A strontium isotopic study of formation waters from the Illinois basin, U.S.A

The isotopic composition of Sr has been measured in 73 formation-water samples from Paleozoic strata in the Illinois basin; 87Sr/86Sr ratios range from 0.7079 to 0.7108. With the exception of four samples, the waters are more radiogenic than corresponding Paleozoic sea-water values. The relatively narrow range of slightly elevated 87Sr/86Sr ratios is uniformly distributed in waters throughout the stratigraphic column and in Silurian waters across the basin. Isotopic analyses of core samples from reservoir rocks show an absence of water-rock Sr isotopic equilibration. Basin lithology and analyses of detrital rock units indicate that clay minerals in shales and in quartz sandstone matrices represent the only significant source of radiogenic Sr for the waters. Silurian and Devonian waters show a two-component mixing relation which suggests that they comprise a single hydrogeological system that evolved when radiogenic water from New Albany shales entered Silurian-Devonian carbonate rocks and mixed with marine interstitial water. Under favorable circumstances subsurface waters are capable of retaining a Sr isotopic record of their evolution. (author)