Fluid Inclusion Study from the Tres Virgenes Geothermal Field, Baja California Sur, Mexico

Abstract

The geothermal field of Las Tres Virgenes is a liquid-dominated system whose wells produce sodium chloride-rich waters. Fluid composition is chemically balanced and suggests that the fluids attained water-rock equilibrium at ca. 280?C, in agreement with the temperatures estimated for gaseous and liquid phases. The gas content is less than 1% in weight and is mostly CO2 (> 90%). Fluid inclusions studies show that ice-melting temperatures (Tmi) display a vertical evolution. Fluids sampled within the granodiorite basement present a large range of Tmi from -2.1?C to -15.7?C, with corresponding salinities between 3.5 to 19.2 wt.% NaCl eq. In the volcanic cover, Tmi have a much narrower dispersion, from -0.3?C to -2.0?C and associated with clathrates melting at Tmclat = +0.4 to +8.7?C. The homogenization temperatures indicate a field-scale thermal evolution from 290?C to 150?C, delineating an up-flow zone with temperatures constrained between 290 and 260?C, and an out-flow zone with a mean temperature of 240?C.