Signature of ongoing brine reinjection on noble gas isotopes and fluid chemistry at Las Tres Vírgenes geothermal field, Mexico

Abstract

A geochemical survey of the Las Tres Vírgenes geothermal field (LTVGF), Baja California Sur, Mexico, was carried out to determine the origin and evolution of its fluids, along with the influence of injection of local brines, and inhibitors intended to manage carbonate scaling issues. Major and minor cations, Cl, Br, Sr, O, H, and noble gas (He, Ne, Ar, Kr and Xe) isotopes were measured in fluids collected from four production wells, two injection wells, and one fumarole (El Azufre) in 2016 and 2018. Variations in the stable isotopes of water (δD and δ18O) and halogens (δ37Cl, δ81Br) over the years suggest that local brine reinjection might have started to modify the pristine chemistry of the fluids. However, relevant information on the original geothermal fluids has been preserved. The Na/Br and Cl/Br ratios suggest that LTVGF brines are seawater having leached evaporite deposits (halite). These deposits are absent in the area, but were likely present during the Miocene, prior to the breakup of the Gulf of California, suggesting that the saline fluid end-member at the LTVGF could be several million years old. Measured 3He/4He ratios (R) normalized to the atmospheric ratio Ra (= 1.384 × 10−6) plotted against the 4He/20Ne ratios suggest mixing between a mantle fluid with a R/Ra of 6.19, typical of the sub-continental lithospheric mantle and an atmospheric helium component. The R/Ra versus the 132Xe/36Ar ratio normalized to air suggests mixing between a mantle-rich pre-production fluid and air. The source of air appears to be linked to local gravity-controlled flow reinjection which causes the entrapment of air bubbles, leading to a fluid supersaturated in atmospheric noble gases. This pattern suggests that reinjection fluids are impacting the production area.