descending CO2 -rich condensate. Rarely, platy calcite and chalcedony occur in epidote-bearing rocks. The deposition of chalcedony at temperatures above 240 ◦ C (Bogie et al., 2003), are suggestive of more intense boiling, which could result from localized pressure drops during fracturing. In contrast, chalcedony is widely distributed at Karaha-Telaga Bodas (Moore et al., 2002, 2004), where its presence is interpreted as evidence of rapid boil off. Fluid inclusion work has been limited by the amount of suitable sample material. Abrenica (2007) reports homogenization temperatures in primary fluid inclusions in quartz from a vein at 590 m asl in well MBD-5 between 228 and 255 ◦ C, with the mode at 235 ◦ C. The current estimated temperature is 246 ◦ C (the downhole logging tool did not reach this depth). Melting point measurements for these inclusions indicate salinities of 0.53–1.05 wt% NaCl equivalent (Abrenica, 2007). These values reflect both the dissolved salt and gas contents of the trapped fluids. The present-day salinities, calculated on a gas free basis, of the deep liquid reservoir is


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∼2 wt% NaCl equivalent. Therefore, these inclusion results likely reflect early liquid reservoir conditions. Secondary fluid inclusions in the quartz are vapour rich and have higher vapour/liquid ratios in successive generations, consistent with the presence of increasing vapour-dominated conditions with time. Homogenization temperatures of the secondary inclusions range from 241 to 334 ◦ C, but as it is unlikely that a single-phase fluid was trapped, they may not reflect temperatures prevailing at the time. Primary inclusions in calcite from a sample collected 2 m below the quartz sample from well MBD-5 (i.e. at 592 m asl) gave higher homogenization temperatures of 264–295 ◦ C (Abrenica, 2007), with the mode at 275 ◦ C. Like the quartz-hosted secondary inclusions, these calcite-hosted inclusions are vapour-rich and have increasing vapour/liquid ratios in successive generations and homogenization temperatures may not be reflecting trapping temperatures. Epidote from 542 m asl in MBA-1 was observed to contain liquid-dominated fluid inclusions (Abrenica, 2007), although homogenization temperatures could not be obtained. As vapourdominated conditions now prevail at this depth, it is inferred that the liquid level was previously higher and that the epidote is relict. 5. Reservoir characteristics and geochemistry Wayang Windu has a deep, hot, neutral pH, liquid reservoir that, in the area drilled, is overlain by a perched vapour-dominated two-phase reservoir (Fig. 6). Throughout the field within the deep liquid reservoir, pressures and temperatures versus elevation are similar (Table 2 and Fig. 5). In the north its top is at 400 m asl, and it deepens towards the south where the top of the liquid is found near sea level elevation. Since the reservoir has almost the same vertical pressure distribution throughout, it can be considered to be a contiguous body, and since it is near pressure equilibrium there is little fluid flow. It is under-pressured with respect to groundwater hydrostatic pressure and there is geochemical evidence for only limited recharge (Suminar et al., 2003). The deep liquid reservoir is possibly recharged from west of G. Bedil and G. Wayang and southwest of G. Windu. In these areas, the decrease in the elevation of the base of the conductive layer is less steep, and thus the early argillic alteration that may have created hydrological barriers on these sides of the vapour-dominated reservoir is more limited. In the areas where recharge may be occurring the shallowest well feed zones from the deep liquid reservoir are cooler than elsewhere in that reservoir, but without actual temperature inversions deeper in the well. This indicates some limited ingress of meteoric waters in the shallow zones, rather than indicating outflows in these areas. Two-phase vapour-dominated reservoirs overlie the deep liquid-dominated regime. The largest in the north appears to be coalesced over two fluid upwelling centres (associated with Puncak Besar and G. Gambung), while the two further south (associated with G. Wayang and G. Windu) appear to be separate, giving three vapour-dominated reservoirs over four fluid upwelling centres. The characteristics of the vapour-dominated zones change progressively towards the south: pressures, temperatures and gas contents increase, they are found at greater depths, and their thicknesses decrease. The degree of communication between the vapour-dominated reservoirs is uncertain. However, if they were all hydraulically connected, the drop in fluid pressure for any given elevation towards the north (i.e. the oldest part of the system if the ages of the spatially related volcanic centres are also temporally related to the fluid upwelling centres) may point to a northerly flow from the inferred youngest part of the system in the south. The fact that the field can be divided up

