More than 300 hot spring areas are located along the Chilean Andes, associated with Quaternary vo... more More than 300 hot spring areas are located along the Chilean Andes, associated with Quaternary volcanism. Systematic geothermal exploration in the northern part of the country started by the end of 1968, followed by basic geological, geochemical and volcanological surveys in many geothermal areas of southern Chile. These studies, together with wells drilled in El Tatio and Puchuldiza (northern Chile), allowed a preliminary assessment of the geothermal potential of the country of several thousands of MW. Renewed interest in geothermal exploration in Chile has been encouraged by the enactment of the Geothermal Law in 2000, which governs the regulatory aspects of the exploration and exploitation concessions, and also by the uncertainties regarding the imports of natural gas from Argentina, the vulnerability of hydropower to periodic droughts, and the environmental cost associated with hydropower and fossil fuels. Recently, a 4-year-long geothermal research project (2000 – 2003) was car...
Andesites are minor rock types in the Taupo Volcanic Zone (TVZ) and most occur near the Tongariro... more Andesites are minor rock types in the Taupo Volcanic Zone (TVZ) and most occur near the Tongariro Volcanic Centre (TVC), White Island, Putuaki and Rolles Peak near Rotokawa. However, several subsurface andesites have also been intersected by wells drilled at the Ngatamariki, Rotokawa, Wairakei, Waiotapu, Kawerau and Tauhara geothermal fields. The Spa Andesite at Tauhara, at least 201 meters thick, comprises interbedded lavas and breccias between 390 and 591 m depth. It was intersected only in well THM18 in the Spa Bowl area. Some of rocks are intensely altered. This study characterizes the Spa Andesite and compares it with other andesites within the TVZ. Twelve cores were chosen to cover the vertical extent of the andesite as well as differences in its hydrothermal alteration. The samples were examined by petrography, X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF) and fluid inclusion geothermometry. The Spa Andesite cores have silica contents ranging from basalt to dacite (52.2 t...
The Wairakei Seismic Network collects high spatial resolution microseismic data from thirteen dow... more The Wairakei Seismic Network collects high spatial resolution microseismic data from thirteen downhole seismometers with depths ranging from ca. 65 m to 1,400 m. This paper examines statistical and spatial characteristics of 5,649 events recorded from March 2009-April 2013. In order to assist spatial analysis, three-dimensional, continuous numeric models of seismic energy are developed. The results show that microseismicity can improve reservoir models by constraining the depth of fluid circulation, by defining deep upflow zones, and locating horizons of higher fracture permeability.
Active volcanism in the Central Taupo Volcanic Zone (TVZ) during the past ~1.8 Ma has changed fro... more Active volcanism in the Central Taupo Volcanic Zone (TVZ) during the past ~1.8 Ma has changed from predominantly andesitic to silicic composition, with widespread rhyolitic volcanism dominating the Central TVZ by ~0.34 Ma. There are few surficial outcrops of andesites in the Central TVZ, although buried andesites have been encountered by drillholes at several TVZ geothermal fields, including 30 geothermal wells in the Wairakei Geothermal Field. Extrusive lavas of the Waiora Valley Andesite (WVA) occur at relatively shallow depth (i.e., less than 1 km depth) in the Western Borefield (WBF) at Wairakei, with deeper andesites (i.e., more than 1.5 mRL) in drillholes to the northwest and southeast of the WBF. In thin section, the WVA is distinguished from the deep andesites by two plagioclase populations (dominated by large, sieved plagioclase), and large plagioclase-pyroxeneamphibole glomerocrysts. In contrast, the deep andesites include rare amphibole, and plagioclase-only glomerocrysts...
Reservoir modeling is a multidisciplinary task aimed at consolidating diverse, and complex, geosc... more Reservoir modeling is a multidisciplinary task aimed at consolidating diverse, and complex, geosciences information, reservoir production, and injection history into a robust numerical model of the system. The resulting model is used for resource management and to assess resource sustainability for a number of development scenarios. The Wairakei-Tauhara and Ohaaki geothermal systems (New Zealand) are operated by Contact Energy Ltd, and have been modeled extensively over past decades using the expertise of University of Auckland Department of Engineering Science. The advent of faster computers and a greater quantity and variety of data has allowed significant improvements in the resolution of these models. One consequence of the increased modeling resolution is that the assignment of reservoir parameters to a numerical model has become increasingly challenging. This has led to a collaborative effort between Contact Energy Ltd and ARANZ Geo Ltd, to improve both the visualization and c...
