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 DV 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.