We present results on regional geochemical analyses of formation waters from Jurassic carbonate strata of the upper Smackover Formation from the southwest Arkansas shelf (Gulf Coast basin). These formation waters have been noted in earlier studies for their exceptional salinities and abnormally Br-rich composition. Our detailed and regionally extensive database shows that chemical heterogeneities occur on a relatively small scale and have spatial relations to faults, which may act as pathways for fluid migration from deeper and hotter portions of the stratigraphic section. One of the most pronounced heterogeneities in water chemistry is an H2S-rich anomaly in the center of the shelf. Enrichments in H2S are accompanied by large increases in alkali elements (Li, K, and Rb) and B, which could be produced by higher temperature diagenesis of clay minerals and feldspars in deeper seated siliciclastic strata.
Bromide enrichments in all Smackover Formation waters are apparent from relations between Br and Cl contents as well as Br and divalent cation (Carpenter function) values. As suggested by earlier studies of these waters, evaporatively concentrated seawater expelled from associated thick monomineralic evaporite sections may be an important source of Br, Cl, and Na, whereas Ca, Mg, and Sr concentrations have been modified by carbonate mineral recrystallization and dolomitization. Mineral equilibria exert significant controls on concentrations of Ca, Mg, Sr, and SO4 in these brines. Br enrichment likely was produced by halite recrystallization in fluids already enriched in Br from seawater evaporation. Elevated Cl/Br ratios occur in regional association with lower salinity water near the South Arkansas fault system, consistent with halite dissolution during meteoric water recharge. Thus, despite relatively uniform reservoir lithology and simple structural setting, processes on small spatial scales have resulted in significant heterogeneity in water chemistry within a small area of the Gulf Coast sedimentary basin. Although movement of fluids along these fault systems is not demonstrated in our study, the regional trends in geochemistry are strongly suggestive of their important role in controlling chemical heterogeneities in Smackover Formation waters.