Technical Luncheons
Acadian-Alleghanian Orogenesis as Revealed by Fracturing Within the Appalachian Foreland
Speakers:
Terry Engelder, Pennsylvania State University
Date/Time: Tuesday, March 25, 2008 - 11:30 am
Location: Telus
Convention Centre - Calgary, Alberta
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ABSTRACT
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.
BIOGRAPHY
Terry Engelde completed his Bachelor's Degree in Geology at the Pennsylvania State University, his Master's in Geology at Yale University, and his Ph.D in Geology at Texas A&M University. He currently a professor of geosciences at Pennsylvania State University.
He has written over 130 research papers, including "Stress Regimes in the Lithosphere". He has also received numerous awards, such as the Fulbright Senior Fellowship and the French-American Foundation Fellowship.
His professional interests include frictional properties of fault gouge, strain relaxation and hydraulic fracture in situ stress measurements, fracture analysis with focus on development of natural hydraulic fractures, strain analysis in foreland fold and thrust belts, and analysis of coupling between ore pressure and stress in basins.
