Ultra High Temperature
Reservoir fluid greater than 300°C is considered by Sanyal to be "ultra high temperature".
"Such reservoirs are characterized by rapid development of steam saturation in the reservoir and steam fraction in the mobile fluid phase upon exploitation. The performance of such reservoirs, specifically the evolution of production enthalpy, is generally difficult to forecast with any confidence. The upper temperature limit for this class may be considered the critical temperature of water (374.1°C). A temperature significantly higher than the critical is unlikely to be encountered in a productive well for a number of physical reasons" (reference: http://www.geothermal-energy.org/pdf/IGAstandard/SGW/2005/sanyal1.pdf).