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.