Eva P. Moldovanyi, Lynn M. Walter
Trace and major element geochemical and isotopic (^dgr18O, D, 87Sr/86Sr) 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 ^dgr18O and ^dgrD (+0.2 to +5.0^pmil and -18.5 to 25.7^pmil 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 eteoric fluids during early diagenesis has been inferred from diagenetic textures, such as oomoldic porosity. Low temperatures (50-60°C), high SO42- (up to 700 mg/L), absence of H2S, and low to moderate 87Sr/86Sr (mean = 0.70799) suggest that chemical SO42- 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. Heaver ^dgr18O and ^dgrD (+ 6.0 to + 7.0^pmil and -6.2 to -13.0^pmil SMOW, respectively), and higher salinites (TDS up to 350 g/L) suggest lack of meteoric flushing. Even here, however, areas of localized exposure are evidenced by light ^dgrD (-16 to -23.8^pmil). High concentrations of boron (up to 360 g/L) and radiogenic 87Sr/86Sr 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.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990