Geochemistry plays a great role when assessing the impact of hydrogen storage. With the purpose of discovering and characterizing minerals-brine interactions as a consequence of hydrogen injection into a depleted gas reservoir, we constructed an equilibrium geochemical model using, GEM-Selektor (GEMS) package. A depleted gas reservoir from upper Austria, Molasse basin, has been selected as a candidate for hydrogen storage. Thermodynamic model of formation water and minerals at in situ condition (P = 107 bar, T = 40°C) was created to represent initial state of the system, then the system re-equilibrated by introducing hydrogen. Over the course of hydrogen injection, decreasing trend in the contribution of H2 to reactions in aqueous, gas, and mineral phases is observed, pH value increases immediately after hydrogen introduced into the system and interactions among minerals and brine are observed. Both minerals dissolution and precipitation are observed to maintain the equilibrium. Brine data reveal slight variations for aqueous species at the beginning of hydrogen injection and rapid changes when higher amount of hydrogen injected into the system. The results are achieved based on some assumptions such as considering hydrogen to be highly reactive and giving infinite time for reactions (equilibrium calculations).