Study of the Water-Rock Interaction Processes in the Hidden Geothermal System of Acoculco Puebla (México) by Using a Geochemometric Modeling

Abstract

Abstract. The study of water-rock interaction (WRI) processes through an integral modeling of fluid and rock geochemistry is being used as a new geochemometric tool for the prospection of geothermal systems. These WRI studies are becoming in a powerful and efficient tool for elucidating the main transport mechanisms of major and trace elements that occur in geothermal systems, including the determination of deep reservoir temperatures with less uncertainty. The present work reports the results of a comprehensive WRI study carried out in the hidden Geothermal System of Acoculco (GSA) which is currently under intensive exploration. This hidden geothermal system is an anomalous geothermal system, characterized by active hydrothermal alteration (argillic alteration), cold springs with acidic fluids, and cold-gas emissions with near ambient surface temperatures (also referred as Kaipohans). Based on an early exploration programme carried out by the Comision Federal de Electricidad (CFE) of Mexico and a high-temperature source of nearly 300 °C, the GSA was proposed as a probably hot-dry rock system. Impermeable layers or faults were most likely sealed by calcite or silica precipitation, which have limited the transport pathways of fluids and gases from deep-seated sources to the surface. As a part of a comprehensive geochemical field work at the GSA, sampling and analysis of rock and fluid samples were conducted. Based on the chemical analysis of major and trace elements in both rock and fluid samples, a new geochemical model is proposed for explaining the main geochemical signatures and the behavior of major and trace elements in this complex geothermal system. A special emphasis has been given to the interpretation of the behavior of the rare earth elements (REE) in the rock and fluid samples. Because REE are elements present at very low concentrations in most minerals, their relative abundances may reflect the interaction of unique chemical properties of the system. Similarly, the pattern of behavior of the elements of REE in surface acidic fluids may contribute to the understanding of fractionation and mineral associations, degree of alteration, and processes of hydrothermal alteration taking place in the study area. With regard to the calculation of reservoir temperatures, gas geothermometry tools have been applied for predicting the deep reservoir temperatures with their associated uncertainty, which were compared with bottomhole temperatures logged in two exploratory wells. From the integration of all these geochemometric studies, a more realistic geochemical model of GSA is proposed in this work.