Elemental and isotope geochemistry of Smackover Formation brine in southwest Alabama

Abstract

This study investigates the geochemical evolution of brines within the Smackover Formation in southwest Alabama through elemental and isotopic analyses, coupled with geochemical modeling techniques. Major ion and trace element analysis displays Na-Cl-Ca dominated hydrochemical facies and a distinct lack of carbonates (HCO3- or CO32- ) in the Alabama brine samples in this study. Lower sulfate concentrations, coupled with high H2S levels in some brines, suggest sulfate removal mechanisms such as bacterial sulfate reduction or precipitation of sulfate minerals. Analysis of Cl/Br, Na/Br, and K/Br ratios indicates the acquisition of salinity through the mixing of meteoric water with evaporated seawater reaching halite saturation, rather than halite dissolution. Isotopic trends of δ18O and δ2H also suggest a mixture of meteoric water with remnant evaporated seawater reaching gypsum and halite saturation. The strontium isotopic ratios of the brine range from 0.707403-0.710026, higher than that of Jurassic seawater (0.7071). The varying extent of water-rock interaction can be understood by the relationship between 87Sr/86Sr and alkali elements (K and Li). Multiple fluid pathways may be taken through the siliciclastic basement rock as water is forced into the Smackover Formation. Geochemical modeling predicts a Cl-Mg facies after halite saturation, contrasting with the observed Na-Cl-Ca dominance in the Smackover Formation brines. This suggests the influence of water-rock interaction such as dolomitization.