Assigning heat sources to a numerical model of a geothermal reservoir for the simulation is a quite challenging task not only because of its challenge to assign proper intensity of heat flux, the proper value of enthalpy and flow rate of the upflow, but also because of their correct assignments to the reservoir model. This paper presents how the assignment of heat flux and upflow to the reservoir model layers will influence the simulation result. The simulation was carried out under three types of cases for evaluating the influence of enthalpy-heat flux intensity difference. In case 1, a high enthalpysmall heat flux combination was used, whereas in case 2 and case 3, medium-medium and lowhigh combinations were used, respectively. In model 1, heat flux was assigned to the bottom layer, and upflow was assigned to the second bottom layer. In model 2, both heat flux and upflow were assigned to the second bottom layer. In model 3, both heat flux and upflow were assigned to the bottom layer. The result shows reservoir simulation is not influenced by whether upflow and heat flow are assigned to the same layer or separated. When the upflow carries high enthalpy fluid, the distance from the origin of upflow will influence temperature and pressure distribution. The existence of high permeable faults and low-temperature cold recharge does not interfere with the converging process of two types of heat sources.