A Lithospheric Plate-Scale Fracture System: Evidence for the Simultaneous Maturation of Coal and Oil in the Appalachian Basin Based on the Correlation between the Development of Face Cleat in Coal and Early Joints in Organic-Rich Shale

Terry Engelder
The Pennsylvania State University, University Park, PA

Coal…. The regional stress field during coalification controls orientation of face cleat (i.e., joints in coal). Closely spaced (i.e., strong) cleat develops later when coal reaches a rank of high volatile A bituminous generated by P-T conditions equivalent to burial just below the top of the oil window and depending on heat flow this may occur at a depth of 2 – 3 km.

Organic-rich Shale…. Early joints propagating within or just above black shale suggest that they were driven as a consequence of maturation of hydrocarbons after burial to the oil window. The regional stress field at maturation controls the orientation of these joints.

A plate-scale stress field (PSSF)…. A PSSF is approximately rectilinear and has a dimension > 1500 km. If joint development occurs within local regions of a lithospheric plate when a PSSF is active, the combined joint sets act as one mega-set leaving the possibility for very large fractured reservoirs.

Dating an Appalachian PSSF using a mega-set of joints…. A pre-Alleghanian PSSF is dated using the time of propagation of face cleat and early joints in black shale based on well-known burial curves for Appalachian sedimentation. Early ENE striking fractures within both Pennsylvanian coal and Devonian black shale appear as one mega-set recording the existence of a rectilinear stress field extending for 1500 km across three promontories separated by oroclinal embayments of the Central and Southern Appalachians. This mega-set defines a PSSF dating from the Pennsylvanian-Permian boundary when these rocks were buried to the oil window during a 10-15 My period.