We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to be... more We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to better understand their roles in controlling the Muara Laboh geothermal system. The oldest rocks in the Muara Laboh region are the metamorphic Paleozoic Barisan Formation. Late Oligocene to Middle Miocene volcanism is represented by the Painan Formation, consisting of mixed volcanic and sedimentary rocks of andesitic to dacitic composition. In the Middle Miocene, granitic and granodioritic rocks intruded the Barisan and Painan Formations. Undifferentiated Silicic Volcanic and volcaniclastic rocks consisting dominantly of dacitic to rhyolitic tuff and sediments, are widely distributed northwest, west and southwest of Muara Laboh. Evidence from exploration wells indicates that this rock sequence is present mostly in the western Muara Laboh basin. In the eastern Muara Laboh Basin Andesitic Volcanics were erupted to the SE of Muara Laboh at about the same time. These sequences are overlain by ...
The purpose of this paper is to evaluate the likelihood of outflow of geothermal water to shoreli... more The purpose of this paper is to evaluate the likelihood of outflow of geothermal water to shoreline hot springs at Rajabasa and the implications of this outflow to conceptual models. Two different models have been proposed to explain the chemistry of the Gunung Botak shoreline hot spring; a mixed seawater – geothermal water and purely steam – heated seawater. The steam – heated seawater model is based solely upon a similarity in Cl/B ratios between the springs and seawater, which are much higher than in the usual liquid geothermal system. Hot springs distant from the shoreline show high Cl/B ratio, however, suggesting that this is a characteristic of the system. The model for a mixed seawater – geothermal water origin proposes an elaborate model of boiling, seawater mixing and repeated boiling, which is testable by modelling fumarole gas chemistry. It was found that the majority of the gas chemistry of the inland Way Merak and seaside Gunung Botak fumaroles can be matched with a boi...
Proceedings 41st New Zealand Geothermal Workshop, 2019
Thick silicic tuff sequences and silicic intrusives are found in deep wells drilled at the Muara ... more Thick silicic tuff sequences and silicic intrusives are found in deep wells drilled at the Muara Laboh and Rantau Dedap Geothermal Fields, Sumatra, Indonesia. Petrographic and petrophysical investigation of cuttings, core, gamma ray and image logs were used to understand the silicic stratigraphic controls on reservoir permeability. Regional geologic constraints and limited U-Pb zircon ages show that silicic explosive volcanism most likely occurred mainly during the Miocene to Plio-Pleistocene, and locally continuing into the Holocene. Wells in both fields show that silicic tuff sequences reach thicknesses of 500 to >1000 meters, and silicic intrusive complexes intrudes to reservoir depths. In Muara Laboh, thick silicic tuffs are found in a basin generated between major strike-slip fault segments. The dominant rock type is variably welded to non-welded silicic ash-flow tuff with a variety of devitrification textures. A long-lived sheared and altered granite-granodiorite-microdiorite intrusive complex representing multiple magmatic episodes (96 to 20 Ma) occurs in the SW sector of the field. In Rantau Dedap, similar silicic tuff sequences are found as caldera fill deposits overlain by debris flows. A weakly altered, poorly deformed granite to granodiorite intrusive complex occurs at depth. The silicic volcanics have high resistivity in image logs, fine fragmental textures (tuffs) to massive textures (intrusives), and high gamma ray counts (65 to 200 API) in both fields. Fracture intensity of the thick silicic tuff sequences increases with welding, primary devitrification, and possibly the thickness and cooling history of individual eruptive units. Major feed zones are associated with faulted lithological contacts and very limited at thick silicic tuffs in Muara Laboh. Fluid entries at Rantau Dedap are most abundant in the relatively thin Upper Rhyolite Tuff and underlying Dacite Tuff, and near the basal contact of the thick Lower Dacite Tuffs with intrusions. Permeable zones are encountered at the margin of the intrusive complexes in both fields.
