Evolution of permeability across the transition from brittle failure to cataclastic flow in porous siltstone

Porous sedimentary rocks fail in a variety of modes ranging from localized, brittle deformationto pervasive, cataclastic flow. To improve our understanding of this transition and its affect on fluid flowand permeability, we investigated the mechanical behavior of a siltstone unit within the Marcellus Forma-tion, PA USA, characterized by an initial porosity ranging from 41 to 45%. We explored both hydrostaticloading paths (r15r25r3) and triaxial loading paths (r1>r25r3) while maintaining constant effectivepressure (Pe5Pc–Pp). Samples were deformed with an axial displacement rate of 0.1lm/s (strain rate of 231026s21). Changes in pore water volume were monitored (drained conditions) to measure the evolutionof porosity. Permeability was measured at several stages of each experiment. Under hydrostatic loading, wefind the onset of macroscropic grain crushing (P*) at 39 MPa. Triaxial loading experiments show a transitionfrom brittle behavior with shear localization and compaction to cataclastic-flow as confining pressureincreases. When samples fail by shear localization, permeability decreases abruptly without significantchanges in porosity. Conversely, for cataclastic deformation, permeability reduction is associated with signif-icant porosity reduction. Postexperiment observation of brittle samples show localized shear zones charac-terized by grain comminution. Our data show how zones of shear localization can act as barriers to fluidflow and thus modify the hydrological and mechanical properties of the surrounding rocks. Our results haveimportant implications for deformation behavior and permeability evolution in sedimentary systems, and inparticular where the stress field is influenced by injection or pumping.