Hydrogeothermal potential assessment in a complex tectonic environment by geophysical and hydrogeochemical tools: a case of the geothermal province of Hajeb Layoun-Jilma-Oued Hjal basins (Central Tunisia)

This paper presents the results of extensive geological, geophysical, chemical, and geothermometry investigations on natural thermal and cold water discharging throughout the main geological domains of Hajeb Layoun-Jilma and Oued Hjal basins. These areas are known by the rise of two hydrothermal springs of Hammam Sidi Maamer S1, Chenama S2, and Ouled Farhane deep water well S3. Their distributions are strongly related to the main active tectonic structures. Composite geo-seismic model highlights the deep structuring of the Cretaceous and Miocene horizons and the fluid pathways from the recharge areas (Mghilla, Hmaiema, Nara outcrops) to discharge zones of Sidi Maamer (Baten Damous outcrop) and Chenama (Zaouia outcrop). Chemical features of cold and hot water are mainly controlled by water–rock interactions and the dissolution of evaporate and carbonate minerals leading to mixed Ca-Mg-SO 4 -Cl, Ca–Cl, and Na-Cl facies. Molar ratios of (Ca 2+ + Mg 2+ )/ (SO 4 2− + HCO 3 − ) and chloro-alkaline indicators implied that ion exchange and reverse ion exchange influenced the hydrogeochemistry of groundwater sources. IIRG diagram reveals two significant aquifer types: one indicating the deep circulation of fluid from the limestone of the Aptian Orbata Formation reservoir and ascends to the surface through S1 and S2 along faults of Hajeb Layoun basin, whereas the second aquifer is characterized by sandstone levels; where water circulates through the late Miocene Saouaf Formation deposits to the surface also along faults of Oued Hjal basin. Silica geothermometers for S1 and S2 give the best results for reservoir temperatures varying from 70.43 to 83 °C and depths range from 1709 to 2153 m. For S3 deep hot water well, K-Mg geothermometer gives the most reasonable temperature estimation of 48 °C and depth up to 910 m. Giggenbach diagram reveals that water samples are not in an equilibrium state with the reservoir rocks.