DETERMINING THE TECTONIC HISTORY OF THE MARCELLUS FORMATION THROUGH ANALYSIS OF FRACTURE POPULATIONS

The Marcellus Formation is a hydrocarbon rich, black shale in the Appalachian Basin characterized by pervasive natural fracturing, the in-situ study of which provides insight into the tectonic history of the region. In this study, we mapped the fractures found in an exposed section of the Union Springs member of the Marcellus Shale located in the Seneca Stone quarry near Seneca Falls, NY to determine the geometry of the 3-D fracture network of the formation. The deformation history was then interpreted using crosscutting relationships of faults, folds and fractures; younger fractures abut against older fractures while younger faults and folds cut older features. In addition, we studied individual fracture characteristics such as orientation, size, mineralization, and connectivity to determine which fractures likely accommodate fluid flow. Large, well-connected fracture networks are more conducive to flow and numerous open fractures also increase the hydraulic conductivity of the network.

We focused our mapping in the hanging wall of a prominent E-W striking, gently dipping thrust fault, with a dip separation of ~10 meters. This section reveals a complex deformation history in sub-horizontal beds. The fault indicates early shortening in the N-S direction. Sets of small N-S striking thrust faults, along with N-S striking folds, and conjugate sets of wrench faults indicate later E-W shortening. Two other joint sets generally oriented at 020° and 350° abut these wrench faults, indicating a later stage of extension in the E-W direction. There are also large, widely spaced E-W striking fractures that almost all other fractures terminate against. Fracture mineralization is pervasive in the study area, indicating fluid flow is not constrained to one fracture orientation; unmineralized, partially mineralized, and completely filled fractures coexist. This strain history illustrates the changing deformation conditions of the Appalachian Basin and the 3-D fracture network it creates has implication for fracture related fluid flow.