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Table 2 Geochemical and pressure–temperature properties of Wayang Windu reservoir areas prior to production Area

Deep liquid reservoir

Two-phase reservoirs NaKCa temperatureb (◦ C)

NCGc (wt%)

NCGc (wt%)

Measured temperaturea (◦ C)

Measured pressurea (bar)

Elevation (m asl)

Puncak Besar Gambung

d

d

d

d

d

d

12,00 –13,000

295–300

0.3–0.6

0.6–2.6

250–260

35–45

?–1120 400–1100

Wayang

12,000 –13,000

295–308

0.5–0.6

2–4.5

255–267

50–55

200–700

Windu

6000 –8000

285–300

3.5

10

260–290

80–85

80–400

a

Prior to production. NaKCa geothermometer of Fournier and Truesdell (1973). c Non-condensable gases in weight percent. d Well MBB-1 drilled in this area produces 20 MWe on initial discharge. It did not penetrate into the deep liquid reservoir and fully stabilized gas, temperature and pressure data are not yet available. b


Reservoir (ppm)

Cla


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geochemically into a minimum of three areas (Table 2), and that high temperatures are maintained at depth and are actually highest beneath G. Wayang rather than in the Windu area, argue against a single heat source in the south. Separation of these three vapour-dominated reservoirs may reflect the distribution of impermeable, regional ash deposits that restrict vertical permeability and the distribution of the more porous pyroclastic-rich deposits that host the two-phase zones. Interpretation of the MT surveys indicate that the Wayang Windu Geothermal Field may extend approximately 7 km to the north of MBE-2 and that this well is located south of two domed structures in the base of the conductor (Fig. 3), with MBE-2 and the major producing wells on the MBA and MBD drill pads associated with the southern dome. In the north, a fourth area associated with the northernmost domed structure in the base of the conductor has now been drilled and found to be productive [well MBB-1 (see Figs. 2–4) produces 20 MWe]. The four possible fluid upwelling centres areas are spatially associated with four eruptive centres (Puncak Besar, G. Gambung, G. Wayang and G. Windu, going from north to south; Fig. 2), whose age decreases generally towards the south. When considering these four particular areas, one finds that the deep liquid reservoir in the G. Windu area has geochemical characteristics similar to that of geothermal reservoirs associated with andesitic stratovolcanoes elsewhere, for example Tongonan in the Philippines (Lovelock et al., 1982). The slightly lower temperatures at depth in the G. Windu area when compared to those measured further north suggests that any concentration of solutes by boiling (as is indicated by the downhole pressure profiles) is being offset by some mixing with meteoric waters. The waters of the deep reservoirs in the G. Gambung3 and G. Wayang areas have similar Cl/B ratios than those in the G. Windu area, but are much more saline and have lower gas contents suggestive of boiling at high temperatures, with very limited mixing with ground waters (a process that requires continuous heat recharge to maintain reservoir temperatures). Parts of the system where the deep liquid reservoir is more saline would therefore have to be older than those at G. Windu in order to provide time for this extent of boiling off to occur, which is consistent with the age of the heat centres (i.e. generally getting younger towards the south). The variation in gas content between the areas (Table 2) may reflect a lower gas flux from the older centres, and may be responsible for the deepening of two-phase conditions as the increase in gas content towards the south increases the depth of first boiling. 6. Discussion As pressure-versus-elevation in the deep Wayang Windu liquid reservoir is the same throughout the drilled area and the deep reservoir is under-pressured with respect to local ground waters, the system’s hydrology cannot be interpreted in terms of the liquid upflows and outflows typical of most geothermal systems associated with andesitic stratovolcanoes. The presence of a deep neutral-Cl reservoir, the low degree of under-pressure in the deep liquid reservoir, preexploitation pressures and temperatures above that of the maximum enthalpy of steam in the vapour-dominated reservoirs and the limited depth range where vapour-dominated conditions prevail at Wayang Windu also mean that the field cannot be strictly compared to the solely vapour-dominated systems of Darajat or Kamojang. Wayang Windu must therefore be regarded as a new type of geothermal field, transitional between liquid dominated and vapour dominated.

