Hydrothermal Alteration of SMN-X,Sumani Geothermal Area, West Sumatra, Indonesia Muhammad Ghassan Jazmi Shalihin 1, Euis Tintin Yuningsih 1, Aton Patonah 1, IldremSyafrie 1 Yuano Rezky 2 1 Faculty of Geology, Universitas Padjadjaran Jalan Raya Bandung-Sumedang Km.21, Jatinangor, Indonesia 2 Kelompok Kerja Panas Bumi, Pusat Sumber Daya Geologi, Badan Geologi Jalan Soekarno Hatta 444, Bandung, Indonesia Email: ghassan105.geounpad10@gmail.com Abstract SMN-X is gradient thermal well located in Solok District, West Sumatra. Stratigrapic unit of SMN-X well are divided into tuff, pyroclastic breccia, andesitic lava, andesitic lava breccia, basaltic lava and basaltic lava breccia. Each lithology is experienced hydrothermal alteration process with various intensity from weak to strong due the change of temperature and chemical environment. Hydrothermal alteration occurs at 1.80 meter depth to the end of borehole at a depth of 700 meter. Based on alteration mineral assemblage, the alteration zonationin SMN-X are divided into three alteration zones, smectite-zeolite zone, zeolite-chlorite-smectite zone and illite-sericite zone which are equal with the alteration types of argillic, sub-propylitic and phyllic. The comparison of paleo-temperature obtained from the occurence of alteration mineral with present well temperature measurement, conclude that temperature on the well has been decreased during time. Paleo-temperature showed the minerals that has the lowest temperature is smectite and zeolite (chabazite, heulandite, mordenite, stilbite) with temperature range from 40 o to 150 o C and minerals that has the highest temperature is illite and sericite with temperature range 200 o -250 o C, while the present temperature measurement showed that the lowest temperature (surface)is ±26 o C and the highest temperature at the bottom of borehole is reached 100.3 o C after 24 hours immersion. This temperature decrease probably caused by development of geological structure at the research area which resulted a large space for heat to migrate at the geothermal system. Keywords : Geothermal, hydrothermal alteration, paleotemperature Introduction Sumani area is located about 60 Km from northeast of Padang, the capital city of West Sumatra Province ( Figure 1). According to geology, geochemistry and geophysic survey by PSDG (2011), this area has geothermal potential characterized by geothermal manifestation such as hot spring waters. The studies also shown the anomalies which indicate the existence of a geothermal potential. SMN-X well were drilled to investigate subsurface condition and emphasize geothermal prospect in this area. Geothermal system at the Sumani area suggested to be related with the last volcanic activity in the Quartenary (PSDG, 2011). This volcanic activity is predicted still keep the heat from magma chamber as a heat source which heating up the fluids on the geothermal reservoir (PSDG, 2011). Furthermore, Sumani area is located at the Great Sumatran Fault zone which resulted many geological structure such asfaults or fractures in this area (PSDG, 2011). Figure 1 Location map of Sumani Geothermal Area Borehole data from SMN-X well were used for this study. Those consist of core samples from 1.80to 700 m depth with analyses of general 265
lithology description, petrography and specterra analysis, which is then correlated with temperature measurement of wells. This study mainly a detailed study of hydrothermal alteration minerals on SMN-X well. Hydrothermal alteration minerals are very useful for geothermal exploration geothermometry as well as assesing the subsurface fluid ph. Stratigraphy Based on megascopic andmicrospic analysis, the lithology on SMN-X well is composed of basaltic lava, andesitic lava with basalt, tuff and pyroclastic breccia at the bottom, then sequently pyroclastic breccia, basaltic lava, tuff, andesitic lava, pyroclastic breccia in the surface layer.each lithology has altered with various intensity from weak to strong due the change of temperature and chemical environment (Table 1). Table 1 Percentageand intensity of alteration mineralsmn-x well SM/TM Alteration Dept (m) Lithology (%) Intesity 0-100 100-200 200-300 300-400 400-500 500-600 Tuff, Andesitic Lava, Basaltic Lava, Basaltic Lava, Tuff, Andesitic Lava, Basaltic Lava, 3-50 15-65 Weak - Moderate Weak Moderare 2-68 Weak Strong 22-61 Moderate Strong 2-67 Weak Strong 15-75 Weak Strong 600-700 Basaltic Lava 38-48 Moderate Alteration Zone Based on alteration mineral assemblage, the lithology in SMN-X divided into three alteration zones, smectite-zeolite zone, zeolite-chloritesmectite zone and illite-sericite zone. These are equal with the alteration type of argillic (clay cap), sub-propylitic and phyllic ( predicted as transisition layer, clay cap reservoir). -Zeolite-Zone -zeolite zones are present at intervals of 1.80 to 105.50 m depth. It is alteration product of tuff, andesitic lava, basaltic lava and pyroclastic breccia. This zone is characterized by smectite abundance ( Figures2 & 3) with zeolite, secondary quartz, calcite and hematite.is included inargillic type (Corbett and Leach, 1998). formed in the fluids with ph 5-6, temperatures from 50 o to 150 o C andbelongs to illite group. Meanwhile, zeolite formed in the fluids with neutral-alkaline ph and wide range temperature because zeolite is temperature sensitive mineral. The type of zeolitepresent at this zone are low temperature, characterized byoccurrence of chabazite and stillbite minerals with temperatures range from 40 o to 120 o C. Clay Iron Oxide Figure 2 Core sample at 23.85 24.00 m depth with clay minerals in the veins and matrix which has been altered to hematite Figure 3 Spectral analysis using specmin ASD-Database at 7.85 m depth which shows the presences of smectite (x axis: wavelength 350-2500nm; y axis:different reflectance for each mineral) 266
Zeolite-Chorite- Zone Zeolite-chlorite-smectite zones are present at intervals 105.50 to 522.00 m depth. It is alteration of tuff, andesitic lava, basaltic lava and pyroclastic breccia. This zone is characterized by zeolite abundance with chlorite and smectite (Figure 4 & 5), illite, secondary quartz, calcite and hematite. Is included by sub-propylitic type (Corbett and Leach, 1998). Zeolite formed in the fluids with ph neutralalkaline. It has wide temperature, from low to moderate temperature and characterized by present of chabazite, stilbite, mordenite, heulandite and laumontite mineral with range temperatures from 40 o to 150 o C. formed in the fluid with ph neutral and temperature >120 o C and belongs to chlorite group, meanwhile smectite formed in the fluids with ph 5-6 with range temperatures from 50 o to 150 o C. Zeolite Illite-Sericite Zone Illite-sericite zones are present at intervals 522.00 to 700.00 m depth. It is alteration of andesitic lava, basaltic lava and pyroclastic breccia. This zone is characterized by minerals illite and sericite ( Figure 6, 7 & 8) withchlorite, smectite, secondary quartz, calcite and hematite. Is included byphyllic type (Corbett and Leach, 1998). Illite and sericite are formed in the fluids with ph 5-6, with illite formed with range temperatures <200 o -250 o C and sericite with temperatures to about>250 o C,both of this minerals belongs to illite group. 0,2 mm // nikol Figure 6Petrographic analysis at 529.80 m depth which dominated by sericite and clay mineral (illite), with chlorite, secondary quartz and iron oxide 0,2 mm S Sericite I Illite x nikol 1 mm 0,5 mm // nikol x nikol Figure 4Petrographic analysis at 343.80 m depth which dominated by zeolite, chlorite and clay mineral (smectite), with a bit of secondary quartz and opaque minerals Sericite Zeolite Figure 7 Spectral analysis using specmin ASD-Database at 628 m depth which shows the presences of sericite (x axis: wavelength 350-2500nm; y axis:different reflectance for each mineral) Figure 5 Spectral analysis using specmin ASD-Database at 134 m depth which shows the presences of zeolite, chlorite and smectite (x axis: wavelength 350-2500nm; y axis:different reflectance for each mineral) 267
