The Cerro Prieto geothermal field is located 35 mi (56 km) south of Mexicali, Baja California, Mexico. The subsurface geology of Cerro Prieto is typical of a deltaic depositional environment. Logging data from more than 70 wells have been analyzed. A comparison of the data derived from these logs with surface geophysics, mineralogic, and stratigraphic information leads to a model of subsurface fluid flow.
Cross sections have been constructed using the values obtained from resistivity and density logs. The resistivity cross sections are consistent with the model derived from surface dipole-dipole resistivity measurements, lithology, production, and mineralogic characteristics. The density cross sections show good agreement with the location of a self-potential anomaly and the location of micro-earthquake hypocenters.
Interpretation of these well logs leads to estimates of porosity, salinity, temperature, and permeability. High porosities are probably due to secondary dissolution. The salinity distribution shows that relatively fresh water extends from near the surface to depths of 4,000 to 5,000 ft (1,220 to 1,525 m) in the eastern part of the field and to shallower depths in the western part. Water of higher salinity underlies the fresh water. Permeability has been hard to estimate because of conflicting evidence from the well logs.
In the proposed model of hydrothermal flow, hot brines enter the field at 5,000 ft (1,525 m) or more at the eastern part of the field and flow upward and westward through the field.