In this paper, I identify a mechanism by which high pore pressures, acting alone, can cause natural hydraulic fractures and keep them open for extended time periods. Such fractures may occur in tectonically extensional, neutral, or compressional settings. Although the fractures' orientations will be governed by the regional stress field, they can form and remain open without a horizontal stress differential large enough to induce regional-scale dilation. Rather, these fractures are a result of compression or shrinkage of the host rock's solid constituents (grains). As such, grain shrinkage caused by a pore-pressure increase is analogous to grain shrinkage caused by temperature decreases, a process that also causes extension fractures in rocks. I call these fractures hydra lic-shrinkage fractures to distinguish them from other types of hydraulic fractures that may be induced by intrusive fluids other than the dominantly present pore fluid.

I present sample calculations to demonstrate the theoretical basis for these fractures, and include various examples to show how fracture potential and orientation are affected by the host rock's mechanical properties and different geological processes. These examples indicate that pore-pressure magnitude cannot be used alone as a fracture criterion. Instead, fracture potential and orientation depend on the geologic processes causing the excess pore pressures and the mechanical properties of the unfractured rock. In general, vertical (shrinkage) fractures are favored and should occur at lower pore pressures in less compressible rocks buried in basins with lower geothermal gradients.