SEAWATER ORIGIN OF CACL2 BASINAL BRINES IN THE ILLINOIS BASIN

CaCl₂ basinal brines in many sedimentary basins inherited their chemistries and salinities from evaporated paleoseawaters when the world oceans were Ca-rich and SO₄-poor (CaCl₂ seas of the Cambrian-Devonian and Jurassic-Cretaceous). CaCl₂ seas coincided with periods of rapid seafloor spreading, high influxes of mid-ocean ridge brines, and elevated sea levels, conditions that favored accumulation of marine CaCl₂ brines in marginal and interior continental basins. During periods of CaCl₂ seas, evaporative concentration in a restricted marine basin would produce "instant" CaCl₂ brines with chemical compositions similar to basinal brines. This explanation for the origin of CaCl₂ basinal brines contrasts with others that assume constancy of seawater chemistry and involve more complex brine-rock interactions.

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.