Origin and chemical evolution of formation waters from Silurian-Devonian strata in the Illinois basin, USA

Abstract

A suite of formation-water samples from Silurian-Devonian reservoirs in the Illinois basin has been analyzed for major, minor and trace element concentrations and for H, O and Sr isotopic compositions in order to interpret origin of salinity and geochemical evolution of brine compositions in this evaporite- and shale-poor cratonic basin. Although chloride concentrations range from 2000 to 137,000 mg/L,Cl/Br ratios (291 +/- 18) are consistent with those of seawater or seawater evaporated short of halite saturation (Cl/Br = 292). Thus, during Silurian-Devonian time, subaerially evaporated, penesaline brine entered the subsurface where it was chemically modified through brine-rock interactions. Cation/Br ratios and mineralogy of associated strata indicate that Na and K were depleted through interaction with clay minerals, Ca was enriched and Mg depleted by dolomitization and Sr was enriched as a result of CaCO3 recrystallization and dolomitization. Although significant dilution of the modified brine with meteoric water is supported by [delta]D-[delta]18O covariance, original marine waters have not been completely expelled from Silurian-Devonian strata. Hydrogen and oxygen isotopes exhibit covariant relations with cation and anion concentrations, implying that isotopic exchange between H2O and minerals has not greatly influenced the [delta]D - [delta]18O trend. Brine 87Sr/86Sr ratios range from 0.7092 to 0.7108; when these ratios are plotted versus 1/Sr, a two-component mixing trend is suggested, although Sr concentrations have experienced local diagenetic modification. A 87Sr-enriched fluid may have accompanied petroleum migration from New Albany shales into adjacent Silurian-Devonian carbonates where it mixed with remnant evaporated seawater. This event probably preceded the influx of meteoric water, as /gdD and [delta]18O are not correlated with Sr isotopic compositions of formation waters.