GRC Transactions, Vol. 37, 2013

Reservoir Review of the Rendingan-Ulubelu-Waypanas (RUW) Geothermal Field, ,

Suharno Geophysical Engineering Department, the Faculty of Engineering, Lampung University, , Indonesia

Keywords and a southern (Ulubelu) section. With extension of the survey Rendingan-Ulubelu-waypanas, hydrothemal-mineral, fluid- area to include the Waypanas manifestations, this combined study inclusions, two phases, cooling area is now called the Rendingan-Ulubelu-Waypanas (RUW) geothermal system. My study of this area used geological, geophysical and pa- ABSTRACT leohydrological methods to obtain a four dimensional picture of the reservoir. Geological assessment consisted of surface studies, The Rendingan-Ulubelu-waypanas (RUW) geothermal sys- including field surveys of hydrothermal manifestations and rock tem contains host rocks that alter very readily because of the sampling; cores and cuttings were examined in hand specimen and great contrast between their hydrothermal environment and the petrographically, boreholes drilled, down hole temperatures and volcanic conditions under which they formed. Most surface rocks pressures measured and interpreted. Geophysical work consisted are weathered but some have also been altered hydrothermally. of micro-earthquake, gravity and magnetic data that have been The alteration includes both replacement of primary phases and analyzed to interpret the RUW reservoir. The microearthquake the products of processes that affected ascending thermal fluids. analysis contributed information that helped characterize the Alkali chloride water of near neutral pH once deposited silica hydrothermal system. The gravity data helped reveal the distribu- sinter at the surface but now acid steam condensate is forming tion and dimensions of host rocks within the geothermal system kaolin, silica residue and other phases. The mineralogy, fluid and nearby, and the magnetic studies the extent of the geothermal inclusions and surface manifestation indicate that conditions in system with respect to its rock alteration intensity. The RUW the RUW geothermal system changed spatially and temporally during its lifetime. The mineralogical evidence incompletely re- cords some of the changes in the thermal regime. The identities of the hydrothermal minerals reflect the new environment in which reservoir rocks find themselves. The main hydrothermal mineral assemblages were produced by neutral pH waters. The Th values are higher than the present well temperatures suggesting cooling has occurred since the inclusions formed. The RUW reservoir contains vapor, two phase and liquid dominated domains but is overall a liquid-vapor dominated system. This is revealed by the well measurements (T and P) and the hydrothermal alteration and fluid inclusions geothermometry.

I. Introduction The Rendingan-Ulubelu-Waypanas (RUW) geothermal system is near circular in shape, as revealed by its magnetic signature, and extends over a distance of 15 km at the southern end of the Su- Figure 1. Surface geology and stratigraphy of the Tanggamus Regency. matra Fault Zone in Tanggamus, Lampung. Formerly this system Semangka Fault is a part of the end SE of Sumatera Fault System. Key to was known as Ulubelu. However, Pertamina (1992) suggested it stratigraphy is given in Table 1. Dashed and full lines are inferred and should be separated into two parts; a northern section(Rendingan) confirmed faults; modified from (Amin et al., 1993; Suharno, 2003).

