Fractures in shale : the signi fi cance of igneous intrusions for groundwater fl ow

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. 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 m day measured from pumping tests. Fracturing was less where dolerite was intruded into sandstones, however, and the measured transmissivity was lower (<0.5 m day). 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. Fractures in shale are essential for controlling fluid migration. They are often studied from the perspective of hydrocarbons as a result of the control they exert on the characteristics of reservoirs (Curtis 2002) and their impact on the coherence of caprocks (Shukla et al. 2010; Bricker et al. 2012). In waste management, the development of fractures within clays and shales is crucial in limiting the migration of contaminants from repositories – particularly for nuclear waste (Hanor 1993; Neuzil 1994). Shales and mudstones are more rarely considered as aquifers, although they can provide a strategic source of drinking water for dispersed rural populations (Jones et al. 2000). Research within shallow shale environments shows that significant fracture permeability can develop within shales at depths <100 m, equivalent to a permeability of >1 m day (MacDonald et al. 2005). Much of the variation in permeability is related to the degree of burial metamorphism the shale has undergone and the corresponding change in clay mineralogy. Shales that have been altered by burial to approach the anchizone have a clay mineralogy dominated by illite and have the highest permeability. Those that are largely unaltered (early diagenetic zone) have the lowest permeability. These unaltered low-permeability shales are often dominated by smectite clays, within which fractures do not remain open (Neuzil 1994; MacDonald et al. 2005). However, the presence of igneous intrusions within low-permeability shales can radically change the hydrogeology. To study the impact of small igneous intrusions within a smectite-dominated shale environment, we drilled and tested 24 exploratory boreholes, nine of which were targeted to small dolerite intrusions and the remainder to unaltered shale.