Investigation of geothermal fluid circulation through the study of CO2 soil flux: an application to the Tivoli quarry area (Rome, Italy)

Geothermal energy represents a renewable energy source exploited for multiple purposes, including electricity, direct use, district heating and heat pumps. One of the most relevant problems in geothermal energy industry is the permeability of the reservoir, both for production and reinjection. Therefore, it is important to assess the fluid circulation in the reservoir and where deep fluids rise. Faults, fractures and active tectonics influence fluid behaviour and fluid-rock interactions in a geothermal context. It is necessary to estimate the role of faults and map as well as their distribution, as tectonic structures could act as barriers to fluid circulation or as preferential conduits. The Acque Albule Basin (AAB) is a case study representing one of the most important hydrothermal manifestations in central Italy. The AAB is a tectonically controlled basin, characterized by a huge hydrothermal manifestation (discharges in the order of m³/s). The deep hydrothermal activity is testified by the presence of a large and thick travertine deposits and several mineralized springs (T_max at the surface up to 23°C) in which warm fluids rise from the geothermal reservoir. These hot fluids circulate through the Meso-Cenozoic carbonate reservoir, highly affected by dissolution and brittle deformation. In this framework, travertine deposition is mainly controlled by the faults activity. Several geophysical surveys were carried out to evaluate the cap-rock of the geothermal reservoir, beneath the travertine plateau. The exploration provided a clearer view of the stratigraphy of the AAB and revealed the carbonate roof at 300-400 m. The carbonate rocks are overlain by some alluvial sediments and a travertine plateau from 10 m to 90 m. Based on the geophysical investigations, the measurement of diffuse CO₂ emissions from the soil was planned to ascertain the faults role in the hydrothermal circulation. Preliminary results show that the fault zone is characterised by an extremely low degassing (5/10 g m^-2d^-1). The low degassing could be related to the low-permeability of the alluvial and travertine deposits and/or by self-sealing processes through the main shear zone, which obstacle the upwelling of fluids and gases. The model will be improved with further regional CO2 surveys and δC¹³ analysis of CO₂ of gaseous samples taken from the soil.