3 Fluid chemistry data on the deep liquid reservoir is lacking for the northernmost wells as they were completed above it. Thus, all the deep liquid geochemistry data come from well MBE-2.

362


This transition is most advanced in the northern parts of the field where the drop in the deep water table has been largest. Towards the south, the vapour-dominated zones are thinner and deeper and make up proportionality less of the resource. This relationship implies a series of steps in the transition from liquid- to vapour-dominated conditions represented in the geothermal field. The Wayang Windu system is largely sealed off from surrounding and overlying ground water aquifers, but still receives deep heat and fluid recharge from possibly four fluid upwelling centres that are inferred to be progressively younger to the south. Vapour-dominated reservoirs have developed over the fluid upwelling centres, with a vapour-dominated reservoir in the north coalesced over the fluid upwelling centres associated with the Puncak Besar and G. Gambung volcanic centres. Two possibly separate vapour-dominated reservoirs further south, lie above separate fluid upwelling centres associated with G. Wayang and G. Windu. Allis (2000) concluded that stock-sized intrusives cannot provide sufficient heat to maintain a long-lived hydrothermal system like the one at Wayang Windu. Thus, it is probable that the field is underlain by a major multiphase intrusion, which is the ultimate heat source. The diorite xenoliths observed in the younger volcanics may be from the older parts of this intrusion. The locations of the four fluid upwelling centres could overlie shallow apophyses of the larger intrusive body, which have been fractured by a combination of secondary boiling and thermal contraction, to provide conduits that channel steam and gas flow from a much larger intrusive source at depth. If the ages of the eruptive centres can be related to the heat centres, it would explain how the geothermal system has been active for possibly 0.23 Ma. Early pulses of acidic, magmatic condensates produced from the intrusive apophyses may be responsible for the formation of the rare advanced argillic alteration observed deep in the Wayang Windu field. The deep liquid reservoir now has a neutral pH; thus there is no evidence for the presence of acidic fluids produced by the condensation of magmatic volatiles reaching shallow levels of the system. The deep magmatic degassing is now heating and partially recharging a large exploitable reservoir. As this deep recharge is composed of water vapour and non-condensable gases, it does not require a large mass, only enough to maintain the large volume of the system under boiling point versus depth curve conditions, and allow the upper part of the original water-dominated reservoir to boil off. Whether or not there is sufficient heat flow to dry out the entire reservoir is another problem, as the volume of the deep contiguous deep reservoir at Wayang Windu may be proportionately larger than that of geothermal systems associated with a single or closely spaced multiple intrusive heat conduits, as may be the case at Kamojang and Darajat. The Wayang Windu field is part of a cluster of Indonesian high-temperature geothermal fields, which include the vapour-dominated reservoirs of Darajat and Kamojang, and may represent the end point of the liquid-to-vapour transition. Vapour capped reservoirs at Patuha (Layman and Soemarinda, 2003) and Karaha-Telaga Bodas (Moore et al., 2002, 2004) exhibit magmatic vapour cores as defined by Reyes et al. (1993), and may represent an earlier stage of transition than found in the Windu part of the Wayang Windu Geothermal Field; the Wayang and Gambung-Puncak Besar areas being successively further advanced in that transition. A common feature of some of these Indonesian geothermal fields is that they underlie sector collapses; this is most strongly the case at Darajat, where it includes most of the field. This may possibly also be the situation at Kamojang, where a partially circular collapse feature is reported, although this has been previously interpreted to be a caldera (Healy and Mahon, 1982). Moore et al. (2002, 2004) consider that the sector collapse of G. Galunggung Volcano triggered the drying out of the Karaha-Telaga Bodas system. The collapse of G. Wayang may have also contributed to the boiling off of at least that part of the Wayang Windu geothermal system. The radiocarbon


