ABSTRACT

The total dissolved solids content of formation water from Plio-Pleistocene reservoirs, offshore Louisiana, is dominated by NaCl derived from the dissolution/recrystallization of diapiric salt. Other solutes, and the water itself, are derived from primary pore water (originally seawater, subsequently modified by sulfate reduction and methanogenesis) from mineralogically immature late Cenozoic marine clastics. ¹⁸O values between 0 and +2 (SMOW), coupled with non-radiogenic ⁸⁷Sr/⁸⁶Sr ratios, demonstrate that the water has undergone little interaction with detrital silicates (smectite or detrital feldspars) prior to emplacement in the reservoirs. Water from Plio-Pleistocene reservoir rocks contrasts with water from nearby, onshore Miocene reservoirs, which is largely derived from more mineralogically mature Cenozoic clastic sediments. Low Ca, Ba, Li, B, and Br in both Miocene and Plio-Pleistocene water samples from offshore Louisiana indicate little contribution from Ca-rich water characteristic of deep-seated Mesozoic reservoirs. A few samples of formation water associated with diapiric salt structures could contain up to about 10% solutes derived from deep-seated Mesozoic sources, however.