REGIONAL SPATIAL AND STRATIGRAPHIC TRENDS OF FORMATION BRINE COMPOSITION IN THE CENTRAL APPALACHIAN PLATEAU PROVINCE

Over 900 published chemical brine analyses were compiled for Ohio, Pennsylvania, and West Virginia. The data were analyzed for trends in spatial and stratigraphic distribution. Formation brine composition and salinity is stratrigraphically zoned, with three megahydrostratigraphic aquifer units bounded by thick shale aquitards. Brines in the lower units are more Ca and Mg rich that the upper units.

The Upper Paleozoic clastic aquifer shows a progressive downward increase in average salinity from the Pennsylvanian (80,000 mg/L) to Middle/Upper Devonian (120,000 mg/L). In this aquifer, log Ca, Na, Mg, Br and I (and less so K) exhibit strongly collinear dependence on log Cl concentration. Divalent cations become progressively more important with increasing TDS. Pennsylvanian brines show little regional variation in salinity while Mississippian brines tend to have lower salinities northern and southwestern West Virginia. There is little regional variation in major cations in Mississippian brines while Pennsylvanian brines tend to be more Ca- and Mg-rich in southwestern West Virginia. Upper Devonian brines in western Pennsylvania are more Ca-rich than to the east.

The Middle Paleozoic carbonate-clastic aquifer is below the thick Devonian shale sequence, and shows a significant jump in salinity (250,000 mg/L) in the Lower Devonian–Silurian formations. Brines in this aquifer show a relatively poor collinear dependence of log Ca, Na, Mg, Br K and I with log Cl concentration. As salinities near 300,000 mg/L, halite saturation is reached and Na concentrations fall, while Ca, Mg, and K concentrations rise. Lower Devonian brines are lowest in salinity in the Valley and Ridge and in central West Virginia. They are Ca- and Mg-rich in north-central OH and more Na-rich to the southeast.

The Cambro-Ordovician carbonate-clastic aquifer is below the Martinsburg shale sequence and has an average salinity of 150,000 mg/L in the Ordovician and 230,000 mg/L in the Cambrian. Salinity is lower in western Ohio than to the east.

Interestingly, the stratigraphic distribution of average salinity for vein mineral fluid inclusions trapped during the Alleghenian orogeny is nearly identical to that determined for modern brines, suggesting that modern brines have been in chemical equilibrium with the host formations since the Paleozoic.