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Figure 2. The reservoir boundary model of the Rendingan-Ulubelu-Waypanas (RUW) geothermal system approximated from Suharno (2000, 2003). Solid ellipse is the Ulubelu caldera. Dashed line ellipses are geothermal prospects (RI, Rendingan; RII, Ulubelu; RIII, Waypanas). Qa: Alluvium, AtR: Altered rocks, TgAl: Tanggamus andesite lavas, KrRl: Kurupan rhyolite lavas, Dt: Da- cite tuff, RdAl: Rendingan andesite lavas, RdPr: Rendingan pyroclastics, TgLb: Tanggamus laharic breccia, KbPr: Kabawok pyroclastics, DdDl: Duduk Da- cite lavas, KkBAl: Kukusan basaltic andesite lavas, SlAl: Sulah andesite lavas, QTr: Ranau Formation, Tmgr: Granodiorite, Tomh: Hulusimpang Formation. Filled circles (Kk1): bore holes; stars: hot springs or fumaroles; triangles: sum- mits of mountains; squares: petrographic samples. Boxes are villages. Solid and dashed lines are confirmed and inferred faults, respectively. reservoir contains vapor, two phase and liquid dominated domains but is overall a liquid dominated system. This is revealed by the well measurements (T and P) and the hydrothermal alteration and fluid inclusion geothermometry. II. Materials and Methods a. Geology of Tanggamus Region The Tanggamus Region is located in Lampung Province, in southern , Indonesia Its western edge occurs at the southern end of the Sumatra Fault Zone, which is marked by the Semangka River (Figure 1). This system trends along the main axis of the western part of the island. The Semangka Fault extends to the Sunda Strait, along the Semangka River into Semangka Bay and southwards. The geology of the Tanggamus Region is illustrated in Fig- within the Tanggamus Regency. Tanggamus volcanic rocks de- ure 1. The stratigraphy (Table 1) is divided into rocks of three rived from a ring of volcanoes comprising Mts. Tanggamus (1.5 broad age ranges: i.e. pre-Tertiary, Tertiary and Quaternary (Amin Ma), Kabawok (1.7 Ma), Waypanas (3.9 Ma), Kukusan (3.9), et al., 1994). The pre-Tertiary successions are the oldest exposed Sulah (4.5 Ma), Rendingan (1.4 Ma) and Kurupan (1.4.Ma); Mt. rocks, and are of regional extent. They include a low to medium Duduk (3.9) is in its center (Figure 1) (Suharno, 2003; Hidayatika grade metamorphic sequence of the Gunungkasih Complex (Amin dan Suharno, 2011). et al., 1994). The oldest rocks (Paleozoic) in this succession The highest part of the RUW study area is in the southeast, are nowhere exposed, however, and have almost certainly been with the summit of Mt. Tanggamus at approximately 2000 m asl, displaced by post-metamorphic faulting. This occurred in the late Table 1. Summary Stratigraphy of the Tanggamus Region. From Suharno (2003). Paleozoic or early Mesozoic. The No Unit (symbol) The Ages Lithology Thickness Menanga Formation is in contact Unconsolidated material, boulders, sand, 1 Alluvium (Qa) Holocene ? with the metamorphic basement silt and clay (Amin et al., 1994). The Quater- 2 Ranau Formation (QTr) Holocene to Pleistocene Pumeceous tuff 100-500 m nary succession comprises late 3 Recent volcanic (Qhv) Holocene to Pleistocene Andesitic basaltic lava, tuff and breccia, ± 800 m Pleistocene to Holocene lavas, 4 Older volcanic (Qv) Pleistocene Volcanic breccia and lava flow ± 400 m breccias and tuffs, reef limestone 5 Semong Formation (QTse) Pleistocene to Pliocene Sandstone-claystone (20-40 m) and Holocene alluvium sedi- 6 Kasal Formation (QTk Pleistocene to Pliocene Poorly consolidated volcaniclastics ± 200 m Pliocene to late- 7 Simpangaur Formation (Tmps) Badded sandstone and claystone (200-700 m) ments. Geological studies have Miocene been made by van Bemmelen 8 Bal Formation (Tmba) Miocene Tuffaceous volcaniclastics (100-200 m) (1949), Katili (1985) and Masd- 9 Granodiorite (Tmgr) Miocene Granitic pluton ? juk (1997). Mocene to late Interbedded sedimen: clay-stone, silt- 10 Seblat Formation (Toms) ± 500 m Oligocene stone & sandstone b. The Tanggamus Mocene to late Interbedded sedimens: clay-stone, 11 Gading Formation (Tomg) 100-500 m Volcanic Area Oligocene siltstone & sandstoene Hulusimpang Formation Mocene to late Volcanism occurs in the 12 Volcanic breccia and lava ? (Tomh) Oligocene northeastern part of the Se- Pandean pluton & related 13 Late Cretaceous Graitic pluton ? mangka Fault. The Semangka intrusives (Kgr) Fault is the SE part of the end 14 Menanga Formation (Km) Early Cretaceous Shale, cllaystone & sandston ? of Sumatra Fault System. The Gunungkasihn 15 Palaeozoic Undifferented metamorphic rocks ? volcanic system partly occurs Complex (Pzg)

