THE DISTRIBUTION OF FRACTURES ABOVE BLACK SHALES IN THE APPALACHIAN BASIN

One concern about production of gas shales is the possibility of hydraulic fracture fluid leaking upward along an interconnected network of fractures to contaminate groundwater. The plausibility of leakage along fractures depends on evidence for the pervasive development of interconnected fractures between gas shales and the surface. Although continuous fracture imaging in the borehole might aid in sorting out whether fractures are interconnected from depth to the surface, the borehole volume is insufficient for the job. This leaves outcrops mapping as the best means of resolving the extent of fracture interconnectivity. Outcrop observation is only effective to the extent that the operator has a strategy for distinguishing between deep-formed fractures and exhumation-related surface fractures. While there is no continuous exposure of the 7000 feet of section between the Marcellus black shale and units bearing fresh water, a complete section can be assembled by examining outcrops in a traverse across the Appalachian Plateau. Outcrops of these units were visited along Route 15 between Williamsport PA and the NY-PA border. In this traverse, fractures were virtually all one or more sets of J₂ joints. Examples of cross cutting J₂ joints were rare. Most cross cutting or abutting was a consequence of the growth of later exhumation-related joints. Fracture density was measured in photographs looking along strike of the J₂ joints. A 100 m² area was identified and the vertical height of each joint was measured within that cross section. Density was the cumulative height of joints per unit area (m/m²). Fracture density varied from 15 m/m² to near zero with black gas shales exhibiting the highest density and Carboniferous clastics exhibiting the lowest density. Outcrops show little evidence for fracture interconnectivity when the density is < 0.25 m/m². The correlation between joint density and proximity to gas shales is consistent with the Lacazette-Engelder (1992) and the McConaughy-Engelder (1999) conclusions that J₂ joints are gas-driven nature hydraulic fractures whose orientation is controlled by late Paleozoic tectonics of the Appalachian Plateau.