Assessment of Petrophysical Rock Properties in North Madagascar: Implications for Geothermal Resource Exploration

Abstract : Geothermal exploration concepts can be improved at an early stage of exploration by assessing the petrophysical rock properties of samples from surface outcrops. The results can be used to better evaluate the proposed geothermal resource and exploration strategies for the region. Our study provides fundamental knowledge on thermohydraulic rock properties to help identify the heat transfer mechanisms associated with geothermal resources in the extensional tectonic settings of the Ambilobe and Ambanja areas in North Madagascar. Thermostratigraphic units were defined according to the thermofacies concept. The geology of both areas is predominantly Quaternary volcanics (mainly basalt) and Permian–Triassic sediments (mainly shale and sandstone) overlying a faulted and fractured Proterozoic metamorphic basement of gneiss and amphibolite. Normal faults related to graben structures, along which hot springs occur, are characteristic of this extensional regime. Permian sandstone and mica schist have the highest porosities, with values up to 0.10–0.11. The matrix permeability of sedimentary and basement rock formations varies from less than 1.0 \(\times\) 10–19 to 1.1 \(\times\) 10–14 m2 (< 0.001 to 11 mD), while the average thermal conductivity is ~ 3.8 W m−1 K−1 and ~ 2.7 W m−1 K−1, respectively. Conductive heat transfer is expected for such low-permeability rocks, except where fractures form permeable conduits that promote forced convection driven by regional groundwater flow. Crustal-scale faults should be targeted for further geothermal exploration.