FORMATION WATERS FROM RESERVOIRS IN THE APPALACHIAN BASIN, NEW YORK, PENNSYLVANIA, AND WEST VIRGINIA: OBSERVATIONS ON MAJOR AND MINOR ELEMENT, AND RADIUM GEOCHEMISTRY

Formation waters from Cambrian through Mississippian-age oil and gas reservoirs in the Appalachian Basin are co-produced with hydrocarbons. Hydraulic fracturing of the Marcellus Shale and other gas-bearing shales also leads to production of significant quantities of fluids during flowback and gas production. Major and minor element analyses of produced water samples from the Middle Devonian Marcellus Shale in NY, PA, and WV have been compared with analyses of produced water from other Devonian reservoirs, including sandstones in the Bradford group (Upper Devonian), the Onondaga Limestone (Middle Devonian), and Oriskany Sandstone (Lower Devonian) using published data augmented by a limited number of new analyses. Total dissolved solids values in water produced during later stages of flowback from the Marcellus are similar to those in water produced from other Devonian formations, and most commonly range from 100,000 to 300,000 mg/L. The water produced from these formations are Na-Ca-Cl dominant, with low HCO₃ and SO₄ concentrations. Low SO₄ is consistent with the minimal barite precipitated from the produced water, but only partially accounts for the high concentrations of dissolved Ba throughout the basin (hundreds to thousands of mg/L), whose solubility remains poorly understood. Na/Br and Cl/Br ratios indicate mixtures of brines, with a major component of salinity derived from evaporatively concentrated seawater.

The Marcellus Shale is known to be enriched in uranium, based in part on its high gamma-ray response on geophysical logs. Radiochemical analyses of produced water from the Marcellus Shale show elevated radium-226, and lesser amounts of radium-228, the decay products of uranium-238 and thorium-232, respectively. Radium activity of produced waters within the basin positively correlates with TDS concentration. Controlling for this TDS dependence, total radium activity in Marcellus produced water samples are on average 2.5 times higher than for non-Marcellus samples. The virtual absence of dissolved uranium in the produced water reflects its low solubility in the reducing environments at depth that characterize most oil and gas reservoirs. Uranium thus remains predominantly in a solid phase while the more soluble element, radium, is brought to the surface with produced water.