472 Suharno and nearby the summit of Mt. Kabawok, at close to 1600 m asl. 100oC. The Waypanas manifestations between 700 and 400 m asl The summit of Mt. Rendingan at the 1700 m asl, is located in the south of Mt. Kukusan to the southwest of the Mt. Waypanas have northern part of the area. The lowest locations are around 100 to the same types of manifestations (Figure 2). 300 m asl, southwest of Mt. Waypanas. Most of the central part Thermal fluids ascend through the host andesites beneath of the study area is about 700 to 800 m asl within the volcanic ter- the sinters within moderately steep terrain in the central part of rains of Mts. Tanggamus, Kabawok, Waypanas, Kukusan, Sulah, the study area, close to Pagaralam village (Ulubelu manifesta- Rendingan and Kurupan (Figure 2). tions). But some out-flows discharge 7 to 15 km to the south and southwest of the study area (Waypanas manifestations) at lower c. Surface Expression of the RUW Geothermal System elevations (Figure 2). Insight into the RUW reservoir can be recognised from its surface expression. The geothermal system is situated within areas III. Result of high relief, around 1600-400 m asl, but moderately steep terrain occurs in its central part, mostly from 700 m to 800 m asl. The a. Extent of the Reservoir lowest places are south and southwest of (the RUW) geothermal The RUW geothermal system (Figure 2) is a large system, manifestations, at about 400 m asl (Figure 2). covering an area of about 150 km2. Evidence for its extent includes surface manifestations that are widespread from the northern part of the Ulubelu manifestations, close to Pagaralam village, south- ward to the southern part of the Waypanas. The manifestations include thermal discharge features and altered rocks within the Ulubelu caldera, extending along the Belu and Ngarip rivers to southern Mt. Waypanas (Figure 1). The Rendingan microearthquake swarm, which occurred in February 1993 (Suharno et.al, 2001; Suharno, 2002; 2005; 2008), is probably a result of hydrothermal activity. Assuming this to be true, I conclude that well Rd penetrated part of the geothermal reservoir. This is also consistent with the occurrence of hydro- thermal minerals at shallow depths in this well (Suharno et. al., 1999; Suharno and Browne, 2000; Suharno, 2003). Suharno et.al, 2005; Hidayatika et.al, 2010). A gravity low situated at the northern part of the study area, below well Rd within the Mt. Rendingan andesite lavas (RdAl) and Mt. Rendingan py- roclastics (RdPr), may represent a permeable fracture zone (Pbr) infered from micro earth quake analysis form Suharno (2003). The magnetic data (Suharno, 2004; Suharno and Soengkono, 2007)) indicate that hydrothermally demagnetised rocks, surrounding well Rd extend from the Ulubelu caldera southward to the vicinity of Mts. Kukusan and Waypanas (Suharno, 2003). b. Present Character of the RUW Reservoir The downhole temperatures and pressures, hydrothermal mineralogy, the thermal characteristics of fluids trapped in fluid inclusions and geophysical data from the RUW geothermal system Figure 3. Structural features of the Rendingan-Ulubelu-Waypanas (RUW) provide information about the thermal character of its reservoir. geothermal field. Dashed ellipses are geothermal prospects (RI, Rendin- The system has perched water or steam condensate above 250 gan; RII, Ulubelu; RIII, Waypanas). Dashed and solid lines are inferred and m depth (450 m asl), vapor occurs between about 250 m and 550 confirmed faults. The lithology symbols are indicated in Figure 1. (modi- m depth (450 m and 150 m asl), two phases from about 600 m to fied from Suharno, 2003). 800 m depth (100 m asl to 100 m bsl) and alkali chloride water below this, near well UBL3. The water level is at 600 m depth in The Rendingan manifestations occur on steep terrain between wells UBL1 (150 m a.s.l.) and UBL3 (100 m asl) and at 400 m 1600 and 900 m asl, from north to south. Only steaming ground depth in well UBL2 (450 m asl). Convection occurs below 800 appears there, but relict hydrothermal minerals occur near well m depth in the reservoir near wells UBL1 and UBL3. However, a Rd (Suharno et. al, 1999; Suharno and Browne, 2000; Suharno, pronounced temperature reversal indicates inflow of cooler water 2003: Kariye and Suharno, 2008). The Ulubelu manifestations, at about 700 m depth, probably meteoric water descending along situated in the central area within moderately steep terrain, are fault F11 (Figure 3). Convection below 800 m depth is also con- mostly between 800 and 700 m asl. Manifestations here include sistent with the occurrence of the mineral characteristic of high fumaroles, hot pools, hot mud pools, H2S discharges, bubbles of permeability (i.e. adularia and albite) (Browne, 1998), in rocks CO2, steaming ground, silicified country rocks, silica sinter, silica below 800 m. In wells UBL2 and UBL3, adularia and albite oc- residue and acid thermal waters at temperatures between 45 and cur from 500 and 600 m depths respectively, consistent with the

