Dissolved Helium Estimation in the Casaglia Geothermal Reservoir: a Geological Active Area

Deep fluids and their isotopic signatures, especially He due to its inert nature, are powerful tools for investigating the processes occurring within the crust, as fluids- rocks interaction, storage and transfer mechanism. Numerous studies have emphasized the connection between local seismicity leading to widespread microfracturing processes and an increase in the release of ⁴He that was previously trapped within minerals. Thus, the episodic degassing of 4He through the crust can sustain the excess of He in natural reservoirs respect to the steady-state diffusion processes.

The aim of this study is to determine whether the quantity of He stored in the geothermal reservoir of Casaglia, an exploited mining concession located at the top of the Dorsale Ferrarese, which is a seismically active area in the Emilia-Romagna Region, can be attributable to long lasting diffusion process and to highlight the contribution that these geochemical approach can provide to enhance the understanding of underground phenomena, revealing changes in the stress field and related earthquakes.

We collect fluids from the reservoir and we analyse their chemical and isotopic composition. In particular, the helium abundance is very high, up to 3956 ppm. It is also characterised by a ³He/⁴He ratio of 0.02Ra, clearly indicating that the dominant component is attributable to radiogenic ⁴He produced by U and Th decay in the crust.

The measured total amount of He stored in Casaglia reservoir vary from 10⁷ to 10⁵ mol/km³ rocks, depending on the porosity. This data has been compared with the expected amount of helium accumulated over time under steady-state crustal degassing, which has been computed taking into account the local stratigraphy, the age of formation of the anticline hosting the reservoir (Upper Pliocene - Pleistocene) and the abundances of U and Th in the rocks. The preliminary result shows that there is at least an order of magnitude difference between the experimental evidence and the calculated data in terms of the amount of helium accumulated in the reservoir, even under the most conservative conditions, assuming that there are no losses of ⁴He due to advection or diffusion processes.

Our study demonstrates that the in-situ production of ⁴He in the crust and a long-lasting diffusion through the crust are not the main processes that rule the He degassing in the Casaglia reservoir but alternative and episodic processes controlling the transport mechanism act simultaneously.

Finally, exploiting the established association between rock deformation and helium degassing, monitoring helium flux in active tectonic settings may yield valuable insights into variations in the stress field and the occurrence of seismogenic processes. This aspect highlights the applicability of our findings in contributing to the understanding of underground phenomena, emphasizing the potential of geochemical approaches to reveal changes in the stress field and seismic activities.