The geological expression of hydraulic fracturing is varied and is controlled primarily by the magnitude of the differential stress and the intrinsic properties of the rock. The orientation and type of fractures that develop within a basin are determined by the state of stress, which in turn is controlled by the geological boundary con ditions. During the early stages of burial and diagenesis the for mation of hydraulic fractures is thought to be an important factor in the movement of fluids through and out of low-permeability, semilithified sediments. Unfortunately, these fractures are not gen erally preserved and are presumed to heal once the fluid pressure is relieved.

The low-permeability Mercia Mudstones of the Bristol Channel Basin, southwest England, however, contain bodies of sand that, during the opening of the basin, were injected along some of the hydraulic fractures in the mudstones, preserving them as sedimen tary dikes and sills. Field observations indicate that fluid pressures within the Mercia Mudstones were also very high during basin in version and that hydraulic fracturing provided a transient perme ability that relieved this excess pressure. The fractures are not visi ble in most of the mudstones but have been preserved within evaporite-rich horizons as a network of satin spar veins. Thus, the chance preservation of the sedimentary dikes and satin spar veins shows that at different times during the evolution of the basin, flu ids migrated through low-permeability units along transient net works of hydraulic fractures. In addition, the orientation and spatial organization of these fractures reflect the boundary conditions op erating at various stages in the basin history.