| Abstract |
The two largest geothermal fields generating electricity in southern California, USA, and adjacent northern Baja California, Mexico, are the Salton Sea Geothermal Field (SSGF), in California, and the Cerro Prieto Geothermal Field (CPGF), in Baja California. Both fields have similar tectonic environments of pull-apart structures developed at right-stepping, right lateral, strike-slip faults. Their reservoir rocks are both largely deltaic and lacustrine sediments deposited by the Colorado River, with a higher content of lake sediments at the SSGF. The sizes of these two geothermal fields are about equal, and both have downhole temperatures reaching as high as 350-370°C at 2-3 km depths. The salinity of their reservoir fluids is the main difference between them. The CPGF brines have a salinity of ~5% TDS, while the salinity of the SSGF brines is ~28% TDS. This results from the SSGF being near the lowest part of the Salton Trough that forms a closed basin with a surface elevation of ~90 m below sea level. In the past, this depression was repeatedly filled by the Colorado River flowing north, forming large lakes. Each time the river resumed its southerly course to the Gulf of California, these lakes dried up, leaving behind evaporites, that are the source of the higher salinity of the SSGF brines. Different socioeconomic factors across the international border led to different development of these two geothermal resources. At its maximum, the CPGF had 720 MWe of electrical capacity installed, but now, after 47 years of generation, its capacity has declined to 570 MWe due to falling reservoir pressures. Development was slower at the SSGF, due to both the higher salinity and the difficulty in getting power purchase agreements. The SSGF currently has an installed generating capacity of only 432 MWe, but a minimum of 395 MWe will be added in the near future. However, the salinity of the SSGF brines, which was a disadvantage for developing electrical generation, is now turning out to be an asset. The SSGF brines contain valuable concentrations of metals, including up to 250-300 mg/kg of lithium, now in great demand for lithium-ion batteries. Preliminary estimates of the value of the lithium in the SSGF and adjacent area brines are >US$10 billion, so today the first steps in developing a world class lithium extraction industry and battery manufacturing plant at the SSGF are underway. The brines produced at the CPGF contain up to 40 mg/kg of lithium. At the CPGF disposal of the majority of spent brine, after steam separation, is in a large evaporation pond of around 14 km2 area. After 47 years of operation, one to two billion tonnes of spent brine have flowed to the evaporation pond, which now should contain 20-40 million tonnes of lithium, with a value of billions of US dollars, at current 2022 prices. |