SEAWATER ORIGIN OF CACL2 BASINAL BRINES IN THE ILLINOIS BASIN
Basinal brines in Silurian-Devonian formations of the Illinois basin are typical CaCl2 oil-field brines with higher Ca and lower K, Mg, and SO4 than modern evaporated seawater. Illinois basin brines show the same compositional trends as those of progressively evaporated CaCl2-rich Silurian seawater. Chemical deviations can be accounted for quantitatively by brine-rock reactions during burial (dolomitization, dolomite and K-feldspar cement). Illinois basin brines and brines predicted on evaporation of Silurian seawater both have lower Na and SO4 than evaporated modern seawater. Illinois basin brines have Ca well above evaporated modern seawater which reflects the Ca-enrichment of Silurian seawater. These brines are depleted in Mg, below that predicted by evaporation of Silurian seawater. The loss of Mg can be accounted for by dolomitization of limestones and formation of dolomite cement, which together transform the Mg and Ca concentrations in evaporated Silurian seawater into their measured amounts in the Illinois basin brines. The nearly constant Ca/Mg ratio of Illinois basin brines suggests that dolomitization of the host limestones buffered the Ca/Mg ratio of the basinal brines at temperatures of 75-100 °C. The K in Illinois basin brines is below that predicted by evaporation of Silurian seawater and modern seawater. K-feldspar cements and diagenetic illites in the Illinois basin host strata explain the low K of the basinal brines.