THE ORIGIN OF WATER AND s...

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, 0, and Sr isotopic com-positions 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 (29 1 Ifr 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 CaC03 recrystallization and dolomitization. Although significant dilution of the modified brine with meteoric water is supported by 6D- 6’sO 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 H20 and minerals has not greatly influenced the 6D- 6’*0 trend. Brine “Sr/*‘Sr ratios range from 0.7092 to 0.7108; when these ratios are plotted versus l/Q, a two-component mixing trend is suggested, although Sr concentrations hav