The Salton Sea geothermal area is in a rift valley of the San Andreas fault system in southern California. The geothermal reservoir consists of more than 2,000 ft of arkosic sandstone containing interstitial concentrated NaCl-CaCl₂-KCl brines. No steam is present underground. The sandstone is overlain by shale from the surface to a depth of 2,000-3,000 ft. The temperature in the subsurface exceeds 300°C at 3,000 ft and 360°C at 7,000 ft. Thermal convection of pore fluids in the sandstone appears to be the primary mechanism of heat transfer in the reservoir. The salinity and temperature of the pore fluids decrease outward from the center of the geothermal reservoir. The enthalpy of the brines in the reservoir ranges from 220 to 275 cal/g, which is 45-9 cal/g less than the enthalpy of pure water at equivalent temperatures and pressures. Although the origin of the brines is obscure, it appears likely that they formed by evaporation of Colorado River water originally trapped in the pore spaces of the reservoir sands.

Thermodynamic calculations indicate that reaction of the interstitial brines with the sediments in the geothermal reservoir (which is below 3,000 ft) has resulted in substantial mass transfer (more than 40 g/1,000 g of water) during metamorphism. Geochemical considerations suggest that the reservoir fluids in the geothermal system are in chemical equilibrium with the mineral assemblage in the enclosing rocks. Chemical reactions between the original pore fluid in the rock and the sedimentary mineral assemblage have led to a relatively low pH and enrichment of K, Ca, Fe cations, and other constituents in the aqueous phase. The total dissolved solids in the brine have been concentrated by removal of water from the system.

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