FRACTURE ANALYSIS OF UTICA AND MARCELLUS SHALES IN THE APPALACHIAN BASIN: A COMPARATIVE STUDY

The Pennsylvania Geological Survey acquired two drill cores from central Pennsylvania’s Valley and Ridge Province: one sampling the Utica shale and the other sampling the Marcellus shale. These cores are separated by a map distance of less than 20 miles; both were drilled on the flanks of major folds and contain economically important black shale formations. Additionally, these cores are not separated by the Silurian-aged Salina Formation, an evaporite formation that serves as a major detachment for Alleghanian tectonics within the Appalachian Plateau. As a result, these cores serve as an excellent natural experiment, and a comparative analysis of them provides insights into the deformation history of the Appalachian Basin, the fractures within it, and their effect on subsurface fluid migration.

The Oak Hall core is made up of the Ordovician-aged Utica calcareous shale and limestone, which dips between 40 and 70 degrees. The core was drilled in the Oak Hall Quarry, located on the southeastern limb of the Nittany Syncline. The Bald Eagle core is made up of Marcellus shale and dips between 10 and 25 degrees. The Bald Eagle core was drilled on the northern rim of the Nittany Anticlinorium, approximately 20 miles northeast of Oak Hall Quarry.

Preliminary data show that, for both cores, the most common type of fracture was calcite veins, followed by shear fractures. Neither core had a significant number of barren joints. The Bald Eagle core had a higher fracture density than the Oak Hall core, including a higher density of both veins and shear fractures. There is a higher concentration of steeply dipping veins in the Bald Eagle core compared to the Oak Hall core. Although both cores appear to have been affected by faulting, a visible fault is present at a depth of 342 feet in the Bald Eagle core. The calcareous Utica shale in the Oak Hall core is more competent than the fissile Marcellus shale in the Bald Eagle core. Stylolites were common in limestone beds found in both cores. Cross-cutting relationships indicate that some of the stylolites and veins formed at the same time. These data appear to support previous work suggesting that the Marcellus shale acts as a regional detachment, as the Marcellus shale is more intensely deformed than the more deeply buried and older Utica shale.