Pre- and post-exploitation variations in hydrothermal activity in Los Humeros geothermal field, Mexico

Los Humeros geothermal field is a remarkable example of the lack of water–rock equilibrium. Significant variations of hydrothermal activity have occurred before and after exploitation started. Presently, discharged water is not in equilibrium with the alteration suite observed in the reservoir rocks. Hydrothermal minerals identified in core and cuttings define the occurrence of several stages of hydrothermal activity. Cooling at depth is inferred from fluid inclusion and alteration mineralogy data from the wells located nearby Los Humeros fault. Most wells produce a two-phase fluid with excess enthalpy, this accounts for the high CO2 content observed in the discharged fluid. Sulfur and carbon isotopic data indicate that volatile species in the geothermal fluid have magmatic as well as sedimentary components, while strontium isotopic composition shows that calcium is provided by the andesitic rocks that form the reservoir. As exploitation of the field started, concentration of HCl increased in the discharged fluids. This shows that recharge of the reservoir is not enough to balance the output for production, and drying out of the field may be taking place at depth. The lack of chemical equilibrium and the presence of gases of magmatic origin suggest that Los Humeros is a relatively young geothermal field related to a recent magmatic intrusion.