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present water levels in these wells (Suharno et.al, 1999; Suharno Table 3. Temperature indicate from well UBL1 using bor hole measured, and Browne, 2000; Suharno, 2000; Suharno, 2003; Kariye and minerals and fluid inclusions geothermometer (from Suharno, 2003). Suharno, 2008). T.bore (oC) T.mineral (oC) Th Well Downhole temperature profiles in wells Rd, Kk1, Kk2, UBL1, o UBL2 and UBL3 (Suharno, 2003) characterize the thermal regime depth (m) Two Four Ep Pr Ilt Sm Ko ( C) 0 34 53 within the RUW geothermal system. The temperature profile in 25 o well Rd shows a temperature gradient of 110 C/km. This value 50 indicates that the heat here moves only by conduction (Björnsson 75 et al., 2000). It is also higher than the regional value heat flow val- 100 57 190 <180 ues in Iceland (80-100)oC/km (Flòvenz and Sæmundsson, 1993). 125 The temperature gradients in wells Kk1 and Kk2 are 140 and 200 150 oC/km respectively. Wells UBL1, UBL2 and UBL3 are character- 175 <180 200 94 198 ized by very high temperature gradients in their uppermost 200 225 250 o m. These are 420, 220 and 500 C/km respectively. 250 Generally, the fluid inclusion data (Suharno, 2003) indicate 275 high reservoir temperatures and boiling conditions. The average 300 191 202 250 <180 homogenization temperatures (Ths) are mostly between about 325 200oC and 250oC. The occurrence of both vapor rich and two- 350 250 phase inclusions in the same samples indicates boiling occurred 375 while the inclusions were being trapped. The water involved in 400 198 203 250 425 the fluid/rock interactions was very dilute with apparent salinities 450 250 from 0.0 to 0.9 wt. % NaCl equivalent (Table 2). 475 250 500 200 202 Table 2. Summary of fluid properties of RUW geothermal system deduced 525 220 <120 from alteration mineralogy and fluid inclusion geothermometry (modified 550 200 203 from Suharno, 2003). 575 250 >220 Type of Indicated 600 201 202 pH Fluid Origin Alteration Temperature 625 100oC (at surface) Chlorite water discharging 650 194 182 Silica Sinter 6 to 7 > 210oC (at depth) at the surface 675 250 >220 <120 700 169 177 Deeply derived upflowing Epidote-Wairakite alkali-chlorite water. 725 > 250 oC Neutral Adularia-Quartz Salinity < 0.9 wt% NaCl 750 144 203 250 >220 <120 equiv. 775 Kaolinite < 140 oC 2 to 5 Steam condensate 800 146 213 825 250 >220 <120 850 168 214 IV. Discussions 875 900 199 215 250 >220 225-245 a. Relations between the Rendingan, Ulubelu 925 and Waypanas Geothermal Fields 950 201 217 975 250 >220 Although the surface manifestations are concentrated in three 1000 202 220 1025 places (i.e. the Rendingan, Ulubelu and Waypanas manifestations; 1050 202 221 Figure 2), thermal activity was once more widespread within the 1075 220 Rendingan-Ulubelu-Waypanas (RUW) geothermal system and 1100 206 221 was probably contiguous between the Rendingan and Ulubelu 1125 areas and also probably between Ulubelu and Waypanas. Past 1150 208 222 thermal activity in the three areas is indicated by the occurrence 1175 250 220 of the surface manifestations and relict hydrothermal minerals. 1200 Therefore, I think the evidence presented in this paper shows T. bore is measured of temperature in bore hole: (oC) that this is one system, as delineated on Figure 2. The location of Two: measured after 2 months heating Four: measured after 4 months heating the northwest boundary is uncertain, probably well Rd, as sug- T.mineral is Thermaly sensitive minerals and their usual gested by microearthquake (Suharno et. al, 2001; Suharno, 2002; temperature range (oC) Suharno, 2005; Suharno, 2008) and gravity data (Suharno, 2003; Ep is epidote : 250oC Suharno et. al, 2005; Hidayatika et. al., 2010). Pr is prehnite : >220oC llt is illite : 220oC The Rendigan geothermal area infered form microearthquake Sm is smectite : <180oC analysis corelated with magnetic and gravity interpretastion. The Ko is kaolinite : <120oC microearthquake activity and gravity data obtained near well Rd Th is homogrnisation temperature of fluid inclusions o( C) reflect the presence of the Rendingan reservoir. However, no data 225-245: homogenisation temperatures

