International audience ; We present the main features of the geothermal production history between 2018 and 2019 at the Balmatt geothermal doublet (3-4 km depth), located in Mol in northern Belgium, and the recorded seismic data set which represents the first return of experience on the seismic response of the fractured Lower Carboniferous Limestone reservoir in the Campine Basin to fluid injection. The study has been motivated by the occurrence of a felt ML = 2.2 event leading to the temporal suspension of the project due to a red-light status of the traffic light system. Even though the seismic data set is comparatively limited (Mc∼0.6), thanks to a precise analysis of the seismic source features, we derive a new conceptual structural reservoir model that includes a hidden fault zone not seen during the exploration and provides new constraints on the main hydro-mechanic processes involved in the triggering of seismicity and seismic hazard.