Proceedings The 4th Indonesia International Geothermal Convention and Exhibition, 2016
The occurrence of high temperature geothermal system in Gunung Rajabasa, Lampung, is indicated by... more The occurrence of high temperature geothermal system in Gunung Rajabasa, Lampung, is indicated by numerous fumaroles and associated boiling SO 4-hotsprings. Surface geologic data provides general framework which simultaneously complements and constrains the possible geophysics-and chemistry-based resource conceptual models. A comprehensive yet interpretative geologic model has been built based on surface geologic data and geophysical models. The structural setting consists of NW-SE trend parallel to the Sumatra Fault Zone (SFZ), and more subtle N-S trend similar to the Sunda Strait structural domain which represents the southern end of SFZ. The regional stratigraphy is closely related to the Rajabasa Caldera and its wide spread ignimbrite products known as Lampung Formation, while the Quaternary Gunung Rajabasa Complex is interpreted as post-caldera volcanism. Those caldera-related volcanic products overlay the Tertiary andesite exposed in the SE edges of the caldera, possible Miocene sediments, and older metasediment rocks. The volcanostratigraphic interpretation reveals the relative age and eruption center migration, which might be related to the evolution of hydrothermal system. The metasediment basement and Miocene sediments, syn-caldera ignimbrite, and Quaternary volcanic pile might control porosity distribution as well as reservoir thickness. NW-SE trending faults control the resource area boundaries, fluid pathways, and potential resource compartmentalization, while the N-S trending faults control the deeper basement configuration.
The purpose of this paper is to evaluate the likelihood of outflow of geothermal water to shoreli... more The purpose of this paper is to evaluate the likelihood of outflow of geothermal water to shoreline hot springs at Rajabasa and the implications of this outflow to conceptual models. Two different models have been proposed to explain the chemistry of the Gunung Botak shoreline hot spring; a mixed seawater-geothermal water and purely steam-heated seawater. The steam-heated seawater model is based solely upon a similarity in Cl/B ratios between the springs and seawater, which are much higher than in the usual liquid geothermal system. Hot springs distant from the shoreline show high Cl/B ratio, however, suggesting that this is a characteristic of the system. The model for a mixed seawater-geothermal water origin proposes an elaborate model of boiling, seawater mixing and repeated boiling, which is testable by modelling fumarole gas chemistry. It was found that the majority of the gas chemistry of the inland Way Merak and seaside Gunung Botak fumaroles can be matched with a boiling-mixing-boiling model, but with surpluses of hydrogen and ammonia at Gunung Botak. The hydrogen surplus appears to be due to regeneration after first boiling, whereas the ammonia surplus remains unexplained. In any case, the modelling supports the existence of a geothermal component in the Gunung Botak hot springs, consistent with the expectation of shoreline hot springs at an onshore liquid geothermal system. This information refines the conceptual model of a liquid dominated system which has upflow just north of the high elevation Pangkul fumaroles then flows to the southward to the Way Merak fumaroles and Gunung Botak hotsprings and fumaroles.
We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to be... more We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to better understand their roles in controlling the Muara Laboh geothermal system. The oldest rocks in the Muara Laboh region are the metamorphic Paleozoic Barisan Formation. Late Oligocene to Middle Miocene volcanism is represented by the Painan Formation, consisting of mixed volcanic and sedimentary rocks of andesitic to dacitic composition. In the Middle Miocene, granitic and granodioritic rocks intruded the Barisan and Painan Formations. Undifferentiated Silicic Volcanic and volcaniclastic rocks consisting dominantly of dacitic to rhyolitic tuff and sediments, are widely distributed northwest, west and southwest of Muara Laboh. Evidence from exploration wells indicates that this rock sequence is present mostly in the western Muara Laboh basin. In the eastern Muara Laboh Basin Andesitic Volcanics were erupted to the SE of Muara Laboh at about the same time. These sequences are overlain by Quaternary andesitic volcanics over almost the entire Muara Laboh area. These products come from several eruption centers respectively from the NW to SE including Mt. Patah Sembilan and Mt. Anak Patah Sembilan. Flow patterns are generally from the eruption centers along the Siulak fault in the south towards the north. Field geologic mapping indicates that all of these volcanoes are composed dominantly of andesitic rocks and consist mainly of lava, tuff, breccia, lahar, and debris flow deposits. The more distal deposits consist dominantly of volcaniclastic equivalents of the eruptive products. The most recent volcanic deposits at Muara