Natural fractures play a crucial role in many sustainable subsurface applications, particularly in reservoirs with low primary porosity and permeability. Therefore, fracture analysis has become a fundamental aspect of geosciences, often starting with fracture interpretation from borehole image logs. It has been widely recognized that interpretations come with uncertainties; nevertheless, there is a current lack of both quantitative and qualitative understanding regarding differences among interpretations. This study aims to address this gap, by performing a statistical analysis on five interpretations of the same well, highlighting their differences and similarities. Based on the results from the analysis, a workflow was created to reduce uncertainties in fracture interpretations from image logs.

The created workflow was then applied to two wells in the Geneva Basin, Switzerland; associated with a geothermal project aiming to produce heat and electricity from the Upper Cretaceous limestone rocks. Research has shown that these rocks form a tight reservoir, and fluid flow will rely on the presence of fracture networks. In the two wells, a total of eight fracture sets were defined based on movement and relative chronology, and used to reconstruct the fracture history of the basin. This led to the identification of four Mesozoic to Cenozoic stress regimes: 1) a normal or strike-slip regime, 2) a NE-SW reverse regime, 3) a normal regime, and 4) a NW-SE reverse regime. The latter is considered the main deformation event of the region: the Late Miocene fold-and-thrust tectonics. In the Geneva Basin, this event is represented by a pair of NE-SW striking conjugates, which forms the majority of observed fractures. The fifth fracture set is believed to be fault-related and was only observed in one of the wells.

Beyond the regional implications, this study emphasizes the importance of uncertainty reduction in the early stages of fracture analysis, and its effects on establishing a reliable fracture history.