Geothermal exploration of high-temperature geothermal systems commonly includes broadband magneto... more Geothermal exploration of high-temperature geothermal systems commonly includes broadband magnetotelluric (MT) measurements, which can be sensitive to subsurface electrical resistivity structures at depths of 10 km or more. At shallow levels (~1-3 km depth), models of these MT data generally image a low-resistivity ‘cap’ (associated with hydrothermal brines and clays) embedded in a more resistive background. At greater depths (3 – 10 km), recent regional MT resistivity models (developed as part of GNS Science’s Deep Geothermal Research Program in the southeastern Taupo Volcanic Zone) have revealed vertical low-resistivity anomalies (plumes) that may indicate the locations of deep upflows. These low-resistivity ‘plumes’ are key to refining conceptual models of geothermal systems. However, the process of inversion modeling (in 2D and 3D) requires that surface measurements cover a spatial extent that is at least twice the intended depth of investigation. With interest increasingly shif...
The Cordón Caulle region (40.5 S) hosts a 15 km long, NW-trending volcanic depression made up of ... more The Cordón Caulle region (40.5 S) hosts a 15 km long, NW-trending volcanic depression made up of Holocene and historic silicic lavas and pumice deposits overlying Late-Pleistocene basaltic flows. Fumaroles occur at the top of the system (~ 1500 masl), ...
The Puyehue-Cordón Caulle area (40.5° S) hosts one of the largest active geothermal systems of So... more The Puyehue-Cordón Caulle area (40.5° S) hosts one of the largest active geothermal systems of Southern Chile, comprising two main thermal foci, Cordón Caulle and Puyehue. Cordón Caulle is a NW-trending volcanic depression dominated by fumaroles at the top ( ...
... Sampling bottles with their reagents were evacuated in the ENAP-Chile laboratory before going... more ... Sampling bottles with their reagents were evacuated in the ENAP-Chile laboratory before going to the field. ... Hydrogen sulfide quickly reacts with CdCl 2 to form CdS, which precipitates in the NaOH solution (eg Tassi et al., 2005), inhibiting oxidation of H 2 S. This sampling ...
ABSTRACT Drill-hole temperature and stratigraphic datasets from the Wairakei geothermal field wer... more ABSTRACT Drill-hole temperature and stratigraphic datasets from the Wairakei geothermal field were used for geostatistical predictions using Kriging. In order to adequately constrain Kriging models, anisotropy and trends associated with temperature and stratigraphy were studied using standard variogram analysis, in combination with new regional and local structural data, revised gravity, and available geoscientific and reservoir data. This combined analysis lead to the incorporation of horizontal anisotropy (horizontal to vertical correlation ranging from 8:1 for regional stratigraphic units to 4:1 for local rhyolite bodies) in the case of stratigraphic models and variable anisotropy in the case of temperature models. In the latter, the variable anisotropy was represented by two end members: an isotropic model (horizontal to vertical correlation of 1:1) representative of depths >2000 mGL, and an anisotropic model (horizontal to vertical correlation of 3:1) representative of depths <1000 mGL. Kriging models of temperature also incorporated a vertical trend which is a combination of two end members at Wairakei: Boiling-Depth-Point Curve (convective) and linear (conductive). The Kriging models succeeded in identifying the primary geological controls on temperature distribution: major upflows largely controlled by structures at depth (>1000 m depth) and shallow (<1000 m depth) outflows stratigraphically channelled through formation contacts and rhyolite edges. A combination of stratigraphy and faults explain local cold downflows in shallow (750–1000 m depth) parts of the field.
Abstract The results of research in the use of Au grain morphological and compositional propertie... more Abstract The results of research in the use of Au grain morphological and compositional properties applied in primary Au ore exploration are presented here. Two different and independent topics are discussed:(1) morphological characteristics of Au grains from ...