Laboh consist of andesitic to dacitic tuffs and debris flows. Carbon in the tuffs yielded ages of ~34 to 41 ka. Debris flows underlying the tuffs may be related to sector collapse and debris avalanches from Patah Sembilan crater, providing a likely minimum age of this event. The Muara Laboh geothermal system is situated within a pull apart basin along the NW-SE Great Sumatera Fault (GSF). About 8 km north of the prospect, the Suliti Fault segment juxtaposes uplifted metamorphic basement with young basin fill deposits. South of Muara Laboh, the Siulak Fault segment has accommodated the magmatic intrusion that provides geothermal heat sources. Cross sections constrained by mapping, gravity, and well data indicate two main grabens in Muara Laboh i.e. West Muara Laboh Basin and East Muara Laboh Basin. The proposed model is for an asymmetric basin system with a narrower and deeper western basin along the Siulak master fault, and a shallower but wider eastern basin within the main step over fault structure. Analysis of structures at surface and in borehole image logs show dominant trends of open fractures are N-S, NW-SE, and NE-SW. The NW-SE fracture trend is associated with the GSF and based on image logs is important in the deeper section of exploration wells (H pad well), while surface mapping found this orientation in areas near the main GSF (Suliti and Siulak Fault segments). The N-S structural trend is considered to be the step over fault trend, and associated with the pull apart basin structures generating a horst and graben system. This N-S set corresponds with extensional fractures that are interpreted as the most important in controlling permeability, fluid flow, and thermal discharges in Muara Laboh geothermal system. The NE-SW fault trend is interpreted to be antithetic to the GSF and is considered to be the youngest structure based on the field mapping data. The image log data also support this interpretation because NE-SW fractures are more abundant at shallower depth. An extensive clay cap overlies the Muara Laboh geothermal system and its outflow area, but thick clay on the eastern flank of the system is in part related to basin fill deposits. The commercial reservoir top conforms to the base of the conductor best near Idung Mancung fumarole, where it hosts a 240°C steam cap. This zone appears to have recently been heated and host quartz, wairakite and prehnite veins open space texture. Some of the edges of the MT conductor have also been shown to approximately parallel the traces of inferred faults. The top of the propylitic alteration zone does not typically conform to the base of the conductor because a zone of transitional alteration defined by mixed-layer clay and chlorite underlies the smectite cap that corresponds with resistivity of ≤ 7 ohm-m. The highest deep permeability in the area is associated with epidote-adularia veins with open space textures that produce fluid at ≥ 270°C, but permeability is locally reduced by late-stage infilling by calcite, quartz, and prehnite.
We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to be... more We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to better understand their roles in controlling the Muara Laboh geothermal system. The oldest rocks in the Muara Laboh region are the metamorphic Paleozoic Barisan Formation. Late Oligocene to Middle Miocene volcanism is represented by the Painan Formation, consisting of mixed volcanic and sedimentary rocks of andesitic to dacitic composition. In the Middle Miocene, granitic and granodioritic rocks intruded the Barisan and Painan Formations. Undifferentiated Silicic Volcanic and volcaniclastic rocks consisting dominantly of dacitic to rhyolitic tuff and sediments, are widely distributed northwest, west and southwest of Muara Laboh. Evidence from exploration wells indicates that this rock sequence is present mostly in the western Muara Laboh basin. In the eastern Muara Laboh Basin Andesitic Volcanics were erupted to the SE of Muara Laboh at about the same time. These sequences are overlain by ...
The purpose of this paper is to evaluate the likelihood of outflow of geothermal water to shoreli... more The purpose of this paper is to evaluate the likelihood of outflow of geothermal water to shoreline hot springs at Rajabasa and the implications of this outflow to conceptual models. Two different models have been proposed to explain the chemistry of the Gunung Botak shoreline hot spring; a mixed seawater – geothermal water and purely steam – heated seawater. The steam – heated seawater model is based solely upon a similarity in Cl/B ratios between the springs and seawater, which are much higher than in the usual liquid geothermal system. Hot springs distant from the shoreline show high Cl/B ratio, however, suggesting that this is a characteristic of the system. The model for a mixed seawater – geothermal water origin proposes an elaborate model of boiling, seawater mixing and repeated boiling, which is testable by modelling fumarole gas chemistry. It was found that the majority of the gas chemistry of the inland Way Merak and seaside Gunung Botak fumaroles can be matched with a boi...
