This paper reviews geological, geochemical and geophysical data on the deeper parts of the high-temperature, water-dominated geothermal system of Cerro Prieto in Baja California, Mexico. It also speculates about the lithology, temperature, pressure and fluid chemistry that might be encountered in a deep (about 6 km) exploration well being considered for the eastern part of this field. Based on the depths of observed seismic activity, it is possible that the proposed well could penetrate below the brittle- ductile transition zone. Above the transition pressures should be hydrostatic and temperatures in the 350-400?C range, with production occurring largely from fractures. Below the brittle-ductile transition fractures would tend to be sealed both by precipitation of silica and the plasticity of the rocks. In such a region fluid pressures could exceed hydrostatic and heat flow would be conduction dominated, leading to higher temperatures but lower permeabilities.

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