Skempton’s poroelastic relaxation: The mechanism that accounts for the distribution of pore pressure and exhumation-related fractures in black shale of the Appalachian Basin

Abstract

Middle and Upper Devonian gas shales of the Appalachian Basin host a sequence of mode I cracks from early veins to late unmineralized joints. These fractures develop during a cycle of burial and exhumation starting with horizontal veins, then vertical veins, then vertical joints, and ending with pancake joints. Veins are most common in the cross-fold orientation in gas shale of the deep hinterland, whereas joints are common in the shallow foreland fringe. Of particular interest for the recovery of natural gas are joints striking northeast to east-northeast in shallow core and outcrops of Appalachian gas shales, particularly in the underpressured, southwestern part of the basin. Northeast to east-northeast fractures are sparse in the deeper, northeastern part of the basin where the Marcellus gas shale is highly overpressured. The common occurrence of unmineralized joints in shallow gas shales is best explained with a stress–pressure–depth model for the evolution of pore pressure in the Appalachian Basin. Our model shows that by Skempton’s poroelastic relaxation, gas-charged porosity can drive natural hydraulic fractures during exhumation. In the shallow foreland fringe of the Appalachian Basin, exhumation was sufficient to favor the propagation of east-northeast–striking neotectonic joints in the contemporary tectonic stress field. These joints appear mainly at shallow depths where they facilitate leakage to form an underpressured gas column trapped by groundwater (i.e., Big Sandy gas field, Kentucky). Exhumation was insufficient for Skempton’s poroelastic relaxation to favor propagation of east-northeast joints in the deep, central part of the basin where capillary forces maintain overpressured gas shale.