Fluid Sources and Rock/Water Interactions During Burial Diagenesis, Upper Jurassic Smackover, Formation, Southwest Arkansas: ABSTRACT

Abstract

Trace and major element geochemical and isotopic ({delta}{sup 18}O, D, {sup 87}Sr/{sup 86}Sr) trends in Upper Jurassic Smackover Formation fluids, southwest Arkansas, correlate with previously defined regional diagenetic gradients and record rock/water interaction during progressive diagenesis. Formation fluids within southwest Arkansas shelf reservoirs are grouped as northern and southern, according to present geochemical patterns. Northern fluids, with light {delta}{sup 18}O and {delta}D (+0.2 to +5.0 and {minus}18.5 to 25.7% SMOW. respectively) and low salinities (total dissolved salt = 150 to 200 g/L), exhibit a significant contribution from meteoric water. Regionally, these brines occur within the northern diagenetic zone where extensive dissolution by meteoric fluids during early diagenesis has been inferred from diagenetic textures, such as oomoldic porosity. Low temperatures (50-60{degree}C), high SO{sub 4}{sup 2{minus}} (up to 700 mg/L), absence of H{sub 2}S, and low to moderate {sup 87}Sr/{sup 86}Sr (mean = 0.70799) suggest that chemical SO{sub 4}{sup 2{minus}} reduction and exchange reactions with siliciclastics have not significantly affected fluid chemistry. Formation fluids occurring within transitional and southern diagenetic zones, record a more complex history. Heavier {delta}{sup 18}O and {delta}D (+ 6.0 to + 7.0% and {minus}6.2 to {minus}13.0% SMOW, respectively), and higher salinities (TDS up to 350 g/L) suggest lackmore » of meteoric flushing. Even here, however, areas of localized exposure are evidenced by light {delta}D ({minus}16 to {minus}23.8%). High concentrations of boron (up to 360 g/L) and radiogenic {sup 87}Sr/{sup 6}Sr isotopic ratios (mean = 0.70987) may occur in southern zone fluids. These trends are related to areas where overlying Buckner anhydrite grades into a more shaly unit, indicating mixing with chemically and isotopically evolved fluids.« less