A brine interface in the Salton Sea Geothermal System, California: Fluid geochemical and isotopic characteristics

Abstract

Data from 71 geothermal production intervals in 48 wells from the Salton Sea Geothermal System (SSGS) indicate that fluids in that system cluster into two distinct populations in terms of their salinity and their stable isotopic compositions. The distinctive, hot, hypersaline brine (typically >20 wt% total dissolved solids) for which the SSGS is known is overlain by a cooler (<260°C) fluid with distinctly lower salinity (typically <10 wt% total dissolved solids). Hypersaline brines have high and rather consistent 18O shifts produced by water-rock interaction and have a very narrow range in δD values. Low TDS fluids, on the other hand, show a wide range in both δD and δ 18 O. Production of both types of fluid from closely spaced geothermal wells in many regions of the SSGS indicates that a relatively sharp salinity interface exists over much of the field. The fluid interface typically cross-cuts sedimentary bedding but is consistently found where reservoir temperatures are approximately 260°C. At these temperatures, hypersaline brines have densities of approximately 1.0 gm/cm 3, while the low TDS fluids have densities as low as 0.85 gm/cm 3. This stable, density-stratified interface acts as a barrier to convective heat and mass transfer in the SSGS, isolating the hypersaline reservoir from overlying dilute fluids. A lithologic “cap” implied by previous SSGS models is unnecessary in such a stratified system since heat and mass transfer across the interface must occur by slow conductive, diffusional and interface mixing processes regardless of local permeability.