363

dating (discussed in Section 2.1) indicating that this sector collapse took place after the formation of the shallow hydrothermal alteration further north, supports this notion. Other areas of major slope failure that may also be interpreted as sector collapses are found on the western side of G. Windu, north of G. Gambang, west of Puncak Besar and north of G. Malabar (Fig. 3). However, that to the north of G. Gambung and that to the west of Puncak Besar may be more closely related to slippage on the Bandung Basin Boundary Fault. Hydrothermal alteration and activity have not been reported from these areas and it is more difficult to relate these to changes in the hydrology of the system. This concentration of vapour-dominated and transitional resources associated with andesite stratovolcanoes in the Bandung area is yet to be satisfactorily explained given their apparent dearth in the rest of Java, or elsewhere in the world. Allis (2000) suggested that the perpendicular subduction beneath Java produced compressive deformation in the upper plate restricting the recharge from depth, but this applies all along Java, not just to the Bandung region. This compression is enhanced by the subduction of the Roo Rise (Whittaker et al., 2007), a thickened section of oceanic crust of the down-going Australian Plate. However, there is a break in the Roo Rise in the Australian Plate immediately adjacent to the Bandung area, which is marked by a large bulk-sound velocity anomaly of the down going slab (Gorbatov and Kennett, 2003), with a gravity high directly above it (Newcomb and McCann, 1987). Subduction roll back (Whittaker et al., 2007) is taking place all along the Javan part of the Sunda Arc because of the subduction of the old (95–135 Ma) slab of the Australian Plate. It is likely that this rollback is locally accentuated by the thinner and denser oceanic crust adjacent to Bandung, and that this produces local extensions in the overlying plate, which may be responsible for the large number of volcanic centres and associated geothermal fields in the Bandung area. The roll back may also be responsible for the occurrence of multiple heat centres in Javan geothermal fields, as the location of the upper crustal plate and the deep magma source will vary with time as rollback proceeds. The fields around Bandung are in terrains of sufficient elevation for there to be room for steam reservoirs to form above the general level of deep meteoric recharge. This recharge may be limited by the poor permeability at depth as the faults in the basement had an initial strikeslip movement that reduced fault permeability due to prolonged shearing. The subsequent fault movement within the volcanic deposits was normal (at least at Wayang Windu) resulting in higher rock mass permeabilities, thus favoring the formation of highly productive vapour-dominated reservoirs. Acknowledgements The authors wish to gratefully acknowledge the permission and support of the management of PT Star Energy to publish this paper and the help of Manfred Hochstein in doing so. Constructive reviews by Rick Allis and Dick Henley are also acknowledged, along with the editorial assistance of Joe Moore, Marcelo Lippmann, Sabodh K. Garg and Greg Bignell. Thanks go to Mariano Gutierrez of Sinclair Knight Merz Ltd. and Iis Dian Indriani for their assistance with the figures. References Abrenica, A.B., 2007. Hydrothermal alteration and fluid inclusion studies in the northern Wayang Windu geothermal field, West Java, Indonesia. MSc Thesis. Geological Engineering Department, Faculty of Engineering, Gadjah Mada University, Yogyakarta, Indonesia, 151 pp.

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Overview of the Wayang Windu Geothermal Field, West Java, Indonesia

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