Geological characterization of potential geothermal reservoirs in complex basin systems: a study case in the Tertiary Piedmont Basin, NW Italy

The exploration of potential geothermal resources in complex basins, such as piggyback or wedge-top basins, requires the integration of geological and geophysical studies to constrain the lithology and architecture of potential reservoir units, to understand possible fluid flow paths and to infer possible heating mechanisms in the basins. This study focuses on the characterisation of the fractured crystalline basement rocks, likely acting as a reservoir, and the above sedimentary cover of the Tertiary Piedmont Basin (TPB) in north-western Italy. The TPB consists of a thick sequence (up to ca. 6 km) of tectonically controlled sedimentary deposits, Eocene to Pliocene in age, overlying the metamorphic units of the Ligurian and Western Alps to the west and the Inner and Outer Ligurian units to the east. The dynamic tectonic framework of the region has caused continuous and progressive deformation of the basement and its covers, promoting complex fracture patterns in the basement and the development of kilometre-wide structures that control the basin architecture (Maino et al., 2013). Evidence for geothermal activity in the basin includes surface heat fluxes between 62 and 84 mW m-2 (Pasquale et al., 2014), the presence of active thermal springs (e.g., La Bollente and Lago delle Sorgenti in Acqui Terme), and locally high geothermal gradients (70 to 90 mKm-1, Pasquale et al., 2011), in the southern part of the basin, where fluid circulation and heating within the fractured Alpine basement is most likely to occur. We show results obtained from the integrated analysis of geological/geophysical data to characterise and classify possible reservoir and seal units in the TPB succession. A 3D model was implemented, based on a merged geologic map of the area and the interpretation of seismic reflection lines, which show the geometry (e.g. thickness and depth) and spatial distribution of basement and sedimentary cover. Field observations, present brittle structures, and the behaviour of hydrothermal systems in the area point out the basement units as the main geothermal reservoir, in agreement with the study of Pasquale et al. (2011). To characterize this metamorphic reservoir, the rocks of the Ligurian Alps, including the Voltri and Valosio groups and Voltaggio units, were studied using field structural analysis, Digital Outcrop Model mapping, and fracture intensity analysis coupled with petrophysical data. The sedimentary cover sequences were classified as seal units or secondary reservoirs through the analysis of sedimentary facies, grain size ratios, and petrophysical data from available well logs. The analysis of the potential reservoir and seal units allowed to provide a preliminary assessment of the geothermal potential system in the TPB, representing a solid starting point for developing a more detailed geothermal resources evaluation