474 Suharno indicate a direct connection between Rendingan and the Ulubelu- c. Changes in Reservoir Conditions Waypanas fields. The microearthquake swarm occurred mainly Table 3 comparetion of bore hole measured temperatures in the Rendingan area. However, the Ulubelu area was also af- (T.bore), mineral geothermometer temperaratures (T.mineral) and fected, as inferred from a few seismic events recorded at stations homogenisation fluid inclusions temperatures (Th), indicates that 5, 6 and 9 (Suharno, 2003). Hydrothermally demagnetised rocks the differences between the average homogenization temperatures surrounding well Rd southward (Suharno, 2003). and present-day temperatures are within 20 oC, while the differ- The spatial and hydrological relationships between the Rend- ences between the hydrothermal mineral deduced temperatures ingan and Ulubelu and Waypanas thermal areas are such that and the present-day temperatures are > 20oC. This implies that they likely comprise a single geothermal system, the Rendingan- cooling has occurred since the minerals were deposited and the Ulubelu-Waypanas (RUW) system. However, an area of high inclusions were trapped (Suharno, 2003; Handoyo and Suharno, resistivity (Suharno, 2000) occurs between the Rendingan and 2008). Ulubelu manifestations, so perhaps it is only at shallow depths The mineralogy, fluid inclusion and surface manifestation as- that they are separate. sessments indicate that conditions in the RUW geothermal system Some of the hydrothermal minerals in drill cuttings (e.g. illite, changed spatially and temporally during its lifetime. Erosion has wairakite, prehnite and epidote) indicate high-temperature hydro- now exposed hydrothermal minerals that formed deep within the thermal fluids (> 225oC), although it is by no means certain that geothermal reservoir during an earlier stage of activity. Neutral pH these are present temperatures consistent with minerals regime. alkali chloride waters close to the boiling temperature water once The deep fluid that produced the observed alteration (Suharno, discharged at the surface, as is shown by the presence of silica 2003) was an alkali-chloride water. The mineralogy and fluid sinter now altered to quartz (Suharno, 2003). At an unknown time inclusion geothermometry results yield the RUW hydrologi- waters discharged at the surface changed to pHs between 2 and cal conditions summarized in Table1 2. Comparison of T.bore, 4, at temperatures between 45 and 100oC. Differences between T.minerals and Th values, indicates that the deep reservoir is still the measured downhole temperatures, which are lower than liquid hotter than 180oC. those indicated by the hydrothermal mineral and fluid inclusion geothermometers, implies cooling of >20oC has occurred since b. Structure, Permeability and Hydrology the minerals deposited. of the Reservoir The mineralogical evidence incompletely records some of The major fault trends are NW-SE and NE-SW (Figure 3). the changes in the thermal regime. Table 1 is a summary of the The principal fault system affecting the Rendingan manifestations hydrological conditions deduced from the mineralogy and fluid (R I) includes faults F8 and F5. Cool meteoric water supplied to inclusion geothermometry. Erosion exposed rocks that contain the reservoir near here could descend, from the Mt. Rendingan calc-silicate minerals (i.e. epidote, wairakite and prehnite) crater into the central Rendingan reservoir, close to well Rd, also produced by neutral pH waters at greater depth than before intersected by fault F5. erotion. The piezometric surface likely dropped in response to The Ulubelu manifestations (R II) (Figure 3) are affected by movements within the caldera or graben collapse and probably faults F1, F2, F3, F4, F5, and F6. F1 is a normal fault that affects other factors such as climate change also affected the hydrology. most of this area. Normal fault F2 occurs in the southwest part I interpret the geothermal evolution of the RUW geothermal of this area. A strike slip fault, F3, provides may be permeable system as follows: zone that connects the Ulubelu and Waypanas reservoirs. Strike The surface manifestations, down hole data, hydrothermal slip fault F4 may allow migration of cool water from higher