Acadian-Alleghanian Orogenesis as Revealed by Fracturing Within the Appalachian Foreland

Terry Engelder
Pennsylvania State University, University Park, Pennsylvania ([email protected])

Assets within the Appalachian Basin range from conventional clastic and carbonate reservoirs to source rocks of Devonian black shale and Pennsylvanian coal, all of which are fractured. These fractures range from coal cleat and cracks around kerogen flakes to natural hydraulic fractures, tensile joints in stiff beds, and late-stage cross joints.
With some exceptions this broad range of fracture types propagated with the help of pressure generation accompanying the positive ΔV reaction during maturation of hydrocarbons.
Before and during maturation fracture orientation in the Appalachian foreland was controlled by an evolving tectonic stress that reflects three important details of Acadian-Alleghanian orogenesis in the Appalachian hinterland.
First, pre-maturation, forebulge-related tensile joints in distal portions of the Acadian Catskill Delta complex reflect initial loading of Laurentia (i.e., North America) by Gondwana (i.e., Africa) at the New York promontory. The earliest syn-maturation fractures are microcracks around kerogen flakes in black shale. Maturation-related pressure was enhanced by compaction disequilibrium.
Maturation continued to elevate pressure within Devonian black shales to the point that macroscopic natural hydraulic fractures (NHF) developed within the source rocks. The orientation of NHF in black shale and early coal cleat in the foreland reflects a basin-wide stress field arising from the oblique convergence of Gondwana and Laurentia, the second detail of Acadian-Alleghanian orogenesis.
This basin-wide joint system supports the emerging view that dextral transpression controlled the kinematics in the mountain belt to a greater extent than previously recognized. Further burial led to the development of a complete fracture network in siltstones and gray shale of the basin. This later system of fracturing evolved in Alleghanian stress fields arising when transpressional tectonics within crystalline basement, the third detail, drove the classic detachment sheets of the Valley and Ridge and Appalachian Plateau.

AAPG Search and Discovery Article #90073 © 2007 AAPG Foundation Distinguished Lecturer Series 2007-2008