A B S T R A C TThe 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 eastnortheast-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-northwestsouth-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.

Analysis of more than 900 wireline logs indicates that the Middle Devonian Marcellus Formation encompasses two thirdorder transgressive-regressive (T-R) sequences, MSS1 and MSS2, in ascending order. Compositional elements of the Marcellus Formation crucial to the successful development of this emerging shale gas play, including quartz, clay, carbonate, pyrite, and organic carbon, vary predictably within the proposed sequencestratigraphic framework. Thickness trends of Marcellus T-R sequences and lithostratigraphic units reflect the interplay of Acadian thrust-load-induced subsidence, short-term base-level fluctuations, and recurrent basement structures. Rapid thickening of both T-R sequences, especially MSS2, toward the northeastern region of the basin preserves a record of greater accommodation space and proximity to clastic sources early in the Acadian orogeny. However, local variations in T-R sequence thickness in the western, more distal, area of the basin may reflect the reactivation of inherited Eocambrian basement structures, including the Rome trough and northwest-striking cross-structural discontinuities, induced by Acadian plate convergence. Episodes of block displacement locally warped the basin into northeast-southwest-trending regions of starved sedimentation and/or erosion adjacent to depocenters in which regressive systems tract deposits were ponded. Block movement

No abstract

No abstract