UNDERSTANDING THE EFFECT OF LARGE SCALE STRUCTURES ON THE REGIONAL FRACTURE NETWORK IN THE APPALACHIAN BASIN

Natural fractures are pervasive throughout the sedimentary rocks of the Appalachian basin. These fractures have been studied in great detail in outcrop and three regional sets of joints have been previously described. In addition to fractures, other geologic structures observed in the Appalachian plateau province of the Appalachian basin include broad salt-cored folds, steeply dipping faults that cut up from these folds, thrust faults, and geologic lineaments. Despite this large body of data, little is known about how the regional fracture fabric interacts with these larger scale structures found in the Appalachian plateau.

Our data show how one of these large structures, the Seneca Stone thrust near Seneca Falls, NY, affects the fracture pattern in the middle Devonian Marcellus Shale. We compare the fracture pattern in the hanging wall and footwall of this fault, determine how the fracture pattern changes with distance from the fault, and compare the fracture pattern around this fault with fractures seen within Marcellus shale outcrops that are not near any known major structure. In outcrops greater than 10km from the Seneca Stone thrust, the dominant fracture sets are steeply dipping fractures that strike 146° and 058° with some fractures that strike 002°; bedding parallel fractures are also observed. In the outcrop around the Seneca Stone thrust, there is a greater complexity to the fracture network and steeply dipping fracture sets strike 001°, 074°, 153°, 044°, and 111°. Gently dipping fractures strike 000° and dip 20° to both the East and West. Despite the variety in the orientations of fractures observed in outcrop, only some of the fractures contain mineralization. In outcrop near the Seneca Stone thrust, preliminary results indicate that steep N-S striking fractures are the most commonly mineralized feature. These fractures are the dominant mineralized feature in the hanging wall, are often filled with calcite and pyrobitumen, and can have widths (mineralized aperture) of over 1cm. In the footwall, a greater variety of fracture orientations show calcite fill. The higher intensity of mineralized fractures and the greater variety of fracture orientations near the Seneca Stone thrust show the role of a major structure in controlling a local fracture fabric that is significantly different from regional patterns.