Structural Traps of Nonvolcanic Hosted Geothermal Field Based On

Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010

Structural Traps of a Non Volcanic Hosted Geothermal Field Based on Geophysical Data of Waesalit Area Buru Island-Indonesia

Alanda Idral Centre for Geological Resources - Indonesian Geological Agency [email protected] or idral_idral @yahoo.com

Keywords: geothermal, geophysics, intrusive, metamorphic, The surface geothermal manifestations that are spread out Waesalit-Bacan, Maluku along Waekedang river consists of hot waters (T: 99.2 – 101.6 0C), fumarols, hot grounds (T 85 - 98 0C ), silica ABSTRACT sinters and altered rocks (figure 3). The altered rocks is Waesalit geothermal field lies aproximately 60 km SW of composed by clays mineral such as kaolinite, illite and Namlea, the Capital City of Buru Regency, Maluku alunite. The present of illite indicates the zone of phyllic Province. hydrothermal with relatively high temperature (240 - 300°C), whilst the allunite indicates zones of advanced Geophysical methods (gravity, geomagnetic and resistivity) argillic. Alunite mineral, generally, associated with acidic were carried out to delineate the structural traps of a non hot water with high sulfide contents. Meanwhile, around vulcanic hosted geothermal field in Waesalit, Buru Island- Waesalit hot waters are also found yellowish sulphuric Indonesia. deposit that spread out from NE to SW.

Geophysical data indicates moderate to high gravity and low 3. HOT WATER GEOCHEMISTRY geomagnetic anomalous around the Waesalit hot water, whilst the resistivity data show the resitivity value around Based on geochemical data, Waesalit hot waters are the hot water is relatively high ( 20 – 110 ohm-m) compare bicarbonate type with normal pH, and SO4 is relatively high to the value of a volcanic geothermal sytem (10 – 20 om- enough for a non volcanic hosted geothermal manifestation. m). Plotting of Na-K-Mg indicated Waesalit hot water lies at upper part of partial equilibrium line. Isotop O18 and Based on geophysical data, the structural traps of a non deuterium data shows the hot water is out site of a meteoric vulcanic hosted geothermal field in the area is relatively line. All of geochemical data indicated the hot water derived similar to volcanic hosted geothermal area, whilst the heat from deep waters. The range of reservoir temperature in source is considered to be associated to volcano-tectonic term of surface chemical geothermometer is 227o - 247oC process on metamorphic rock. (SiO2-conductive cooling), PMG-2007 and is classified as high enthalpy, (Badan Standarisasi Nasional, 1999). 1. INTRODUCTION Waesalit geothermal field lies aproximately 60 km SW of 4. GEOPHYSICS Namlea, the Capital City of Buru Regency, Maluku Province-Indonesia, and the UTM coordinates are 9615000 4.1 Gravity mU – 9620000 mU dan 261000 mT – 271000 mT, (figure 1). The manifestation consists of hot waters, alteration rocks, Regional and bouguer gravity anomalous trending E-W and silica sinters and fumarols. The reconnaissance survey of the value of both anomallous tend to decrease to the north. geothermal manifestations in Buru Island was carried out in of Waesalit hot waters, and the altered zone are covered by 2005 by Center for Geological Resources-Indonesian intermediate to high of regional and bouguer gravity Geological Agency, and in the following year an integerated anomalous (figure 4a-4b.). study was established by the same institution The liniation of residual gravity anomaly trending NW-SE and NE-SW in the west north and east south of survey The paper try to study the structural traps of a non volcanic lines. High lenses residual gravity anomalous are found hosted geothermal field in Waesalit Buru Island-Indonesia around Waesalit and Waekedang hot waters and in the north based on geophysical data that is correlated to geological northeast of survey lines, (figure 4c). The high residual and geochemical investogations and their correlation with gravity anomaly is considered to be associated with hydrogeology, termal structure and geothermal reservoir. concealed volcanic intrusive rocks, while relatively low residual gravity anomaly in the southeast survey line is 2. GEOLOGY AND GEOTHERMAL coused by depression zone that is filled with sedimentary MANIFESTATIONS deposit The geothermal manifestation in the area is covered by The present of concealed volcanic intrusvive rock Permian metamorphic rocks and Quartenary sedimentary underneath Waesalit hot water is clearly supported by the deposit. The metamorphic rokcs consist of schist, phylite, present of sulphur deposit around the manifestations and. 2.5 and quarzite, whilst sedimentary deposit is composed by D gravity modelling (figure 5) claystone, river deposit and alluvium. Gravity anomaly is also identified 5 fault structures which The geological structures that are developed in the area are trending NW-SE and NE-SW, those faults control the consists of Waekedang and Waesalit oblique transform geothermal manifestation in the area. faults, Waetina strike slip fault and normal fault of Debu.

