A conceptual model of The Geysers ＇＇High-Temperature Reservoir＇＇ (HTR) has been developed that incorporates existing field measurements and observations, including fluid inclusion salinities, core and cutting analyses, and the non-uniform depth to the top of the HTR. A numerical model based on the conceptual model successfully reproduces observed HTR characteristics. Results of the numerical model indicate that the HTR may be a long-lived feature of the ＇＇normal＇＇ vapor-dominated reservoir. Formation of the HTR is dependent on a spatial variation in petrophysical properties, a relatively rapid venting of a significant fraction of the mass in place, and the presence of a saline reservoir fluid.