Proceedings 41st New Zealand Geothermal Workshop, 2019
Thick silicic tuff sequences and silicic intrusives are found in deep wells drilled at the Muara ... more Thick silicic tuff sequences and silicic intrusives are found in deep wells drilled at the Muara Laboh and Rantau Dedap Geothermal Fields, Sumatra, Indonesia. Petrographic and petrophysical investigation of cuttings, core, gamma ray and image logs were used to understand the silicic stratigraphic controls on reservoir permeability. Regional geologic constraints and limited U-Pb zircon ages show that silicic explosive volcanism most likely occurred mainly during the Miocene to Plio-Pleistocene, and locally continuing into the Holocene. Wells in both fields show that silicic tuff sequences reach thicknesses of 500 to >1000 meters, and silicic intrusive complexes intrudes to reservoir depths. In Muara Laboh, thick silicic tuffs are found in a basin generated between major strike-slip fault segments. The dominant rock type is variably welded to non-welded silicic ash-flow tuff with a variety of devitrification textures. A long-lived sheared and altered granite-granodiorite-microdiorite intrusive complex representing multiple magmatic episodes (96 to 20 Ma) occurs in the SW sector of the field. In Rantau Dedap, similar silicic tuff sequences are found as caldera fill deposits overlain by debris flows. A weakly altered, poorly deformed granite to granodiorite intrusive complex occurs at depth. The silicic volcanics have high resistivity in image logs, fine fragmental textures (tuffs) to massive textures (intrusives), and high gamma ray counts (65 to 200 API) in both fields. Fracture intensity of the thick silicic tuff sequences increases with welding, primary devitrification, and possibly the thickness and cooling history of individual eruptive units. Major feed zones are associated with faulted lithological contacts and very limited at thick silicic tuffs in Muara Laboh. Fluid entries at Rantau Dedap are most abundant in the relatively thin Upper Rhyolite Tuff and underlying Dacite Tuff, and near the basal contact of the thick Lower Dacite Tuffs with intrusions. Permeable zones are encountered at the margin of the intrusive complexes in both fields.
Proceedings The 4th Indonesia International Geothermal Convention and Exhibition, 2016
The occurrence of high temperature geothermal system in Gunung Rajabasa, Lampung, is indicated by... more The occurrence of high temperature geothermal system in Gunung Rajabasa, Lampung, is indicated by numerous fumaroles and associated boiling SO 4-hotsprings. Surface geologic data provides general framework which simultaneously complements and constrains the possible geophysics-and chemistry-based resource conceptual models. A comprehensive yet interpretative geologic model has been built based on surface geologic data and geophysical models. The structural setting consists of NW-SE trend parallel to the Sumatra Fault Zone (SFZ), and more subtle N-S trend similar to the Sunda Strait structural domain which represents the southern end of SFZ. The regional stratigraphy is closely related to the Rajabasa Caldera and its wide spread ignimbrite products known as Lampung Formation, while the Quaternary Gunung Rajabasa Complex is interpreted as post-caldera volcanism. Those caldera-related volcanic products overlay the Tertiary andesite exposed in the SE edges of the caldera, possible Miocene sediments, and older metasediment rocks. The volcanostratigraphic interpretation reveals the relative age and eruption center migration, which might be related to the evolution of hydrothermal system. The metasediment basement and Miocene sediments, syn-caldera ignimbrite, and Quaternary volcanic pile might control porosity distribution as well as reservoir thickness. NW-SE trending faults control the resource area boundaries, fluid pathways, and potential resource compartmentalization, while the N-S trending faults control the deeper basement configuration.
The purpose of this paper is to evaluate the likelihood of outflow of geothermal water to shoreli... more The purpose of this paper is to evaluate the likelihood of outflow of geothermal water to shoreline hot springs at Rajabasa and the implications of this outflow to conceptual models. Two different models have been proposed to explain the chemistry of the Gunung Botak shoreline hot spring; a mixed seawater-geothermal water and purely steam-heated seawater. The steam-heated seawater model is based solely upon a similarity in Cl/B ratios between the springs and seawater, which are much higher than in the usual liquid geothermal system. Hot springs distant from the shoreline show high Cl/B ratio, however, suggesting that this is a characteristic of the system. The model for a mixed seawater-geothermal water origin proposes an elaborate model of boiling, seawater mixing and repeated boiling, which is testable by modelling fumarole gas chemistry. It was found that the majority of the gas chemistry of the inland Way Merak and seaside Gunung Botak fumaroles can be matched with a boiling-mixing-boiling model, but with surpluses of hydrogen and ammonia at Gunung Botak. The hydrogen surplus appears to be due to regeneration after first boiling, whereas the ammonia surplus remains unexplained. In any case, the modelling supports the existence of a geothermal component in the Gunung Botak hot springs, consistent with the expectation of shoreline hot springs at an onshore liquid geothermal system. This information refines the conceptual model of a liquid dominated system which has upflow just north of the high elevation Pangkul fumaroles then flows to the southward to the Way Merak fumaroles and Gunung Botak hotsprings and fumaroles.
