Downward penetration and mixing of sedimentary brines and dilute hot waters at 5 km depth in the granite basement at Soultz-sous-Forêts (Rhine graben, France)

Fluids trapped in granite microfissures around 5 km depth in the GPK2 drilling from the European Hot Dry Rock (HDR) Energy program at Soultz-sous-Forêts, can be related to the rather recent (post-Oligocene) fluid flow due to the rifting of the Rhine graben. Most of fluid inclusions planes were healed by fluids similar to those trapped in hydrothermal quartz veins from the shallower levels of the granite. The fluid inclusion dataset indicates a model of mixing between a sedimentary brine rich in divalent cations (around 20 wt.% eq. NaCl) and a dilute fluid slightly hotter than the brine, and characterized by a temperature close to the present-day measured temperature. The penetration of brines down to 5 km is significant, as well as the lack of homogenization of the reservoir fluids, i.e., each fluid is characterized by a specific salinity in between the two fluid end-members.