Steam field reservoirs are special cases where the fluid is predominantly found in a gas phase between 230°C to 240°C.
"This special class of resource needs to be recognized, its uniqueness being the remarkably consistent initial temperature and pressure (approximately 240°C and 33.5 bar-a) displayed by all such fields in the world: Kamojang (Indonesia), The Geysers (California), Larderello (Italy), Matsukawa (Japan), Darajat (Indonesia), etc. Furthermore, the enthalpy of the resource in such a field is the maximum enthalpy possible for saturated steam (2,800 kJ/kg). Since a pervasively superheated steam reservoir is physically unlikely (notwithstanding occasional cases of superheated steam production at the wellhead), a Class 7 resource typically has the largest available energy per unit mass of all classes. Therefore, wells of 30 to 50 MWe capacity are not uncommon in such fields. Ignoring geothermal resources in the National Parks, The Geysers is the only such field in the U.S. Given the pivotal importance of The Geysers to the U.S. geothermal industry, this class is recognized separately" (reference: http://www.geothermal-energy.org/pdf/IGAstandard/SGW/2005/sanyal1.pdf).