Multi-disciplinary approach to geothermal systems in foredeep basins: the Acquasanta Terme case study (Marche, Italy)

The continuous growth of energy consumption, the progressive and fast depletion of hydrocarbon reserves, the environmental pollution, in addition to frequent political crises, have generated a global importance energy emergency. At the same time, the growth in energy production has to be ‘sustainable', meaning a social and economic development respectful of the natural times and of the resources of the Planet. Within this general framework, one of the most important ‘renewable' is the geothermal, being the energy originated from the heat released by the Earth. In the Country that launched the geothermal energy in the world, initiating the era of energy production from this source (Larderello 1904), the aim of this Ph.D. thesis is to investigate the less promising east side of Italy in a region with surface thermal manifestations, in order to characterize the potential geothermal system. The study area was identified in the Acquasanta anticline (Acquasanta Terme, Southern Marche Region, Central Italy), located within the foredeep Laga basin with low heat flow values and water temperatures ranging between 20º C and 30º C at surface. To achieve the purpose, we integrate different prospections (geochemical, seismic and structural) that allow for a comprehensive picture of the geothermal system. The model of a geothermal system characterized by water of meteoric origin flowing from south to north is proposed. Water infiltrates at altitudes ranging between 1400 m and 1700 m, identifying the recharge area with the Laga Mountains, located in the southernmost part of the anticline, about 30 km South of Acquasanta Terme. From geochemical data collected along more than one year of monitoring three ''end member'' waters with different chemical features and circuits are defined. The first water type shows higher temperatures (25.3º C 30.0º C), and salinity and chemical composition stable during time. It displays a clear contact with the Burano Anhydrites Fm and with carbonatic rocks, confirmed by Sr isotopic values. Furthermore, it has a residence time greater than sixty years (0 T.U.), indication of long circulation period, and no mixing with younger waters. Moreover, some indications (i.e. Rubidium) of a possible slight contribution from deep fluids are present. The second water end member likely infiltrates at lower quotes than the previous one, following a shorter deep circuit within the same formations as suggested by ion concentrations. It was also recognized within the Varoni_1 deep well (AGIP s.p.a., 1987). The third type is inferable from samples distribution on correlation graphs and from identified mixing trends, characterized by warm temperature, but lower ions concentrations than the others. This water likely flows only within the Calcare Massiccio Fm, just interacting with anhydrites, probably following a slightly faster circuit. Except for the easternmost springs (first end member), all the other collected springs are influenced by mixing phenomena both among the three end-members and also with surface freshwater. The lateral confinement of the easternmost springs and the peculiar mixing processes are strictly linked to the complex structural setting, in particular to N S oriented faults. The reservoir is identified mainly within the Calcare Massiccio Fm (CM). In detail, by seismic interpretation, it is slightly N-plunging, following the anticline geometry, and laterally bounded by a N S oriented thrust to the east, a related backthrust to the west, and by a WNW ESE normal fault to the north. Its bottom depth ranges between 3000 m and 3800 m below the surface. A first estimation gives back a computed volume of about 66 km3. The mentioned WNW ESE extensional discontinuity is also considered to be the main rising conduit for the deep hot waters. In the final part of the fluid circuit, hot waters flow within the shallow reservoir of the Scaglia Rossa Fm (SR) creating relevant karst caves phenomena. Finally, all the thermal manifestations occur at the base of the low permeability Scaglia Cinerea Fm (reservoir cap rock), on the right bank of the Tronto River, as confirmed by relevant travertine deposits. Detailed quantitative structural analyses were performed both in the lower (CM) and shallower (SR) reservoirs, in order to create the DFN model and to compute their connectivity properties. In particular, for the CM buried reservoir, an analogue example has been used to perform the field analyses. The highest permeability is shown along the bottom up and the longitudinal directions, whose calculated permeability value is 0.148 mDarcy. Moreover, fractured volume derived from these structural data allows to calculate an estimation of the total profit volume of the reservoir, resulting in 1.32 km3. Concerning the temperature reached by the system, K/Mg geothermometer applied on the most stable samples gives back an estimated temperature at the bottom of the reservoir (top of the Burano Anhydrites Fm) of about 90º C, consistent with the normal geothermal gradient of the sedimentary Laga Basin.