Hydraulic fracturing during the formation and deformation of a basin: A factor in the dewatering of low-permeability sediments

The geological expression of hydraulic fracturing is varied and is controll ed 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 cont rolled by the geological boundary conditions. During the early stages of bu rial and diagenesis the formation of hydraulic fractures is thought to be a n important factor in the movement of fluids through and out of low-permeab ility, semilithified sediments. Unfortunately, these fractures are not gene rally preserved and are presumed to heal once the fluid pressure is relieve d. The low-permeability Mercia Mudstones of the Bristol Channel Basin, southwe st England, however, contain bodies of sand that, during the opening of the basin, were injected along some of the hydraulic fractures in the mudstone s, preserving them as sedimentary dikes and sills. Field observations indic ate that fluid pressures within the Mercia Mudstones were also very high du ring basin inversion and that hydraulic fracturing provided a transient per meability that relieved this excess pressure. The fractures are not visible in most of the mudstones but have been preserved within evaporite-rich hor izons as a network of satin spar veins. Thus, the chance preservation of th e sedimentary dikes and satin spar veins shows that at different times duri ng the evolution of the basin, fluids migrated through low-permeability uni ts along transient networks of hydraulic fractures. In addition, the orient ation and spatial organization of these fractures reflect the boundary cond itions operating at various stages in the basin history.