HYDROGEOLOGIC CHARACTERIZATION OF A FRACTURED TEST HOLE IN AN AREA OF MARCELLUS SHALE GAS DEVELOPMENT, BRADFORD COUNTY, PENNSYLVANIA

The geohydrology and water quality of a 1,664-foot deep test hole penetrating fractured sedimentary rock in Bradford County, Pennsylvania was characterized through the integrated analysis of core, geophysical logs, and discreet specific-depth water and gas isotope samples. The investigation, completed in 2012 as a cooperative project between the U.S. Geological Survey and the Pennsylvania Department of Conservation and Natural Resources, Bureau of Topographic and Geologic Survey (T&GS), provided an evaluation of the depth and composition of fresh and saline groundwater in an area undergoing natural gas development from the Marcellus Formation.

According to the T&GS core analysis, the test hole penetrated 98 feet of the Pottsville Formation of Pennsylvanian age, 584 feet of the Huntley Mountain Formation of Mississippian and Late Devonian age, and 982 feet of Catskill Formation of Late Devonian age. Fractures in the rocks decreased with depth, with the upper 150 feet being most heavily fractured and few fractures penetrated below 800 feet. About 7 gallons per minute of freshwater entered the test hole from multiple fractures between 50 and 294 feet below land surface. The freshwater inflow, with total dissolved solids concentration of 87 milligrams per liter, moved downward and exited the hole through fractures at 553, 661, and 712 feet below land surface. Below the 712-foot fracture, the temperature log approached the geothermal gradient, indicating little ambient fluid flow and minimal fracture transmissivity below this depth. A petrophysical log analysis indicated that the formation water in sandstone intervals below 880 feet was highly saline. The integrated analysis indicates active, fresh groundwater circulation to a depth of 700 to 900 feet below land surface.

Samples of seepage from fractures at 914 and 1,026 feet were collected with a wireline point sampler. The seepage from both fractures was highly saline with a minimum total dissolved solids concentration of 12,700 milligrams per liter. Methane in three samples collected from the seeps ranged from 7.8 to 37 milligrams per liter, and isotopic ratios of ¹³C/¹²C and ²H/¹H of the methane indicated a thermogenic origin.