259 9. Defining high enthalpy geothermal drilling target with multi-physics The second phase of the study (2007-2008) focused on Petite-Terre, specifically the CO₂ degassing zone near the airport and the Dziani Dzaha Lake area, previously noted for gaseous emissions. Geophysical surveys (gravity, magnetism, and electrical resistivity) were conducted to detect potential heat sources such as hypovolcanic intrusions or magmatic chambers, as well as the presence of a hydrothermal system. Key results are presented in Pajot et al. (2007). Based on the findings, including evidence of a dense and magnetic shallow body beneath the CO₂ degassing zone near the airport, a second stage of the study extended the resistivity surveys southward for deeper investigation. This stage also refined geochemical characterizations and natural gas flux measurements. The results, documented by Sanjuan et al. (2008), ruled out the presence of geothermal resources within the first 1000 m of depth and deemed it unlikely up to 1500 m – depths considered economically viable for electricity production. However, given Petite-Terre’s recent volcanism and its geological, geochemical, and geophysical context, the presence of a thermal anomaly or hydrothermal system at greater depths could not be excluded. To confirm the existence and location of deeper geothermal resources, a comprehensive exploration program, including exploratory drilling, is required. Recognizing the cost and complexity of such an undertaking, Darnet et al. (2019) analyzed previous studies to evaluate the five elements of an active geothermal system (Figure 9.2): 1. heat source presence, 2. a cap rock preventing fluid escape, 3. a sustainable water recharge system, 4. a permeable medium (e.g. fractures), 5. the hydrothermal system’s age. Their analysis indicated a likelihood greater than 50% of finding an active geothermal system, though additional data were necessary. A customized exploration program was therefore defined, incorporating various methods to evaluate parameters such as temperature, volume, porosity, and permeability. Key components included: • Geological data acquisition to analyze rock permeability and fracturing. • Geochemical studies, including gas geothermometry, to estimate source temperatures. • New onshore and offshore magnetotelluric (MT) surveys for 3D imaging of subsurface conductivity. • Integration of data into GeoModeller™ (Lajaunie et al., 1997; Calcagno et al., 2008) to create a consistent 3D geological model. • Hydrothermal simulations using the ComPASS (Lopez et al., 2018) platform to locate the optimal exploratory well site. Dezayes et al. (2023) followed the exploration program described above. This chapter will focus on how the various geophysical methods of the program were used to define the geothermal drilling targets on Petite-Terre.
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