The faults trending, generally, NE-SW and NW-SE, (PMG, 2007), figure 2.

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Survey area

Figure 1: Waesalit survey area

Figure 2: Geological map of Waesalit area (PMG)

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Figure3: Waesalit hot water, hot ground and silica sinter

(a) Regional gravity anomaly map

(b) Bouguer gravity anomaly map

Figure 4: Gravity anomaly map

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Figure 5: 2.5-D gravity model along cross section A-B

.2 Geomagnetic zone at AB/2 750 m and AB/2 1000 m showed tongue shape that is trending NW to the Weasalit hot waters or to the high In general, low geomagnetic value (negative) is surrounded topographic zone and open to the SE or to the low high geomagnetic value (positive) in the midle of survey topographic area. This condition indicates up flow zone in area. (figure 6). Low geomagnetic value in the NW is the moderate resistivity zone of geothermal system. associated with metamorphic rock such as schist, phylite Meanwhile, zone of moderate resistivity tends to increase and quarzite (non magnetic rocks). However, very low with increasing the distance electrode current of AB/2. Low geomagnetic value (> 500 nT) surrounding Waesalit hot lenses resistivity area that covers Waesalit hot waters is waters is associated with altered rocks as seen in the field. associated with altered rocks that is caused by hot fluids The altered rocks is caused by hot hydrothermal liquid that goes up through foliations, faults and rock fractures of demagnetized the rocks, and so give low geomagnetic value metamorphic rocks. Low resistivity zone in the south SW or respons, whilst low geomagnetic value in the SE survey survey area is associated with sedimentary deposit. area is associated with depresion zone that is filled with sedimentary deposits. High geomagnetic value in the midle 4.3.2 True resistivity cross section of survey area is considered to be associated with phyllite rocks that poor of quartz compare to schist. Such as gravity Figure 8 shows the true resistivity cross section that is data, geomagnetic data is also identified faults structure that trending NW-SW (A..3100-E.5500). The section indicates is ttrending NE-SW, NW-SE and N-S, those fault control the vertically and laterally: resistive layer with very high geothermal surface manifestations. resistivity value; semi resistive layer with high resistivity value; semi conductive layer with moderate resistivity value; 4.3 Resistivity and conductive layer with low resistivity value. Resistive layer ( 100 - >500 ohm-m) is composed by fresh and 4.3.1 Apparent Resistivity maps weathered metamorphic rocks (schist, phyllite and quartzite). The layer is up to 100 m depth. Semi resistive Apparent resistivity map for AB/2: 250 m – AB/2: 1000 m layer (100-200 ohm-m), the thickness is > 700 m. The layer (figure 7) showed three zone of apparent resistivity, such as: is interpreted as fresh metamorphic rocks. Semi conductive high apparent resistivity or resistive zone (> 110 ohm-m) in layers (50 – 90 ohm-m), the layer is considered as weathered the north NW; moderate apparent resistivity or semi and slightly altered of metamorphic rocks that is found at conductive zone (60 – 110 ohm-m) in the middle of survey 100 – 450 m depth of local ground, with the thickness of > area; and low apparent resistivity or conductive zone (< 60 750 m. Conductive layer (20 – 30 ohm-m) or clay cap is an ohm-m) in the south SE and around Waesalit hot waters altered metamorphic rock and or concealed intrusive Low resisitivity value around Weasalit hot waters indicate volcanic body. The clay cap is found at the depth of > 850 the manifestation is associated with upflow geothermal m with the resisitivity value of 20 ohm-m. Underneath this fluids (Anderson et. al . 2000). layer , at the depth of > 1000 m, is found another semi resistive layer with the resisitivity value of 125 ohm-m. The High apparent resistivity zone is considered to be associated layer is interpreted as a reservoir. with fresh metamorphic rocks. Moderate apparent resistivity

