Joint sets that enhance production from Middle and Upper Devonian gas shales of the Appalachian Basin

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Abstract
The marine Middle and Upper Devonian section of the Appalachian Basin includes several black shale units that carry two regional joint sets (J(1) and J(2) sets) as observed in outcrop, core, and borehole images. These joints formed close to or at peak burial depth as natural hydraulic fractures induced by abnormal fluid pressures generated during thermal maturation of organic matter. When present together, earlier J(1) joints are crosscut by later J(2) joints. In outcrops of black shale on the foreland (northwest) side of the Appalachian Basin, the east-northeast-trending J(1) set is more closely spaced than the northwest-striking J(2) set. However, J(2) joints are far more pervasive throughout the exposed Devonian marine clastic section on both sides of the basin. By geological coincidence, the J(1) set is nearly parallel the maximum compressive normal stress of the contemporary tectonic stress field (S-Hmax) Because the contemporary tectonic stress field favors the propagation of hydraulic fracture completions to the east-northeast, fracture stimulation from vertical wells intersects and drains J(2) joints. Horizontal drilling and subsequent stimulation benefit from both joint sets. By drilling in the north-northwest-south-southeast directions, horizontal wells cross and drain J(1) joints, whenever present. Then, staged hydraulic fracture stimulations, if necessary, run east-northeast (i.e., parallel to the J(1) set) under the influence of the contemporary tectonic stress field thereby crosscutting and draining J(2) joints.

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