Fractures in shale: the significance of igneous intrusions for groundwater flow

Research in Cretaceous shales from West Africa has demonstrated that significant permeability can develop within shales at shallow depths (<100 m), equivalent to a permeability of >1 m day−1. Much of the variation in permeability is related to the degree of burial metamorphism, with shales that have been altered and that approach the anchizone having the highest permeability and those that are largely unaltered (early diagenetic zone) having the lowest permeability. However, further research targeting largely unaltered shales dominated by smectite clay has shown that the presence of small igneous intrusions can radically alter the hydrogeology. Twenty-four exploratory boreholes were drilled into smectite-dominated shale and nine of these boreholes were targeted to small dolerite intrusions within the shale. The dolerite was intensely fractured at the intrusion edge, with significant zeolite growth along the fracture surfaces. The permeability in the fractured dolerite was the highest measured in any shale borehole, with transmissivities of up to 60 m2 day−1 measured from pumping tests. Fracturing was less where dolerite was intruded into sandstones, however, and the measured transmissivity was lower (<0.5 m2 day−1). We postulate that the low permeability and high water content of the shale enabled high pressures to develop during intrusion, facilitating the development of fractures along the intrusion contact zone.