Strontium Isotopic Study of Subsurface Brines from Illinois Basin

Ernest A. Hetherington, Alan M. Stueber, Paul Pushkar

The abundance of the radiogenic isotope ⁸⁷Sr in a subsurface brine can be used as a tracer of brine origin, evolution, and diagenetic effects. We have determined the ⁸⁷Sr/⁸⁶Sr ratios of over 60 oil-field waters from the Illinois basin, where brine origin is perplexing because of the absence of any significant evaporite strata. Initially, we analyzed brines from 15 petroleum-producing sandstone and carbonate units; waters from Ordovician, Silurian, Devonian, and Mississippian strata have ⁸⁷Sr/⁸⁶Sr ratios in the range 0.7079-0.7108. All but those from the Ste. Genevieve Limestone (middle Mississippian) are more radiogenic in ⁸⁷Sr/⁸⁶Sr than seawater values for this interval of geologic time. The det ital source of the more radiogenic ⁸⁷Sr may be the New Albany Shale group, considered to be a major petroleum source rock in the basin. The ⁸⁷Sr/⁸⁶Sr ratios of Ste. Genevieve brines apparently evolved without a contribution from fluid-shale interaction.

Additional work has emphasized Silurian brines, primarily from Niagaran reefs. Samples from 12 producing fields, widely distributed in the basin, yield ⁸⁷Sr/⁸⁶Sr ratios from 0.7092 to 0.7108 and Sr concentrations from 1.8 to 908 µg/ml. For these and all the other brines, no relationship exists between ⁸⁷Sr/⁸⁶Sr and Sr concentration. This suggests that the isotopic ratios were established within a relatively narrow range in these waters by fluid-rock interaction, and then the brines were diluted to varying degrees by mixing with a fluid (probably meteoric water) containing negligible dissolved Sr. Diagenetic effects of Silurian brines have been investigated through isotopic analyses of dolomitized reef rock.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.