Geochemical Survey of Medium Temperature Geothermal Resources in the NW Zone of Mexico

Abstract

In order to evaluate medium temperature resources to be exploited in the near future in the NW zone of M?xico, a geochemical survey of samples from wells and springs, was carried out. Five well samples from the Ri?to (Sonora) zone, one well sample from the Cerritos (Baja California) zone, and six samples from thermal springs located in the Pen?nsula de Baja California coast (Punta Estrella, San Felipe, Puertecitos, El Coloradito, and Valle Chico) were considered. Chemical and isotopic data were processed in order to classify the fluids, to estimate reservoir temperatures, and to investigate possible mixing relationships. Considering that some springs are located in coastal zones, the fraction of sea water component in samples was also estimated.

Maxium reservoir temperatures were found to be 192?C for Ri?to zone, 130?C for the Cerritos zone, 135?C for Valle Chico zone, and 225?C for Punta Estrella and San Felipe zones. Reservoir temperature estimations for the Puertecitos and El Coloradito zones were confusing due to the presence of sea water component in the samples. Isotopic (d 18O, d D) data showed that San Felipe and Punta Estrella fluids lie along an evaporating line. The Ri?to deep well samples exhibited very high salinities (more than 40,000 mg/kg Cl) and they seem to originate from dissolution of evaporitic sediments while fluids from artesian wells present low salinity (less than 1,000 mg/kg Cl). The Cerritos sample plot along the isotopic composition of Cerro Prieto geothermal fluids indicating a similar origin for both fluids. Volcanic steam heated waters were found in the Valle Chico zone sample.

Based on reservoir temperature estimations, drilling in San Felipe and Punta Estrella sites is recommended in order to obtain more representative samples. Electricity could be generated in these zones by conventional or binary cycle. Direct use of geothermal waters is proposed for sites with lower reservoir temperatures.