ABSTRACT The Tauhara geothermal field is located within the Taupo Volcanic Zone, New Zealand and ... more ABSTRACT The Tauhara geothermal field is located within the Taupo Volcanic Zone, New Zealand and has undergone subsidence in three localized areas referred to as the Crown, Rakaunui and Spa Sights bowls, with measured subsidences of 0.9, 2.4 and 2.9 m, respectively. These subsidence bowls are situated close to Taupo township and are of concern to the public and for geothermal developers. Therefore, an intensive subsidence study at the Tauhara field was undertaken to better understand and mitigate further subsidence. Tauhara and Wairakei are often referred to as one system as a shallow, low resistivity anomaly extends continuously across both fields. However, they are two individual fields with separate up-flows. Tauhara is located in the south of the system, while Wairakei is located in the northern area. The Wairakei field also has subsidence bowls that reach up to 15 m in localized areas. Extraction of fluids from the Wairakei field began in 1958 (currently 171 MWe) but did not begin at Tauhara until 2010 (currently 23 MWe). At Wairakei, initial fluid withdrawal was from the Waiora Formation which extends under both the Wairakei and Tauhara fields. Since 1958, fluid pressure in the Waiora Formation has dropped and this pressure decline extends under both the Wairakei and Tauhara fields. A pressure drop has also been detected in the Mid Huka Falls Formation which is a permeable stratigraphic unit present at shallower depth (relative to the Waiora Formation) in both fields. The Tauhara subsidence investigation included drilling, with continuous core recovery, at selected sites located inside (THM 16), outside (THM 13, THM 14) and on the periphery (THM 12) of known subsidence bowls. Cored samples representative of the seven formations encountered were analyzed to establish their stiffness by determining their constrained modulus (CM) value. On the same samples, the effect of hydrothermal alteration was established using scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), petrography and X-ray diffraction (XRD). Key findings include the following: (1) CM values ranged from 20 to 1800 MPa; (2) THM 16 revealed the lowest CM values of the study (< 100 MPa) at 50 to 100 m of depth, where there has been a change in the subsurface to more acidic conditions; (3) Samples that revealed no clay minerals attached to crystal surfaces produced significantly higher CM values (THM 13, CM = 1730 MPa), than those samples where clay minerals were attached to and altering the crystals (THM 12, CM = 84 MPa); (4) Fracturing of crystals was observed in some samples from drillholes THM 12 and THM 13 located on the periphery and outside the Rakaunui subsidence bowl respectively, which may be a response to localized stress induced by subsurface compaction; (5) No crystal fracturing was observed in THM 14 located outside all subsidence bowls. The pressure drop across both the Tauhara and Wairakei geothermal fields has resulted in compaction of the Huka Falls Formation at depth with consequent subsidence at the surface. At Tauhara, compaction of the Huka Falls Formation at 130 to 400 m of depth occurs within the Rakaunui subsidence bowl. Furthermore, intense hydrothermal alteration at the Crown subsidence bowl (THM 16) has weakened a hydrothermal breccia deposit where compaction occurs at 35 to 200 m of depth. The combination of techniques used in this study proved a useful tool for unraveling complex geothermal processes altering the subsurface rocks. By establishing the hydrothermal alteration processes and coupling them with CM values we gained insights into rock stiffness and fluid–rock interactions within the Tauhara geothermal field.
Contact Energy Ltd (Contact) undertook a magnetotelluric (MT) survey at the Wairakei-Tauhara geot... more Contact Energy Ltd (Contact) undertook a magnetotelluric (MT) survey at the Wairakei-Tauhara geothermal system during early 2010. This study presents MT interpretations for the Wairakei system. Both one-dimensional (1D) and three-dimensional (3D) MT models were developed to maximize confidence in MT data interpretations. While the 3D model is useful to study field wide variations of resistivity, the 1D model allows a more detailed examination of the shallow conductive layers. The availability of drill-hole data and geophysical information (e.g. magnetism) has allowed an integrated interpretation of MT in the context of stratigraphy, structure, hydrothermal alteration and temperature distribution. Some features that stand out in the Wairakei system are the: 1. Correlation between the base of conductive layers as defined by MT, the base of swelling clay zone (as indicated by Methylene Blue tests) and the 200°C isotherm; 2. Lack of conventional, doming shape for conductive layer at upf...