Figure 8 Spectral analysis using specmin ASD-Database at 676.30 m depth which shows the presences of illite (x axis: wavelength 350-2500nm; y axis:different reflectance for each mineral) SMN-XWell Temperature The SMN-X well temperature estimation is conduction from temperature data of alteration minerals and the direct present temperature measurement from logging activities. Paleotemperature Paleotemperature obtained from the occurence of alteration mineral assemblage.there are three different zones such as -Zeolite Zone with temperature range from 40 o to 150 o C (Table 2), Zeolite-- Zone with temperature range >40 o to 220 o C (Table 3) and Illite-Sericite Zone with temperature range from 150 o to >200 C (Table 4). Table 2 forming temperatureof smectite-zeolite zones(brown, 1999 and Reyes, 2000) Temperature ( 0 C) Alteration Chabazite Stilbite Illite 50 100 150 200 250 300 Table 3 forming temperature of zeolite-chloritesmectite zones(brown, 1999 and Reyes, 2000) Alteration Chabazite Laumontite Mordenite Heulandite Stilbite Table 4 forming temperatureof illite-sericite zones(reyes, 2000) Alteration Illite Sericite Temperature ( 0 C) 50 100 150 200 250 300 Temperature ( 0 C) 50 100 150 200 250 300 Subsurface Temperature Present well temperature SMN-X obtained from direct measurement using logging tool. Measurement were taken on a four phases at 150 m, 315 m, 500 m and 700 m depth. The result of measurement on logging temperature in the SMN-X shows the temperature increasing with depth. On the first phases at 150 m depth, surface temperature showed the temperature of 16.7 o C and after immersion at the bottom of borehole (150 m) tem perature increase to reach 54.4 o C. On the second phases at 315 m depth, surface temperature showed the temperature of 28.8 o C and after immersion at the bottom of borehole (315 m) temperature increase to reach 68.8 o C. On the third phases at 500 m depth, surface temperature showed the temperature of 24.4 o C and after immersion at the bottom of borehole (500 m) temperature increase to reach 79 o C. And the last phases at 700 m depth, surface temperature showed the temperature of 27.4 o C and after immersion at the bottom of borehole (700 m) temperature increase to reach 100.3 o C. 268
Correlation of Paleotemperature With Present Temperature Measurement Correlation of paleo- and present temperatures on the well is conducted to determination the condition of SMN-X. Paleotemperature showed the mineral with lowest temperature are smectite and zeolite (chabazite, heulandite, mordenite, stilbite).they have temperature range from 40 o to 140 o C. On the other hand, mineral with the highest temperature are illite and sericite with temperature range >200 o - 250 o C. Thedirect measurement on well temperature showed that the lowest temperature (at surface) is ±26 o C and the highest temperature is up to 100.3 o C. Thus it is concluded that there has been decreasing of temperature on the well during the time ( Figure 9). The phenomena probably caused by geological structure formed on Sumani area that resulting large space for heat to migrate in the geothermal system. Conclusion Stratigrapic unit of SMN-X well consist of tuff, pyroclastic breccia, andesitic lava, andesitic lava breccia, basaltic lava and basaltic lava breccia. Each lithology has altered with various intensity from weak to strong. Hydrothermal alteration minerals of SMN-X well consist of smectite, illite, sericite, zeolite and chlorite. alteration assemblage in this well was formed in under 5-neutral ph fluid condition. Based on mineral alteration assemblage, there are three alteration zones, smectite-zeolite zone, zeolite-chlorite-smectite zone and illitesericite zone. These are equal with the alteration type of argillic (clay cap), transisition layer sub-propylitic and phyllic (predicted as transisition layer, clay cap reservoir). Comparison between paleo-temperature with the present temperature measurement showed that there has been decreasing temperature on the well. This phenomena most probably caused by development of geological structure on Sumani area which resulted heat to migrate in the geothermal system. Acknowledgments I would like to show my gratitude to Pusat Sumber Daya Geologi that has given me the opportunity publishing this paper. And all the people who provided insight and expertise that greatly assisted the writing process then always encouraging and supported.. References Figure 9 Paleotemperature and present temperature measurement comparison of SMN-X Browne, P.R.L. (1999)Hydrothermal Alteration. Lecture Handout. The University of Auckland, Geothermal Institute., 32-35 Corbett & Leach. (1997)Southwest Pasific Rim Gold-Copper System: Structure, Alteration and ization. Short Course Manual, Society of Economics Geologist, Inc., 68-73 269
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