Jointing Within the Outer Arc of a Forebulge: A Mechanism for Additional Reservoir Complexity at the Subseismic Scale

Engelder, Terry¹, Gary Lash² (1) The Pennsylvania State University, University Park, PA (2) State University of New York – College at Fredonia, Fredonia, NY

In froreland basins, jointing as a consequence of outer-arc stretching over a forebulge can add an additional complexity to the fracture pattern of the subseismic reservoir. Here, we report an example of such complexity. NW-trending joints are pervasive throughout the Upper Devonian shale succession of the Appalachian Basin in New York State while ENE-trending joints are pervasive but more common in black shale. Both NW and ENE joints formed within a regional stress field related to the Alleghanian clockwise rotational transpressive collision of Gondwana against Laurentia. However, this tectonic scenario is complicated by north-trending (NS) joints that predate both prominent Alleghanian joint sets and are most densely formed at the contacts of gray shale and overlying black shale units. The NS joints appear to have originated in the higher modulus carbonate driven by either a joint-normal stretching or thermoelastic contraction mechanism. A uniform extensional strain over the entire Upper Devonian shale succession caused the enhanced tensile stress in the stiffer carbonate concretions of the gray shale. The uniform extension and subsequent propagation of these early joints required an axis of stretching oriented ~EW, consistent with formation of a forebulge in advance of oblique plate convergence in New England at the onset of the Alleghanian orogeny. Upper Devonian shale hosting the NS joints crops out in that area of the Appalachian Basin where the modeled tensile stress induced by uplift and lithospheric flexure related to the forebulge was optimum.