More than 300 hot spring areas are located along the Chilean Andes, associated with Quaternary vo... more More than 300 hot spring areas are located along the Chilean Andes, associated with Quaternary volcanism. Systematic geothermal exploration in the northern part of the country started by the end of 1968, followed by basic geological, geochemical and volcanological surveys in many geothermal areas of southern Chile. These studies, together with wells drilled in El Tatio and Puchuldiza (northern Chile), allowed a preliminary assessment of the geothermal potential of the country of several thousands of MW. Renewed interest in geothermal exploration in Chile has been encouraged by the enactment of the Geothermal Law in 2000, which governs the regulatory aspects of the exploration and exploitation concessions, and also by the uncertainties regarding the imports of natural gas from Argentina, the vulnerability of hydropower to periodic droughts, and the environmental cost associated with hydropower and fossil fuels. Recently, a 4-year-long geothermal research project (2000 – 2003) was car...
Andesites are minor rock types in the Taupo Volcanic Zone (TVZ) and most occur near the Tongariro... more Andesites are minor rock types in the Taupo Volcanic Zone (TVZ) and most occur near the Tongariro Volcanic Centre (TVC), White Island, Putuaki and Rolles Peak near Rotokawa. However, several subsurface andesites have also been intersected by wells drilled at the Ngatamariki, Rotokawa, Wairakei, Waiotapu, Kawerau and Tauhara geothermal fields. The Spa Andesite at Tauhara, at least 201 meters thick, comprises interbedded lavas and breccias between 390 and 591 m depth. It was intersected only in well THM18 in the Spa Bowl area. Some of rocks are intensely altered. This study characterizes the Spa Andesite and compares it with other andesites within the TVZ. Twelve cores were chosen to cover the vertical extent of the andesite as well as differences in its hydrothermal alteration. The samples were examined by petrography, X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF) and fluid inclusion geothermometry. The Spa Andesite cores have silica contents ranging from basalt to dacite (52.2 t...
The Wairakei Seismic Network collects high spatial resolution microseismic data from thirteen dow... more The Wairakei Seismic Network collects high spatial resolution microseismic data from thirteen downhole seismometers with depths ranging from ca. 65 m to 1,400 m. This paper examines statistical and spatial characteristics of 5,649 events recorded from March 2009-April 2013. In order to assist spatial analysis, three-dimensional, continuous numeric models of seismic energy are developed. The results show that microseismicity can improve reservoir models by constraining the depth of fluid circulation, by defining deep upflow zones, and locating horizons of higher fracture permeability.
Active volcanism in the Central Taupo Volcanic Zone (TVZ) during the past ~1.8 Ma has changed fro... more Active volcanism in the Central Taupo Volcanic Zone (TVZ) during the past ~1.8 Ma has changed from predominantly andesitic to silicic composition, with widespread rhyolitic volcanism dominating the Central TVZ by ~0.34 Ma. There are few surficial outcrops of andesites in the Central TVZ, although buried andesites have been encountered by drillholes at several TVZ geothermal fields, including 30 geothermal wells in the Wairakei Geothermal Field. Extrusive lavas of the Waiora Valley Andesite (WVA) occur at relatively shallow depth (i.e., less than 1 km depth) in the Western Borefield (WBF) at Wairakei, with deeper andesites (i.e., more than 1.5 mRL) in drillholes to the northwest and southeast of the WBF. In thin section, the WVA is distinguished from the deep andesites by two plagioclase populations (dominated by large, sieved plagioclase), and large plagioclase-pyroxeneamphibole glomerocrysts. In contrast, the deep andesites include rare amphibole, and plagioclase-only glomerocrysts...
Reservoir modeling is a multidisciplinary task aimed at consolidating diverse, and complex, geosc... more Reservoir modeling is a multidisciplinary task aimed at consolidating diverse, and complex, geosciences information, reservoir production, and injection history into a robust numerical model of the system. The resulting model is used for resource management and to assess resource sustainability for a number of development scenarios. The Wairakei-Tauhara and Ohaaki geothermal systems (New Zealand) are operated by Contact Energy Ltd, and have been modeled extensively over past decades using the expertise of University of Auckland Department of Engineering Science. The advent of faster computers and a greater quantity and variety of data has allowed significant improvements in the resolution of these models. One consequence of the increased modeling resolution is that the assignment of reservoir parameters to a numerical model has become increasingly challenging. This has led to a collaborative effort between Contact Energy Ltd and ARANZ Geo Ltd, to improve both the visualization and c...
