Evidence of Deep Acid Fluids in the Los Humeros Geothermal System, Mexico

Abstract

Los Humeros geothermal field is one of three geothermal fields under exploitation in Mexico. Los Humeros, hosted in a volcanic caldera, is located in the central-eastern portion of the country, within the Mexican Volcanic Belt, near the limit of this province with the Sierra Madre Oriental province. One special feature in the Los Humeros is the occurrence of fluids of low pH in wells drilled in the area known as the Colapso Central, particularly at more than 1800 m depth. No evidence of a proper of acidic reservoir has been found; instead, the formation of low pH fluids has been explained as a post-exploitation process related to the migration of deep magmatic volatile species. Minerals such as lazulite, tourmaline, pyrophyllite and zunyite have not been found in reservoir rocks; instead, typical mineralogy of neutral to basic environments has been reported. In a recent ongoing research project, a core from a depth of 2000m was studied. Its physical characteristics show the evidences of its interaction with low pH fluids. Three fragments of the core show a rock of cream color, it is like a porous rock mostly composed by residual quartz. It has been classified as andesite even it does not contain primary or hydrothermal minerals commonly found in this type of rock. By petrography and X-ray diffraction abundant fine crystalline quartz and plagioclases were identified. Traces of chlorite and pyrite were also present. At the time the well was drilled, besides of the circulation lost and high temperature (378 °C), the well was abandoned; so no production data were recorded. Mineralogical, textural, chemical and petrophysical data supported the evidence of local deep acid fluids in the Los Humeros geothermal reservoir.