Refined Conceptual Modeling and a New Resource Estimate for the Salton Sea Geothermal Field, Imperial Valley, California

Interim results of a new conceptual modeling effort for the Salton Sea geothermal field (SSGF), in the Salton Trough of southernmost California, show that this resource: (1) is hotter at depth (up to at least 389 degrees Centigrade at 2 km) than initially thought; (2) is probably driven by a still-cooling felsic intrusion rather than (or in addition to) the primitive mafic magmas previously invoked for this role; (3) may be just the most recent phase of hydrothermal activity inititiated at this site as soon as the Trough began to form ~4 m.y. ago; (4) is thermally prograding; and (5) in spite of 30 years' production has yet to experience significant pressure declines. Thick (up to 400 m) intervals of buried extrusive rhyolite are now known to be common in the central SSGF, where temperatures at depth are also the hottest. The considerable thicknesses of these concealed felsic volcanics and the lack of corresponding intermediate-composition igneous rocks imply coeval granitic magmas that probably originated by crustal melting rather than gabbroic magmatic differentiation. In the brine-saturated, Salton Trough sedimentary sequence, granitic plutons inevitably would engender convective hydrothermal systems. Results of preliminary numerical modeling of a system broadly similar to the one now active in the SSGF suggest that a still-cooling felsic igneous intrusion could underlie deep wells in the central part of the field by no more than a kilometer. The model results also indicate that static temperature profiles for selected Salton Sea wells could have taken 150,000 to 200,000 years to develop, far longer than the 20,000 years cited by previous investigators as the probable age of the field. The two viewpoints conceivably could be reconciled if the likely long hydrothermal history here were punctuated rather than prolonged. Configurations of the temperature profiles indicate that portions of the current Salton Sea hydrothermal system are still undergoing thermal expansion. A newly consolidated, field-wide reservoir database for the SSGF has enabled us to reassess the field's ultimate resource potential with an unprecedented level of detail and confidence. The new value, 2330 MWe (30+ year lifetime assured) closely matches an earlier estimate of 2500 MWe (Elders, 1989). If this potential were fully developed, the SSGF might one day satisfy the household electrical-energy needs of a fourth the present population of the State of California.