Stable chlorine isotopes in halite and brine from the Gulf Coast Basin: brine genesis and evolution

Stable chlorine isotope data, used in conjunction with other geochemical parameters, are useful in determining the origin of solute in formation waters with salinity different from that of seawater. Jurassic salt in the Gulf Coast Basin has δ 37Cl ranges of −0.5‰ to 0.3‰ (bedded) and 0.0‰ to 0.5‰ (diapiric). The values for bedded salt are consistent with a δ 37Cl value of 0.0‰ for Jurassic seawater chloride, as in modern oceans. The slightly higher values for diapiric salt are possibly caused by incongruent solution of halite. Formation waters have a δ 37Cl range of −1.9‰ to 0.7‰. Waters with δ 37Cl−0.2‰. Low-salinity water from geopressured aquifers appears to have resulted from shale dewatering. Low δ 37Cl values in such water are related to low m Cl/ m Na (a parameter indicating dewatering) but show no general relationship to Cl/Br. They are consistent with diffusion of chloride from allochthonous brine. Diffusion can generate domains of water with negative δ 37Cl on a scale of hundreds of meters in 10 4–10 6 years, and may also generate positive- δ 37Cl chloride in residual source brine.