The Illinois basin provides an opportune setting for elucidating the roles of remnant evaporite brines and meteoric waters in the evolution of formation waters in an intracratonic sedimentary basin. Formation waters from carbonate reservoirs in the Upper Ordovician Galena Group have been analyzed geochemically to study the origin of their salinity, their chemical and isotopic evolution, and their relationship to paleohydrologic flow systems. Chloride/bromide ratios and CI/Br-Na/Br relations indicate that initial brine salinity resulted from subaerial evaporation of seawater rather than from halite dissolution. Subsequent subsurface dilution of the brines by meteoric waters is disclosed by δD-δ18O covariance; however, the remnant evaporite brine has not been completely expelled from these Ordovician strata. Galena formation waters have 87Sr/86Sr ratios that range from 0.708 17 (a value nearly equal to that of coeval seawater) to 0.710 43. This is the greatest range of Sr isotopic ratios found in waters from any stratigraphic unit in the basin. Two fluid mixing events are revealed in plots of 87Sr/86Sr vs. 1/Sr: introduction of 87Sr-enriched fluids from a siliciclastic source, probably overlying Maquoketa shale, and a later event that only affected reservoir waters in the western shelf of the basin. General covariance between Sr and H-O isotopes suggests that the later event is related to meteoric water recharge. The point of intersection of the δD-δ18O trend with the meteoric water line implies that this mixing event involved Pleistocene glacial meltwater that recharged Galena reservoirs near outcrops along the western margin of the basin. Ordovician Galena formation waters are geochemically distinct from those in both Silurian-Devonian and Mississippian-Pennsylvanian strata, a distinction that has evidently been maintained by the overlying Maquoketa regional aquitard.

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