THE PERSISTENCE OF SALINE WATER IN LATE PALEOZOIC ROCKS OF THE APPALACHIAN PLATEAUS

Brine is commonly produced by oil and gas wells completed in Permian and Carboniferous rocks of the Appalachian Plateaus. Drilling depths for freshwater on the plateaus are generally limited, in part because of the presence of saline water at relatively shallow depths. The shallow persistence of high salinity water is an enigma as these sedimentary rocks are more than 275 million years old and possess fracture permeability enhanced by tectonic stress, erosion and uplift. Two-dimensional, variable-density flow simulations indicate that meteoric recharge could have nearly flushed the brine from a 1-km section of rock within 200 million years, even at permeabilities as low as 10^-19 m². However, by including the effects of estimated uplift and erosion, the saline-water zone reaches a dynamic equilibrium with simulated brine-flushing balanced by uplift and erosion. This configuration with brine at shallow depths is stable and impedes cross-formational flow. It explains a mechanism for the persistence of saline water and suggests the present system evolved without upward migration of brine from underlying, earlier Paleozoic rocks (including the Marcellus and Utica Shales) to land surface. It also suggests that the Appalachian Plateaus may not have been a source of recharge for underlying, topographically-driven brines migrating and transporting ore fluids into adjacent basins, as has been suggested by some previous studies.