el- mineralogy and fluid inclusions record changes in the hydrology of evations in the northwest. Normal faults F5 and F6 northeast of the reservoir, although the sequence and directions of the changes this reservoir, fault F1 may also be permeability and supply cool are incompletely known. The presence of hydrothermal feldspars water to the system.. and other hydrothermal minerals (Suharno et. al., 1999; Suharno The Waypanas manifestations are intersected by a fault system and Browne, 2000; Suharno, 2003; Kariye and Suharno, 2008) that includes faults F3, F7, F10, F11, F12, F14, F15 and F16. The indicate that the altering water had a near neutral pH. Epidote, Ulubelu and Waypanas areas are probably connected by strike and prehnite (in veins) could only have formed from a near neu- slip faults F3 and F11. Faults F10 and F11 may allow cool water tral pH of alkali chloride water. The waters were undersaturated to migrate from higher elevations in the southeast to Waypanas in sulphate, as indicated by the absence of anhydrite and other Resevoir. Cool water may also move through faults F3, F7 and sulphate minerals. The widespread calcite in veins indicates CO2 F13 (Figure 3). loss from boiling or effervescence. Fluid in the permeable fracture zone surrounding well Rd The earliest thermal activity produced by assemblages with probably derive from near the summit of Mt. Rendingan to the chlorite, illite, smectite and vermiculite. These waters ascended north and the area surrounding Mts. Kukusan and Waypanas in deeply penetrating fractures generated around the Ulubelu cal- the southern part (along section NW-SE in Figure 3). The Ulubelu dera or graben faults. These waters were hotter than 260oC in the manifestations close to Pagaralam village and well UBL3 may lie reservoir and discharged at the ground surface as hot pools and close to an upflow zone. The microearthquake activity and grav- hot springs that deposited silica sinter (opal-A that later changed ity data may indicate well Rd lies close to a permeable fracture to quartz). zone. Temperatures measured in well Rd indicate a conductive A progressive lowering of the piezometric surface caused gradient, in the caprock here. This conclusion is consistent with steam condensate to occupy shallow levels in the reservoir, as the resistivity data of Suharno (2000). revealed by overprinting of the silica sinter.

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V. Conclusions Björnsson, G., Thordarson, S., and Steingrimsson, B., 2000, Temperature distribution and conceptual reservoir model for geothermal fields in and around the city of Reykjavik, Iceland, Proceedings Twenty-fifth Workshop The RUW reservoir contains vapor, two phase and liquid domi- on Geothermal Reservoir Engineering, Stanford University, Stanford, nated domains but is overall a liquid dominated system. This is California, p. 112-118. revealed by the well measurements (T and P) and the hydrothermal Browne, P. R. L., 1998. Note Book Lecture, The Geothermal Institute of The alteration and fluid inclusions geothermometry. In the area near University of Auckland, New Zealand. UBL3 vapor occurs at about 250 m to 550 m, two-phase condi- Clark, J. P., and Browne, P. R. L., 2000, Past and present-day thermal activity tion from about 600 m to 800 m depths and dilute alkali chloride between the Orakeikorako and Te kopia gethermal areas, New Zealand, water below this. Perched rain water occurs above 250 m depth. Twenty-fifth Workshop on Geothermal Reservoir Engineering Stanford, At present, surface manifestations are characterized by fumaroles University: Stanford, California, p.271-276. and boiling acid springs, some of which once discharged neutral Darmawan, I. G. B., Suharno dan D. A. Munandar, 2011. Menentukan Sistem pH water as indicated by the presence of ancient silica sinter Sesar di Area Prospek Panasbumi Menggunakan Metode Gayaberat. PIT overprinted by kaolin. RUW is a liquid dominated system with API XI, Bandar Lampung 13-14 Desember 2011. a two-phase heat transfer zone. Cooling has affected this system Flòvenz, Ó. G., and Sæmundsson, K., 1993, Heat flow and geothermal pro- and the piezometric surface has descended. cesses in Iceland: Tectonophysics, v. 225 (1993), p. 123-138. The measured drillhole temperatures are generally lower than Handoyo dan Suharno, 2008. Analisis suhu dan tekanan untuk mengetahui those indicated by the fluid inclusions and hydrothermal mineral karakteristik resiervoir panasbumi. Prosiding Pertemuan Ilmiah Tahunan geothermometers. This implies that cooling has occurred since Asosiasi Panasbumi Indonesi, Yogyakarta, 2008. the minerals were deposited. Overprinting locally of hydrother- Hidayatika, A., Suharno dan M. Sarkowi, 2009. Analisis Karakteristik mal quartz by kaolinite and calcite also supports the suggestion Anomali Gayaberat di Wilayah Prospek Panasbumi Ulubelu Tanggamus, Lampung. Prosiding PIT HAGI ke-34. Yogyakarta, 10 – 13 November that the thermal system has cooled since the alteration occurred. 