Geothermal exploration of high-temperature geothermal systems commonly includes broadband magneto... more Geothermal exploration of high-temperature geothermal systems commonly includes broadband magnetotelluric (MT) measurements, which can be sensitive to subsurface electrical resistivity structures at depths of 10 km or more. At shallow levels (~1-3 km depth), models of these MT data generally image a low-resistivity ‘cap’ (associated with hydrothermal brines and clays) embedded in a more resistive background. At greater depths (3 – 10 km), recent regional MT resistivity models (developed as part of GNS Science’s Deep Geothermal Research Program in the southeastern Taupo Volcanic Zone) have revealed vertical low-resistivity anomalies (plumes) that may indicate the locations of deep upflows. These low-resistivity ‘plumes’ are key to refining conceptual models of geothermal systems. However, the process of inversion modeling (in 2D and 3D) requires that surface measurements cover a spatial extent that is at least twice the intended depth of investigation. With interest increasingly shif...
The Cordón Caulle region (40.5 S) hosts a 15 km long, NW-trending volcanic depression made up of ... more The Cordón Caulle region (40.5 S) hosts a 15 km long, NW-trending volcanic depression made up of Holocene and historic silicic lavas and pumice deposits overlying Late-Pleistocene basaltic flows. Fumaroles occur at the top of the system (~ 1500 masl), ...
The Puyehue-Cordón Caulle area (40.5° S) hosts one of the largest active geothermal systems of So... more The Puyehue-Cordón Caulle area (40.5° S) hosts one of the largest active geothermal systems of Southern Chile, comprising two main thermal foci, Cordón Caulle and Puyehue. Cordón Caulle is a NW-trending volcanic depression dominated by fumaroles at the top ( ...
... Sampling bottles with their reagents were evacuated in the ENAP-Chile laboratory before going... more ... Sampling bottles with their reagents were evacuated in the ENAP-Chile laboratory before going to the field. ... Hydrogen sulfide quickly reacts with CdCl 2 to form CdS, which precipitates in the NaOH solution (eg Tassi et al., 2005), inhibiting oxidation of H 2 S. This sampling ...
ABSTRACT Drill-hole temperature and stratigraphic datasets from the Wairakei geothermal field wer... more ABSTRACT Drill-hole temperature and stratigraphic datasets from the Wairakei geothermal field were used for geostatistical predictions using Kriging. In order to adequately constrain Kriging models, anisotropy and trends associated with temperature and stratigraphy were studied using standard variogram analysis, in combination with new regional and local structural data, revised gravity, and available geoscientific and reservoir data. This combined analysis lead to the incorporation of horizontal anisotropy (horizontal to vertical correlation ranging from 8:1 for regional stratigraphic units to 4:1 for local rhyolite bodies) in the case of stratigraphic models and variable anisotropy in the case of temperature models. In the latter, the variable anisotropy was represented by two end members: an isotropic model (horizontal to vertical correlation of 1:1) representative of depths >2000 mGL, and an anisotropic model (horizontal to vertical correlation of 3:1) representative of depths <1000 mGL. Kriging models of temperature also incorporated a vertical trend which is a combination of two end members at Wairakei: Boiling-Depth-Point Curve (convective) and linear (conductive). The Kriging models succeeded in identifying the primary geological controls on temperature distribution: major upflows largely controlled by structures at depth (>1000 m depth) and shallow (<1000 m depth) outflows stratigraphically channelled through formation contacts and rhyolite edges. A combination of stratigraphy and faults explain local cold downflows in shallow (750–1000 m depth) parts of the field.
Abstract The results of research in the use of Au grain morphological and compositional propertie... more Abstract The results of research in the use of Au grain morphological and compositional properties applied in primary Au ore exploration are presented here. Two different and independent topics are discussed:(1) morphological characteristics of Au grains from ...