We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to be... more We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to better understand their roles in controlling the Muara Laboh geothermal system. The oldest rocks in the Muara Laboh region are the metamorphic Paleozoic Barisan Formation. Late Oligocene to Middle Miocene volcanism is represented by the Painan Formation, consisting of mixed volcanic and sedimentary rocks of andesitic to dacitic composition. In the Middle Miocene, granitic and granodioritic rocks intruded the Barisan and Painan Formations. Undifferentiated Silicic Volcanic and volcaniclastic rocks consisting dominantly of dacitic to rhyolitic tuff and sediments, are widely distributed northwest, west and southwest of Muara Laboh. Evidence from exploration wells indicates that this rock sequence is present mostly in the western Muara Laboh basin. In the eastern Muara Laboh Basin Andesitic Volcanics were erupted to the SE of Muara Laboh at about the same time. These sequences are overlain by Quaternary andesitic volcanics over almost the entire Muara Laboh area. These products come from several eruption centers respectively from the NW to SE including Mt. Patah Sembilan and Mt. Anak Patah Sembilan. Flow patterns are generally from the eruption centers along the Siulak fault in the south towards the north. Field geologic mapping indicates that all of these volcanoes are composed dominantly of andesitic rocks and consist mainly of lava, tuff, breccia, lahar, and debris flow deposits. The more distal deposits consist dominantly of volcaniclastic equivalents of the eruptive products. The most recent volcanic deposits at Muara Laboh consist of andesitic to dacitic tuffs and debris flows. Carbon in the tuffs yielded ages of ~34 to 41 ka. Debris flows underlying the tuffs may be related to sector collapse and debris avalanches from Patah Sembilan crater, providing a likely minimum age of this event. The Muara Laboh geothermal system is situated within a pull apart basin along the NW-SE Great Sumatera Fault (GSF). About 8 km north of the prospect, the Suliti Fault segment juxtaposes uplifted metamorphic basement with young basin fill deposits. South of Muara Laboh, the Siulak Fault segment has accommodated the magmatic intrusion that provides geothermal heat sources. Cross sections constrained by mapping, gravity, and well data indicate two main grabens in Muara Laboh i.e. West Muara Laboh Basin and East Muara Laboh Basin. The proposed model is for an asymmetric basin system with a narrower and deeper western basin along the Siulak master fault, and a shallower but wider eastern basin within the main step over fault structure. Analysis of structures at surface and in borehole image logs show dominant trends of open fractures are N-S, NW-SE, and NE-SW. The NW-SE fracture trend is associated with the GSF and based on image logs is important in the deeper section of exploration wells (H pad well), while surface mapping found this orientation in areas near the main GSF (Suliti and Siulak Fault segments). The N-S structural trend is considered to be the step over fault trend, and associated with the pull apart basin structures generating a horst and graben system. This N-S set corresponds with extensional fractures that are interpreted as the most important in controlling permeability, fluid flow, and thermal discharges in Muara Laboh geothermal system. The NE-SW fault trend is interpreted to be antithetic to the GSF and is considered to be the youngest structure based on the field mapping data. The image log data also support this interpretation because NE-SW fractures are more abundant at shallower depth. An extensive clay cap overlies the Muara Laboh geothermal system and its outflow area, but thick clay on the eastern flank of the system is in part related to basin fill deposits. The commercial reservoir top conforms to the base of the conductor best near Idung Mancung fumarole, where it hosts a 240°C steam cap. This zone appears to have recently been heated and host quartz, wairakite and prehnite veins open space texture. Some of the edges of the MT conductor have also been shown to approximately parallel the traces of inferred faults. The top of the propylitic alteration zone does not typically conform to the base of the conductor because a zone of transitional alteration defined by mixed-layer clay and chlorite underlies the smectite cap that corresponds with resistivity of ≤ 7 ohm-m. The highest deep permeability in the area is associated with epidote-adularia veins with open space textures that produce fluid at ≥ 270°C, but permeability is locally reduced by late-stage infilling by calcite, quartz, and prehnite.
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