2009. Boiling has occurred in the reservoir, as shown by the pres- ence of coexisting vapor-rich and liquid-rich inclusions, as alkali Hidayatika, A., S. Soengkono, Suharno and M. Sarkowi, 2010. Specific Characteristics of the Gravity Aanalysis Within the Ulubelu Geothermal chloride water close to boiling temperature at the surface once System Tanggamus, Lampung Indonesia. World Geothermal Congres discharged but have not done recently. So the water level in the 2010, Bali 25-30 April 2010. main reservoir has lowered, as revealed by the occurrence together Hidayatika, A. dan Suharno, 2011. The Chronology of the Volcano Surounding of silica sinter and acid waters, discharging CO2 with low chloride the Ulubelu Geothermal System. PIT API XI, Bandar Lampung 13 – 14 at temperatures between 45 and 100oC. Its indicated that a part of Desember 2011. the flowing water as ividence of the boiling. Epidote, wairakite, Kariye, M. dan Suharno, 2008. Menentukan permeabilitas dan suhu reservoir prehnite and laumontite could only have deposited directly near panasbumi menggunakan mineral hidrotermal. Prosiding Pertemuan Ilmiah Tahunan Asosiasi Panasbumi Indonesi VIII, Yogyakarta, 2008. neutral pH of alkali chloride water and low in dissolved CO2 (Browne, 1998). Widespread calcite indicates CO2 loss from Katili, J. A., 1985, Geotectonic Advancement of Geoscience in Indonesian Region, The Indonesian Assoc. of Geologists, 248 p. boiling or the effervescing CO2 rich water. Erosion has exposed hydrothermal minerals at the surface that Masdjuk, M., 1997, Laporan geology detil daerah Ulubelu, Lampung: Jakarta, was formed within the geothermal reservoir during an earlier stage Pertamina. of activity. This is similar to events that occurred at the Te Kopia Pertamina, 1992, Provisional report of combination geophysical survey (resis- field where uplift along the Paeroa Fault and erosion have exposed tivity, head-on, CES, gravity, magnetic and SP: Jakarta, Pertamina, p. 96. hydrothermal minerals that formed several hundred meters below Suharno, P.R.L. Browne and S. Soengkono, 1999. Hydrothermal mineral in the former ground surface (Bignall and Browne, 1994; Clark and the Ulubelu Geothermal Field, Lampung, Indonesia. Proceedings the Browne, 2000). 21st New Zealand Geothermal Workshop. Soengkono, S., Y. Daud, Suharno and S. Sudarman, 2000. Interpretation of Self-potential Anomalies Over the Ulubelu Geothermal Prospect, South VI. Acknowledgment Sumatra. Indonesia, Proceedings The 22nd New Zealand Geothermal Workshop. The author wishes to thank Pertamina that for consent to pub- lish this paper and Prof. PRL Browne of the Honorary Research Suharno dan S. Sudarman, 2000. Analisis hasil studi geofisika dan geologi area panasbumi Ulubelu dalam rangka penafsiran permeabilitas reser- Scientist Institute of Earth Science and Engineering, Auckland, voir. Prosiding PIT-HAGI 2000. New Zealand for his help. Suharno, 2000. A geological and geophysical studies of the Ulubelu geo- thermal field in Tanggamus, Lampung, Indonesia. M.Sc Thesis of the VII. References University of Auckland. Amin, T. C., Sidarto, Santosa, S. and Gunawan, W., 1993, Geological Map Suharno and P.R.L. Browne, 2000. Subsurface hydrothermal alteration at of Kota Agung Quadrangle, Sumatra: Department of Mines and Energy the Ulubelu Geothermal Field, Lampung South Sumatra, Indonesia. Directorate General of Geology and Mineral Resources. Twenty-Fifth Workshop Geothermal Reservoir Engineering, Stanford, California, USA. Amin, T. C., Sidarto, Santosa, S. and Gunawan, W., 1994, Geological Report of the Kota Agung Quadrangle, Sumatra: Bandung, Department of Mines Suharno, 2001. Study of Tanggamus Lithology, Volcanism and Stratigraphy. and Energy Directorate General of Geology and Mineral Resources, p. 113. Sain dan Teknologi ISSN: 0853-733X. Bignall, G., and Browne, P.R.L., 1994, Surface hydrothermal alteration and Suharno, S. Soengkono and S. Sudarman, 2001. 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