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100

50 200

150 30 0 0 500 1000 1500 2000 350 50

Very low geomagne ti c anomaly (neg atif): > - 500 nT

Waemetar Moderate ge omagne ti c anom aly (ne gati f): 0 to min. 50 0 nT 350

20 0 Hi gh geo mag neti c anomal y (p osi ti ve) : 0 to 200 nT

Geomagnetic contour interval 100 nT

Met ar Roads

Rivers

A 2250 Observer points

Wapsalit Interpreted faults

Hot waters

Figure 6: Total geomagnetic map

G-2 500 G-25 00 G-3000 G-3000 G-3500 G- 3500 F -2500 G-4 000 F-2500 G-4 000 F-3000 F -3000 G- 4500 G-4500 F- 3500 F- 3500 E-4000 G-5000 E-4000 G-5 000 F-40 00 F-4 000 D-2000 E-4500 D-2000 E-4500 D-2500 D-2500 E-5050 F- 4500 E-5050 F-4500 D- 3100 E-5500 F-5000 D-3100 E-5500 F -5000 C-2500 D-3500 E- 6000 C-2500 D-3500 E-6000 B-2750 C- 3000 D- 4150 E-6500 B-2 750 C-3000 D-4150 E-6500 B-3150 D-4450 B-3150 D-4 450 B-3500 C-3 500 B-35 00 C- 3500 D-4950 D-4 950 A- 20 00 B-4000 C-410 0 A- 20 00 B-4000 C-4 100 A -2500 C-4500 A-2500 C-4500 B-4500 B- 4500 A-3 100 C-5000Wam etar A-3 100 C-5000Wametar B-5050 B-5050 A-3500 C-550 0 A-3500 C-5500 B- 5 500 B-5500 A- 40 0 0 A- 4 000 B-6000 B-6000 WapA- sa450it l 0 WapA- sa4500it l

AB/2= 250m AB/2= 500m

G-250 0 G-2 500

G-3000 G-3 000 G-3500 G-3500 F-25 00 F- 2500 G- 4000 G-4 000 F-3 000 F-3000 G-4 500 G-450 0 F -3500 F-3 500 E-4000 G-5000 E-4 000 G- 5000 F- 4000 F- 4000 D-2 000 E- 4500 D-2 000 E-4500 D-2500 F-450 0 D-2500 F-4500 E-505 0 E- 5050 F- 5000 D- 3100 E-5 500 D-310 0 E-5500 F-500 0 D-3500 D-3 500 C- 2500 E -6000 C-2 500 E-6 000 Legend: B -2750 B- 2750 C-3000 D-415 0 E- 6500 C-3000 D- 4150 E-6500 B- 3150 D-4 450 B-3150 D-44 50 C-35 00 C-3 500 B-3 500 B-350 0 < 60 oh m- m (lo w re sistiv ity zone) or al ter ed ro cks D-495 0 D-4 950 A-2000 B-4000 C-41 00 A-2000 B-4000 C- 4100 and sedimentary deposits (very conductive rocks) A- 2500 C-45 00 A- 25 0 0 B-45 00 B-4 500 C-4 500 C-5 000Wa metar Wa me t ar 60-110 ohm-m (moderate res.zone) or silghtly A-3100 B-5050 A-3100 B-5 050 C-5000 A-3500 AB/2= 1000m C-5500 A- 3500 C-5500 altered and weathered rocks (conductive rocks) B- 5 50 0 B- 5500 A- 4000 A-400 0 B- 6000 B-60 00 Wapsalit A-450 0 WapA-4500 salt i > 110 ohm-m (high resistivity zone) or fresh metamorphic rocks/resistive rocks

AB/2= 750m AB/2= 1000m Figure 7: Apparent resistivity map

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baratdaya

A 3100 timurlaut

213 B-4500 C 4100 250 D 4150 113 E 5500 80 ohm-m80 -m 00 ohm 300 0->5 500 13 180 ohm-m180 11000 50 150 35 35-50 ohm-m -87 50 150 100-200200 ohm-m 150 -187 80 ohm-m 70 -287 100 100-200 ohm-m -387 80 5050-90 ohm-m 60 200 -487 125ohm-m125 90 30 ohm-m 60 100 -587 60

30 - 687

- 787 20 ohm-m

- 887 20 125hm-m -987 ? ? ? ? ? 0 250 500 750 1000 1250 15001750 2000 2250 2500 2750 3000125 3250 3500 3750

Legends 100 - > 500 ohm-m/top soil 100-200 ohm-m/fresh metamorphic rocks 30-90 ohm-m/slightly altered and weathered metamorphic rocks 30 ohm-m/altered rocks 20 ohm-m/ clay cap 125 ohm-m/reservoir faults

