The Karaha-Telaga Bodas Geothermal System, Indonesia Joe Moore Energy & Geoscience Institute

Location Map of Karaha-Telaga Bodas

1982 Eruption of Galunggung Volcano Photo by J. Lockwood, 1982

Kawah Galunggung Photo by J. Lockwood, 1982

View of Kawah Saat and Telaga Bodas

Exploration History In 1980s, Pertamina drilled 4 shallow gradient holes and conducted MT, resistivity, gravity and microearthquake surveys. KBC initiated an aggressive 2 1/2 year program in Dec. 1994 that included: Chemical sampling of springs, fumaroles and non thermal waters; Interpretation of aerial photographs New MT surveys Reassesment of existing geophysical data Drilling of core and production wells

Geologic Studies Extensive cover precluded detailed geologic mapping Existing geologic maps and available whole-rock ages were compiled. Lineaments were mapped using aerial photographs at a scale of 1:50000

Geologic Results Andesitic to Basaltic stratovolcano. Epiclastic and pyroclastic deposits dominate; lava flows less abundant; intrusive rocks occur at depths of <~3 km. K-Ar ages of surface rocks range from 1.75 to .32 Ma. Flank collapse of Kawah Galunggung occurred at 4200 years BP. subsequent 14C dating of well samples indicates the system may be <6000 years old.

Lineament Map Geothermex It was considered that many of the lineaments represent expressions of recent faulting. lineament analyses indicated that NNW structures dominate but that a NE trend is also common. The density of structures is greater in the south than in the north, suggesting that the southern area is more tectonically active. Geothermex

Geochemical Surveys Spring and fumaroles were sampled for major and minor elements, oxygen and deuterium isotopes and 3He/4He:

Geochemical Results Discharges occur at two widely separated locations, at Telaga Bodas/Kawah Saat and Kawah Karaha. Springs (all non boiling) Telaga Bodas: (T<145oF) acidic Cl-SO4 waters Karaha: are dilute, cool (<100oF) neutral pH and bicarbonate rich Outlying areas: dominantly bicarbonate- rich (T<150oF) SO2 found in TLG2-1 and high F in T-2. Native S is present at Telaga Bodas Most springs discharge mixed cation - HCO3 waters. No surface discharges indicative of a deep NaCl reservoir were found 3He/4He ranges from 7.1-7.7 Ra in concession; 5.1-5.9 Ra outside

Telaga Bodas Water Composition (mg/l) Lab pH 0.40 Mg 206 Na 122 K 30 SO4 30500 Cl 8850 SiO2 348 Fe 2320

Geophysical Surveys MT Surveys 1980s; broadly spaced stations (2-3 km apart) 1996-97; 103 stations occupied in a 90 km2 area DC Resistivity (Slumberger Arrays) 214 locations at 500 m intervals along several lines covers western part of MT survey area depth of penetration ~300 m Gravity Survey regional survey across Karaha volcanic massif Microearthquake Survey Natural seismic activity recorded by 5 seismographs in Karaha area 95 day monitoring period Resistivity, gravity and microearthquake data collected in 1980s, reprocessed in 1996-97

MT Station Locations

Locations of Survey Sites Near Telaga Bodas 103 MT

MT Model of Low-Resistivity Conductor The thickness of the conductive layer is coded by elevation, cold colors show where the section is thick. (Raharjo et al, 2002)

MT Model of Low-Resistivity Conductor (Raharjo et al, 2002)

(Raharjo et al., 2002)

Schlumberger Apparent Resistivity RESULTS 1) A well developed low near Telaga Bodas (<5 ohm-m) 2) 7 ohm-m contour encloses surface features in vicinity 3) A weak low (>15 ohm-m) is located near Karaha Karaha AB/2 = 1000 m

Bouguer Anomaly Map Data reduced using density of 2.3 g/cc (GENZL) North-trending high follows ridge axis Circular gravity high near Telaga Bodas Remodeled by Tripp (2002), Raharjo (2002) The gravity data generally follow the trend of topography. The most prominent feature is the presence of a large elliptical high located about 2 km southeast of the lake, modeled by Tripp et al. (2002), suggested it represent shallow granodiorite.

East-West Gravity Profile Of Karaha-Telaga Bodas This slide shows a positive anomaly coincident with the volcanic axis but have provided little insight into system. (Raharjo et al., 2002)

Microearthquake Survey 200 events (83 between .2 to 2.4) were recorded; 73 centered south of the fumarole field; 10 to the north

Exploration and Drilling History 9 core holes (877-2150 ft.) were drilled between Nov. 1995 and May 1996. First deep production test drilled in May 1996 at Karaha fumaroles, based on thermal gradient data. From Sept. 1996 to Dec. 1997, 10 new core holes (3340-6621 ft) and 7 deep production tests were drilled. Drilling of production wells continued until March 1998. Well temperatures and pressures were measured and the compositions of the discharged fluids determined. The project is currently inactive.

Key Results The surface manifestations consist of fumaroles and springs reflecting boiling and gas movement at depth. No springs discharge water typical of a deep NaCl reservoir. The resource may be vapor-dominated. Gas compositions indicate a magmatic contribution to the fluids. Geophysical studies identified a low-resistivity conductor extending from Kawah Karaha to Telaga Bodas and a positive gravity anomaly near Telaga Bodas. The electrical data suggest the presence of an extensive geothermal system beneath the volcanic ridge.

Key Results Early core holes provided good thermal data but were too shallow to reach reservoir; later core holes were deeper and provided reservoir information. Drilling success improved with time as data from core holes was incorporated into the drilling program. The data do not define the limits of the geothermal system. Eastern, southern and western boundaries are not delineated.

What We Know Now Despite lack of thermal expressions, the system is continuous beneath the volcanic ridge from Telaga Bodas to Kawah Karaha. The reservoir has the shape of a broad arch. The conductive layer outlines, in a general way, regions of high temperature. The base of the conductive layer approximates the base of the caprock.

This slide shows a cross section of the geothermal system beneath the ridge. The vapor dominated zone is in gray, it is underlain by a liquid dominated reservoir.

Telaga Bodas is the surface expression of a magmatic vapor chimney. Heat is provided by young intrusions emplaced during the last 6000 years. The deep fluid has low salinity, suggesting meteoric waters are recharging the geothermal system. Magmatic gases combined with extensive drying out of the vapor zone lead to the formation of acidic fluids in the southern wells. The fluids are neutralized/diluted to the north. Northern and central wells produce near neutral waters.

Pyroclastic rocks dominate the volcanic section Pyroclastic rocks dominate the volcanic section. Productive zones occur in fractured pyroclastic rocks, lavas and intrusives. Secondary permeability in the pyroclastic deposits developed after the rocks were altered and fractured. Rocks with high primary porosities appear to be minor contributors to the total fluid production.

THE END