Faulting and Fracture Heterogeneity In Black Shales of the Appalachian Basin of New York State

Numerous fault systems in the Appalachian Basin of NYS were reactivated during the development of the Ordovician and Devonian shale basins. These faults influenced not only the depositional patterns of the black shales, but also influenced later localized fracturing of the shales. These fault systems include (among others) reactivated intra-Grenvillian sutures and Iapetan-opening faults. The reactivated and newly developed fault systems resulted in local stress deviations of a regularly varying far-field SH. Over 100,000 measured fractures and other structural features in outcrop and well logs demonstrate the local spatial complexity and heterogeneity of fracturing, folding and mesoscale faulting that developed in response to the local stress deviations. Finally, changes in stress orientations through time can add an even greater complication to the fracture patterns, as observed in the Utica of eastern NYS. The fractures systems that developed in a regionally regularly varying far-field stress have been well studied by many researchers (e.g., Engelder, Geiser, Lash), and are not the subject of this presentation.

Examples of the heterogeneity include duplexing in Devonian Marcellus black shale outcrops in Otsego County (eastern NYS) that indicate multiply oriented SH directions over relatively short distances, consistent with local fault control and/or multiple phases. Cores of the Marcellus in western NYS show worked surfaces indicative of low angle faulting. In Ordovician Utica black shale outcrops in the Mohawk Valley region of eastern NYS, the fracture frequency increases toward some faults and not others. In the Devonian black shales in the Finger Lakes, numerous Fracture Intensification Domains (FIDs) are not parallel to Set I or Set II fractures; rather, they are coincident with faults proposed on the basis of stratigraphic offsets and/or seismic data. Also in western NYS, a curving-parallel intersection of fracture sets on the scale of the entire creek bed shows the intermediate scale possible