ABSTRACT The Tauhara geothermal field is located within the Taupo Volcanic Zone, New Zealand and ... more ABSTRACT The Tauhara geothermal field is located within the Taupo Volcanic Zone, New Zealand and has undergone subsidence in three localized areas referred to as the Crown, Rakaunui and Spa Sights bowls, with measured subsidences of 0.9, 2.4 and 2.9 m, respectively. These subsidence bowls are situated close to Taupo township and are of concern to the public and for geothermal developers. Therefore, an intensive subsidence study at the Tauhara field was undertaken to better understand and mitigate further subsidence. Tauhara and Wairakei are often referred to as one system as a shallow, low resistivity anomaly extends continuously across both fields. However, they are two individual fields with separate up-flows. Tauhara is located in the south of the system, while Wairakei is located in the northern area. The Wairakei field also has subsidence bowls that reach up to 15 m in localized areas. Extraction of fluids from the Wairakei field began in 1958 (currently 171 MWe) but did not begin at Tauhara until 2010 (currently 23 MWe). At Wairakei, initial fluid withdrawal was from the Waiora Formation which extends under both the Wairakei and Tauhara fields. Since 1958, fluid pressure in the Waiora Formation has dropped and this pressure decline extends under both the Wairakei and Tauhara fields. A pressure drop has also been detected in the Mid Huka Falls Formation which is a permeable stratigraphic unit present at shallower depth (relative to the Waiora Formation) in both fields. The Tauhara subsidence investigation included drilling, with continuous core recovery, at selected sites located inside (THM 16), outside (THM 13, THM 14) and on the periphery (THM 12) of known subsidence bowls. Cored samples representative of the seven formations encountered were analyzed to establish their stiffness by determining their constrained modulus (CM) value. On the same samples, the effect of hydrothermal alteration was established using scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), petrography and X-ray diffraction (XRD). Key findings include the following: (1) CM values ranged from 20 to 1800 MPa; (2) THM 16 revealed the lowest CM values of the study (< 100 MPa) at 50 to 100 m of depth, where there has been a change in the subsurface to more acidic conditions; (3) Samples that revealed no clay minerals attached to crystal surfaces produced significantly higher CM values (THM 13, CM = 1730 MPa), than those samples where clay minerals were attached to and altering the crystals (THM 12, CM = 84 MPa); (4) Fracturing of crystals was observed in some samples from drillholes THM 12 and THM 13 located on the periphery and outside the Rakaunui subsidence bowl respectively, which may be a response to localized stress induced by subsurface compaction; (5) No crystal fracturing was observed in THM 14 located outside all subsidence bowls. The pressure drop across both the Tauhara and Wairakei geothermal fields has resulted in compaction of the Huka Falls Formation at depth with consequent subsidence at the surface. At Tauhara, compaction of the Huka Falls Formation at 130 to 400 m of depth occurs within the Rakaunui subsidence bowl. Furthermore, intense hydrothermal alteration at the Crown subsidence bowl (THM 16) has weakened a hydrothermal breccia deposit where compaction occurs at 35 to 200 m of depth. The combination of techniques used in this study proved a useful tool for unraveling complex geothermal processes altering the subsurface rocks. By establishing the hydrothermal alteration processes and coupling them with CM values we gained insights into rock stiffness and fluid–rock interactions within the Tauhara geothermal field.
Contact Energy Ltd (Contact) undertook a magnetotelluric (MT) survey at the Wairakei-Tauhara geot... more Contact Energy Ltd (Contact) undertook a magnetotelluric (MT) survey at the Wairakei-Tauhara geothermal system during early 2010. This study presents MT interpretations for the Wairakei system. Both one-dimensional (1D) and three-dimensional (3D) MT models were developed to maximize confidence in MT data interpretations. While the 3D model is useful to study field wide variations of resistivity, the 1D model allows a more detailed examination of the shallow conductive layers. The availability of drill-hole data and geophysical information (e.g. magnetism) has allowed an integrated interpretation of MT in the context of stratigraphy, structure, hydrothermal alteration and temperature distribution. Some features that stand out in the Wairakei system are the: 1. Correlation between the base of conductive layers as defined by MT, the base of swelling clay zone (as indicated by Methylene Blue tests) and the 200°C isotherm; 2. Lack of conventional, doming shape for conductive layer at upf...
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