Chemical Characteristics of Saline Water from the Catskill Formation in Test Wells in Northeastern Pennsylvania
Saline water was encountered in three test wells drilled to depths of 1400 to 1664 feet in upland settings in Bradford, Tioga, and Sullivan counties in northern Pennsylvania. Geophysical logging of the test wells identified elevated specific conductance of water from fractures in the Catskill Formation at depths ranging from 914 to 1026 feet below land surface. Groundwater samples, collected by the use of a point sampler, verified inflows of saline water having total dissolved-solids concentrations of at least 2470 to 12,700 milligrams per liter. Chemical analyses showed that the saline water in the Catskill Formation had chemical characteristics similar to oil and gas well brines diluted with freshwater.
The predominant major ions in samples of saline groundwater from the test holes were sodium, chloride, and calcium. The predominant trace constituents were strontium, bromide, barium, lithium, iron, manganese, zinc, fluoride, boron, molybdenum, and arsenic. Mass ratios of chloride to bromide in saline-water samples were about 100, similar to reported values of brines from western Pennsylvania and Marcellus flowback waters.
The saline water from the Catskill Formation contained hydrocarbon gases. Methane concentrations ranged from 7.8 to 55 milligrams per liter in five samples. The isotopic ratios of 13C/12C and 2H/1H of the methane indicated a thermogenic gas, but one which was much more depleted in 13C and 2H than gas produced from the Marcellus Formation.
The analyses showed that the inorganic chemical composition of saline waters found at relatively shallow depths in the Catskill Formation can be difficult to distinguish from theoretical mixtures of freshwater with deeper brines. Thus, inorganic constituents should be used with caution as indicators of contamination from shale-gas operations, even for elements such as barium, strontium, and bromide that have been suggested as highly specific indicators of Marcellus waters.