Fluid chemistry of the Baca geothermal field, Valles caldera, New Mexico
— Art F. White, Joan M. Delany, Truesdell, Alfred, Janik, Kathy, Goff, Fraser E., and Harrison Crecraft

The Baca geothermal field is located on the western flank of Redondo Peak, a resurgent-dome structure near the center of the Valles caldera in northern New Mexico. The geology of the Valles caldera region is described by Smith and Bailey (1968). The regional hydrology has been presented by Trainer (1974, 1975), Trainer and Lyford (1979), Laughlin (1981), Goff and others (1981), and Goff and Grigsby (1982). Production characteristics of the Baca geothermal field are discussed by Bodvarsson and others (1982) and Union Oil Co. (1982).

To date, 24 geothermal wells have been drilled into the Baca geothermal field in the Sulphur Creek area (Fig. I) and along the Redondo Creek. The wells at Sulphur Creek penetrated high temperature (>260°C) but low-productivity zones. Wells completed along Redondo Creek, in a large northeast-trending central graben within the caldera, encountered similar high temperatures in a more permeable fracture and stratigraphically controlled reservoir.

After the initial completion of five commercial wells in the Bandelier Tuff by Union Oil Co. of California (Union), a 50 MWe power plant was proposed in 1977 by the Public Service Co. of New Mexico in cooperation with the U.S. Department of Energy Geothermal Demonstration Power Plant Program. However, drilling of 13 additional wells aimed at completing the required steam capacity resulted in only two production wells. Subsequent attempts to find additional steam in deeper Paleozoic limestones and Precambrian granite, as well as steam derived from hydraulic fracturing in the tuff, failed to adequately increase production and the project was terminated in January 1982.  

The fluids tapped by the Union wells represent the deepest and highest- temperature fluids encountered within the Valles caldera. Their chemistry is important in the context of understanding both the overall hydrothermal recharge and discharge system in the caldera and the potential reservoir contributions to thermal springs outside the caldera, which add significantly to surface flow in the Jemez River. In addition, the Valles caldera is a principal target for proposed deep continental drilling into a magma—hydrothermal regime. The geothermal fluids in the Baca reservoir are probably the best available indicators of potential interaction with magma-derived components at greater depth.

Although Union had previously collected geochemical data applicable to their exploration program, final flow tests at a number of the wells during the summer and fall of 1982 represented an excellent opportunity to collect a more comprehensive suite of chemical data. This paper presents preliminary major- and trace-element data and reinterprets some aspects of the regional hydrothermal system. Additional work is underway in interpretation of existing data for other wells, interpretation of isotopic data, and modeling water/rock interaction within and beyond the geothermal reservoir.