Strontium isotope geochemistry of oil field waters in southeastern Mississippi: evidence for fluid/rock interaction and application as hydrologic tracer

Variations in strontium concentration and 87Sr/86Sr in oil field waters from Jurassic and Cretaceous reservoirs in southeastern Mississippi provide evidence about the chemical evolution of pore fluids as a result of diagenesis and about their use as a hydrologic tracer. Water samples have been analyzed from seven producing fields. Strontium concentrations range from 14.3 mg/kg to 1766 mg/kg. 87Sr/86Sr ranges from 0.7071 to 0.7119. Total dissolved solids range from 3000 mg/1 to 301,000 mg/1. Most of the samples may be classified as brine (TDS greater than 100,000 mg/1). Each formation within a producing field occupies a distinct area on plots of 87Sr/86Sr versus strontium concentration. These plots provide a useful fingerprint of oil field waters within a producing field and can be used to identify and evaluate mixing of fluids between reservoirs, both through geologic processes and during resource development. With the exception of waters from the Selma Chalk, all are more radiogenic than Jurassic or Cretaceous seawater. In general, 87Sr/86Sr values increase with depth to about 3000 meters and then level off. Dissolved strontium also increases but levels off at a greater depth. These systematic variations with depth can be explained by progressive alteration of rubidium-bearing detrital minerals andmore » dissolution and/or recrystallization of carbonate minerals.« less