15Shales are ubiquitous rocks in sedimentary basins, where their low permeability makes them 16 efficient seals for oil and gas reservoirs and underground waste storage repositories (waste 17 waters, CO2, nuclear fuels). Moreover, when they contain organic matter, they form source 18 rocks for hydrocarbons that may escape towards a more porous reservoir during burial, a 19 process referred to as primary migration. And when the hydrocarbons cannot escape, these 20 rocks can be exploited as oil or shale gas reservoirs. While the presence of fractures at the 21 outcrop scale has been described, the existence of fractures at smaller scales, their link with