Abstract: Water Rock Interaction and the Distribution of Salinity in the Illinois Basin

J. A. Sathaye, D. E. Grandstaff

Available data on the chemical composition of Illinois Basin formation waters was analyzed to identify the diagenetic reactions that are occurring in the basin. The distribution of salinity was visualized in three dimensions and correlated with the present day groundwater flow system in the basin. Formation waters in the entire basin are enriched in calcium and depleted in magnesium, sodium, potassium and sulfate, relative to the seawater evaporation trend. However, there are different compositional trends in waters from Devonian-Silurian and Mississippian-Pennsylvanian strata which are probably maintained by the regional aquitard separating them. Extensive analysis of brines reveals that their composition is influenced to a large degree by processes such as seawater evap ration, halite dissolution, ion-exchange on clays, dolomitization, authigenic magnesium-silicate formation, and bacterial sulfate reduction. Groundwater salinity generally increases with depth, and towards the southern and eastern regions of the basin. Freshwater recharge occurs along the basin's northern and western margins.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas