Abstract. Failure behaviors can strongly influence deformation-related changes in volume, which are critical in the formation of fault and fracture porosity and conduit development in low-permeability rocks. This paper explores the failure modes and deformation behavior of faults within the mechanically layered Eagle Ford Formation, an ultra-low permeability self-sourced oil and gas reservoir and aquitard exposed in natural outcrop in southwest Texas, USA. Particular emphasis is placed on

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... is of the relationship between slip versus opening along fault segments and the associated variation in dilation tendency versus slip tendency. Results show that the failure mode and deformation behavior (dilation versus slip) relate in predictable ways to the mechanical stratigraphy, stress field, and specifically the dilation tendency and slip tendency. We conclude that dilation tendency versus slip tendency patterns on faults and other fractures can be analyzed using detailed orientation or structural geometry data and stress information and employed predictively to interpret deformation modes and infer volume change and fluid conduit versus barrier behavior of structures.