A model is described for planar geothermal reservoir charging by upflow in a permeable fault zone in the earth's crust. Liquid water, heated at depth in basement material, rises in the fracture system associated with the faulted region. When the fault is intersected by a horizontal layer of permeable material, charging of hot water into an adjacent aquifer can occur. The heat and mass transfer process for this system is defined in terms of natural and forced convection in a thermally active porous medium. Solutions to the basic mathematical model are obtained by finite-difference methods. The fluid mechanics and energy transport are resolved in both the narrow fault zone and the adjacent aquifer. The results show that previously developed asymptotic solutions, derived for large Rayleigh numbers and narrow fault zones, provide quite good predictions of the system heat and mass transport. The pressure field induced by fluid motion, typically about 50% of the cold hydrostatic head, is found to be quite sensitive to the input mass flux when the system Rayleigh number R is between 100 and 500. When R=100, the convective upflow in the fault zone is cooled by conduction to the surface down to 60% of the system depth. In contrast, when R=500, only about 20% of the fault is affected by cooling. The temperature variation with depth in the adjacent aquifer is nearly the background conductive form at about 5 fault depths away from the fault zone when R=500. In comparison, when R=100, the analogous distance is reduced to about 1 fault depth. The surface heat flux distribution is strongly dependent on the R value. Above the fault the heat flux value increases linearly with R in the range 100 to 500.