13 Shale Entrapment into Fault Zones (Shale Smearing) and Flow Impact
The sketch and photograph (inset) in Figure 40 show the entrainment of shales into fault zones in an underground mine shaft in central California, USA. The sketch depicts a shale body (shale smear) about 1.6 m thick on the lower left, entrained along a normal fault of about 7 m offset (Eichhubl et al., 2005). Figure 41 shows sketches of smeared shales produced in the laboratory using sand and clay (Noorsalehi-Garakani et al., 2013; Vrolijk et al., 2016). The sketch in Figure 41b illustrates multiple shale layers entrained into the shear zone, similar to the natural example shown in Figure 40. Additional subsurface examples of shale entrainment are provided by Yielding and others (1997) and Koledoye and others (2003). Shale smears along faults may completely prevent fluids from flowing across the faults and may form major barriers to groundwater flow. In hydrocarbon exploration, fault smearing is one of the most common forms of traps in hanging walls of normal faults within sandstone-shale sequences.
Figure 40 – Map and photograph showing shale entrained along a normal fault with a throw of 7 m in a mineshaft at Black Diamond Mine, California, USA. Modified from Eichhubl and others (2005).
Figure 41 – Shale smear produced in experiments with sand and clay-like plaster in the laboratory by a) Noorsalehi-Garakani and others (2013); and, b) Vrolijk and others (2016).
When shale smearing occurs along a fault within an aquifer, it can cause a strong hydraulic head gradient across the fault as shown in the example of Figure 42 and Figure 43 from the Santa Susana Field Laboratory (SSFL) in Simi Valley, southern California, USA. Here shale is present along a fault zone called the Shear Zone Fault; its influence on groundwater flow is shown in Figure 43. Figure 43 indicates a difference of about 75 m in hydraulic head across this fault, which indicates the effective sealing behavior of the shale smear. See Box 3 for details.
Figure 42 – Case study location. a) Hydrogeological map of the Santa Susana Field Laboratory which is the rectangular inset as indicated in b) that shows the area surrounding the Laboratory site, modified from MWH (Montgomery, Watson, and Harza) Global (2009 and 2014). The Shear Zone Fault is indicated in yellow, hydraulic head elevation by blue lines, and well locations by solid circles. c) Schematic cross-section A-A′ with the location indicated by a green line in (a), showing the hydraulic head drop recorded in five wells located along an approximately E-W direction on either side of the Shear Zone Fault. Modified from MWH (2009). The inferred kinematics of the fault zone (west side down and toward viewer) also are shown. Modified from Cilona and others (2015).
Figure 43 – Hydraulic head and hydraulic conductivity data for geologic features in the area (mb = member). a) Hydraulic head contours from the first quarter of 2014 are shown by blue lines, from MWH (2014). Representation of Shear-Zone Fault in the groundwater flow model is indicated by the red line and was redrawn from AquaResource/MWH (2007). Values of the hydraulic conductivity of the 6 m thick fault core are listed adjacent to each section of the fault, from AquaResource/MWH (2007). b) Box-and-whisker diagram of the hydraulic conductivity of the Shear Zone Fault and the surrounding sandstones, modified from Cilona and others (2015).