This study focuses on hydrothermal alteration, and more specifically clays, in the geothermal reservoir of Cerro Pabellón in Andean Cordillera, Northern Chile. Although it is the first commercial geothermal power plant in South America with 48 MWe installed and an additional 33 MWe during 2022, the alteration mineralogy of the deep reservoir remains poorly investigated. Cuttings from two deep wells drilled between 500 and 3000 m depth were sampled. The fine-grained clay fraction was analyzed using X-ray diffraction to observe the mineral distribution and variations of crystal structure of the clays. Scanning electron microscopy coupled with energy dispersive spectroscopy allowed us to perform microanalysis of the clays. The results suggest a high heterogeneity of the reservoir in term of alteration: a pervasive alteration dominated by trioctahedral clays was observed in all the samples, whereas a fracture-controlled alteration was only observed locally, dominated by dioctahedral clays. Illite-smectite mixed layers with more than 90% illite typically occur in permeable fracture and fault as a typical clay signature. This study completed an integrated conceptual model of the high enthalpy reservoir (> 250 °C). Circulations of a neutral pH Na-K-Cl fluid occur in a wide fracture system connected to highly dipping and NW-striking faults and could extend beyond the Pabelloncito graben.
This study focuses on hydrothermal alteration in the geothermal reservoir of Cerro Pabellón (Andean Cordillera, Northern Chile). It is based on CP2A and CP5A production wells drilled above a local normal fault and presenting unlike hydraulic properties. Cuttings from 300 to 1555 m depth were sampled and analyzed using X-ray diffraction (XRD) to observe distribution of hydrothermal minerals and crystal chemistry variations of clays (fraction < 5 μm). Then, scanning electron microscopy coupled with energy dispersive spectroscopy (SEM–EDX) allowed to perform microanalysis of hydrothermal minerals. These results highlight a mineral assemblage that was not observed before, composed of adularia + Ba-rich feldspar + feathery quartz + chalcedony + calcium arsenates + illite. They are characteristics of high-temperature hydrothermal alteration in epithermal settings and are restricted to shallow permeable fracture zones of the active part of the reservoir. Another fracture-controlled event related to a typical illitization is observed in all permeable fracture and fault zones of the geothermal system. This multi-event alteration seems strongly controlled by the western graben fault and the associated interconnected fracture network.
During 1997 ten magnetotelluric (MT) soundings were recorded in single-site mode above the Mt. Somma-Vesuvius volcanic area. A first campaign of MT measurements was carried out, during spring, by the researchers of the University of Padua with their MSPM acquisition system. During autumn, the researchers of the International Institute of Geothermal Research (CNR Pisa) with their Phoenix equipment performed a second campaign. A teach site, the horizontal components of the electrical and magnetic fields were recorded in the frequency band between 300-0.003 Hz. The MSPM system could record signals up to the frequency of 800 Hz. Data were recorded at one common site with both the different equipments to verify the compatibility of the two different acquisition systems. The soundings over the area of the volcano's caldera show a continuous morphology of the apparent resistivity and phase curves with small error bars: it means a good correlation between the orthogonal electrical and magnetic fields. The quality of data decreases going further from the caldera and approaching the sources of electromagnetic incoherent noise such as villages, antennas and repeaters. After a very accurate data analysis, the apparent resistivity and phase curves were interpreted with a 1D modelling instead a 2D one as it seems a more appropriate interpretative approach looking at the morphology of the curves and taking into account the 3D geological conditions of the area. The results show an extended conductive structure at a depth of 0.3-1.2 km. It could be connected with a change in the physico-chemical characteristics of the volcano-sedimentary cover (alteration paragenesis and possible hydrothermalism). A 3D MT forward modelling was then used to define the response MT curves for sites above this particular volcanic structure. This approach seems to be very interesting in view of specific interpretative targets, such as dimension and position of the magma chamber, when planning future MT surveys.
During 1997, within the framework of an Italian-funded scientific cooperation between Italy and the Czech Republic, a series of magnetotelluric (MT) soundings was carried out in the region of Northwest Bohemia (Czech Republic). This is one of the most seismically active areas in Central Europe, where micro-earthquake swarms frequently occur during the apparently quiescent intervals between large macro-seismic swarms. Fifteen MT stations were installed in an area of about 15 ´ 20 km2 where 80% of the seismicity of the entire region has been recorded since 1986. The area showed a high electromagnetic noise, possibly of high cultural origin from the nearby industrial zone of the Sokolov basin, which affected both the electric and the magnetic signals. The final data, carefully selected, were modeled by 2D and 3D techniques. The results show an extensive conductive structure in the depth range from 0.5 to 3 km. This structure could be connected with the locally buried granitic massif in the inhomogeneous metamorphic basement, probably accompanied by fracturation, thermo-metamorphism or paleofluids. Moreover, the presence of a conductive anomaly in the northern part of the investigated region could be linked to a lithological change in the metamorphic rocks (prevalence of phyllites over mica schists), which would even increase the effect of the granite.
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