Clay formations are present in reservoirs and earthquake faults, but their mechanical behaviors are still poorly understood, as they can vary from plastic (aseismic) to brittle (seismic). A decametric scale experiment, which aims to reactivate a natural fault by fluid injection was performed in shale materials. The injection area monitored by sensors measuring pressure, deformation and ground motion. Few tens of events were recorded with magnitude ranging between -4.3 and -3.7. Their spatio-temporal distribution, shows that most of the deformation induced by the injection is aseismic. Whether the seismicity is controlled by the fault architecture, mineralogy of fracture filling, fluid and/or stress state is then discussed. The fault damage zone architecture and mineralogy are of crucial importance as seismic slip mainly localizes on the sealed-with-calcite fractures which predominates in the fault damage zone. As no seismicity is observed in the close vicinity of the injection areas, the presence of fluid seems to prevent seismic slips. The fault core acts as an impervious hydraulic barrier that favors fluids confinement and pressurization. Therefore, the seismicity seems to be driven by the stress changes induced by the pressurized volume and the hydraulic permeability heterogeneity strongly influences the seismicity distribution.