Low Salinity Waters in Deep Sedimentary Basins
While the composition and origin of brines in deep sedimentary basins has been extensively discussed, the composition and origin of low salinity waters is not as well documented. Since the 1960s, the presence of deep, low-salinity waters has been observed in some sedimentary basins and is commonly present in overpressured sections. The episodic release of low salinity, overpressured fluids upward into sediments containing high salinity formation waters likely occurs at <100-year intervals. Because there is a growing body of evidence that suggests mixing formation waters of varying salinity could induce the dissolution and precipitation of minerals, it is important to have detailed information about chemical compositions of both end-member fluids to adequately predict such mixing results. This study concludes that low salinity waters (<35 g/l) are generally not unique in major solute composition when compared to high salinity waters (>35 g/l). On log-log plots, monovalent cations plot along 1:1 slopes with respect to salinity and total anionic charge, while divalent cations plot along 2:1 slopes. However, this study concludes that more compositional variability exists at salinities less than seawater and lower anionic charge values. Also, major cations and Cl correlates better with anionic charge than with salinity. While Cl plots on a 1:1 slope as a function of salinity in all waters in southwest Louisiana, other basins in this study begin to show variability at salinities <10,000 mg/l. Spatial analyses from this study suggest the origin of low salinity fluids in southwest Louisiana present at depths greater than 2500 m, are not likely meteoric in origin. Southwest Louisiana water compositions are most likely controlled by rock-buffering with ambient mineral phases. A calculation performed as part of this study showed that the smectite to illite transition could reduce salinity up to 43% in shales. Considering this and the documented presence of illite along the Gulf Coast as well as the stability of illite in the waters presented in this study, it is reasonable to conclude deep, low-salinity waters in southwest Louisiana originate in substantial part from the smectite to illite transition.
Advisor:Jeffrey Nunn; Jeffrey Hanor; Philip Bart
School:Louisiana State University in Shreveport
School Location:USA - Louisiana
Source Type:Master's Thesis
Keywords:geology and geophysics
Date of Publication:04/10/2003