Figure 8: True resistivity crossection

5. DISCUSSION Meteoric water penetrate into the rocks after passing through the high permeability rocks such cracks and joints, 5.1 Sructural traps of Waesalit geothermal field and then keep the water in high porosity rocks as deep or short aquifers, called recharge area and its found at The occurence of geothermal system in Indonesia is, moderately deeping morphology around the manifestation. generally, associated with quarternary or recent volcanism Discharge area occured around river flow zones (WaeApo and non volcanism. Buru island (Waesalit geothrmal field) and Pemali rivers) or at relatively flate morpohology. Run lies in the western end of a non volcanic Outer Banda Belt. off water occured when meteoric water relatively small or The belt is built up from small islands, such as Buru, Seram, can not penetrate into the rocks and will flow as run off Tanimbar, Timor and Sumba Islands, that strech along waters or into the river. Banda sea. As Buru island is not part of a volcanic belt, Waesalit hot waters occured along Pemali river or between therefore the Waesalit geothermal prospect is considered to a discharge area and run off water. The meteoric water, that be associated with tectonic activity and or a cancealed penetrate into the rocks via primery and or secondary volcanic intrusive body (vulcano-tectonic). The assumption permeability of rocks, and then the waters is heated up by is supported by geology, and geophysics data as mention heat source via conductor rocks by convection and or in section 4.1. Both activity caused the occurence of faults conduction, after that it goes up to the surface as hot waters. and joints in the area. The present of faults structures such Waekedang and Waemetar faults, that is trending nearly E- 5.1.3 Geothermal system W and NE-SW respectively, control the manifestation to the surface. The cross cut structures and metamorphic foliation The present of silica sinters and sulphur deposit around the that open when the stress realeased caused the hot fluids manifestation, and supported also by geochemical data as goes up to the surface and so hot ground and altered rocks mention previously in section 3., therefore Waesalit hot occured. The present of altered zone around Waesalit hot water is up flow type, whilst the geothermal system in the water is supported by low geomagnetic value around the hot area is considered to be hot water dominated system. water as mention previously. All those conditions are a good indicators for the development of geothermal sytem in the 5.1.4 Clay cap Waesalit area. Therefore, based on geoscientific data, the structural traps of a non volcanic geothermal field in the area The clay cap is considered to be altered metamorphic and or consists of hydrogeology, geothermal system, clay cap, altered cancealed volcanic intrusif body as a result of reservoir, and heat source (figure 9). contact between hot fluids and those both rocks. Figure 7 showed a clay cap is found at the depth of 850m below the 5.1.2 Hydrogeology observed point (D-4150) with the resisitivity value

Hydrogeology of the area is classified into three category, such: recharge, discharge areas and run off waters

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Figure 9: Cartoon Model of Waesalit geothermal sytem (modified from PMG 2006)

of 20 ohm-m. Geomagnetic data also indicated very low The Waesalit hot waters are charcterized by moderate to geomagnetic anomaly occured around the manifestation. high gravity and low geomagnetic values, whilst the Low geomagnetic value around waesalit hot water is resistivity data show the resitivity value around the hot associated with demagnetised rocks in the area. The type of water is relatively high ( 20 – 110 ohm-m) compare to the alteration rocks is considered to be the same as the type of resistivity value of a volcanic geothermal system (10 – 20 altered zone found in the surface ( advance argillic and om-m). Based on geophysical data, the structural trap of a phyllic ?). non vulcanic hosted geothermal field in the area is relatively similar to a volcanic hosted geothermal area, whilst the 5.1.5 Reservoir heat source ( concealed volcanic intrusive body) is considered to be associated to volcano-tectonic process on A reservoir is aplace where accumulate fluids are heated up metamorphic rock. by heat source. Resevoir occured as a result of tectonic processes, inwhich joints, cracks and faults (Waekedang REFERENCES and Waemetar faults) that acts as secondary reservoir Anderson, F., Crosby, D. and Ussher, G.,: Bulls-Eye – permeabilities, developed during the time. Based on Simple Resistivity Imaging to Reliably Locate the resistivity data the top of reservoir layer with the resisitivity Geothermal Reservoir, Proceeding, World Geothermal value of 125 ohm-m exist at the depth of > 1000 m below Congress, May 28 - June 10, 2000, Kyushu-Tohuku, p. D-4150 (figure 7) and the thickness of reservoir is unknown 901-914. as the resistivity data is only identified the top of reservoir. The figure is also showed the resorvoir zone occured at Badan Standardisasi Nasional: Klasifikasi Potensi Energi semi conductive area on resistivity map (figure 6). The Panasbumi di Indonesia, Standar Nasional Indonesia, reservoir rock is considered to be metamorphicand rocks SNI 13-5012-1998, ICS 73.0202, Dit. Jend. Geologi and or concealed intrusive volcanic body that has a good dan Sumberdaya Mineral, Departemen Pertambangan secondary and or primary permeabilities. dan Energi, Indonesia, 14 halaman.

5.1.6 Heat sources Badan Standardisasi Nasional: Metode Estimasi Potensi Energi Panasbumi, StandarNasional Indonesia, SNI 13- Waesalit hot waters occured at Permian age of metamorphic 6171-1999, ICS 73.020, Dit.Jend. Geologi dan environment rocks, inwhich the rocks can not be acted as a Sumberdaya Mineral, Departemen Pertambangan dan heat source. Based on geophysical data there is an indication Energi, Indonesia, 11 halaman of intrusive body underneath. The present of intrusive body is also supported by the present of sulphur deposit around the Bemmelen, R.W. van, 1949,: The Geology of Indonesia, Vol manifestations. All those data indicates that the heat source IA, Netherlands, The Hague. is considered to be a concealed intrusive volcanic body? 6. CONCLUSION 7 Idral

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