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Preliminary assessment of the highest potential transmissivity of fractures in fault zones by core logging
Engineering Geology 221 (2017) 124–132
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Technical note
Preliminary assessment of the highest potential transmissivity of fractures in fault zones by core logging Eiichi Ishii Horonobe Underground Research Laboratory, Japan Atomic Energy Agency, Hokushin 432-2, Horonobe-cho, Hokkaido 098-3224, Japan
a r t i c l e
i n f o
Article history: Received 9 November 2016 Received in revised form 21 February 2017 Accepted 25 February 2017 Available online xxxx Keywords: Damage-zone fracture Core logging Failure mode Fault zone Transmissivity
a b s t r a c t Fault zones are representative flow paths in rock masses, and the highest transmissivity of fractures in fault zones is important for conservative assessment of groundwater flow velocity for the purpose of radioactive waste disposal. Based on previous hydromechanical studies of fault zones, fault zones without tensile/hybrid damage-zone fractures are unlikely to have experienced faulting in a highly brittle manner, and the highest potential transmissivity of fractures in such fault zones is inferred to be relatively low (transmissivity (T) ≤ 10−8 m2/s). To verify this inference, this study investigates the relationship between the failure mode (tensile/hybrid/shear) and the highest transmissivity of fractures in fault zones, using results from core logging and flowing-fluids electric conductivity (FFEC) logging in boreholes penetrating a Neogene siliceous mudstone (Wakkanai Formation) of the Horonobe area, northern Hokkaido, Japan. In 96% (35/36) of fault zones where tensile/hybrid fractures were not observed, the transmissivities of flow anomalies detected within the fault zones by FFEC logging are within the range of ≤10−8 m2/s, whereas in 95% (145/153) of fault zones where tensile/hybrid fractures were observed, the transmissivities are within the range of ≥10−8 m2/s. This result supports the above-mentioned inference, suggesting that core observations of whether tensile/hybrid fractures develop in fault zones allow preliminary assessment of the highest potential transmissivity of fractures in fault zones (i.e., potentially T ≥ 10−8 m2/s or likely T ≤ 10−8 m2/s). Such assessment increases the efficiency of hydrogeological borehole investigations of fault zones. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Transmissivity of fault zones, which are representative flow paths in low-permeability rocks (e.g., Gutmanis et al., 1998; Mazurek et al., 1998; Follin and Stigsson, 2014; Ishii, 2015), can be directly measured by in situ hydraulic tests in boreholes that penetrate fault zones (Tsang et al., 2015). Such tests include packer tests, flowing-fluids electric conductivity (FFEC) logging (Tsang et al., 1990), and the Posiva flow log (PFL) measurements (Öhberg and Rouhiainen, 2000). The transmissivity data are important for site selection, design, and safety assessment of radioactive waste disposal areas (NAGRA, 1997; Japan Nuclear Cycle Development Institute, 2000; Nuclear Waste Management Organization of Japan, 2013; Tsang et al., 2015). However, when time and cost are somewhat limited in borehole investigations, abundant hydraulic tests may not be feasible, and hydraulic tests themselves may also be impossible, depending on borehole conditions (e.g., borehole diameter, dissolved gas, pore pressure, etc.). Although the aperture of fractures measured from borehole wall images can also be used to assess the transmissivity (e.g., using the parallel plate model (cubic
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http://dx.doi.org/10.1016/j.enggeo.2017.02.026 0013-7952/© 2017 Elsevier B.V. All rights reserved.
law); Taylor et al., 1999; Flodin and Durlofsky, 2001; Jourde et al., 2002), the application of optical borehole-wall imaging for the precise measurement of fracture aperture requires limited borehole fluid and wall conditions (Williams and Johnson, 2004). Additionally, measurements of fracture aperture by acoustic/resistivity borehole-wall imaging have relatively poor detection limits (Genter et al., 1997). To efficiently conduct hydrogeological investigations of fault zones, even under such conditions, full utilization of core logging is necessary, as it enables geological information to be obtained easily and quickly. To elucidate the relationship between geological information obtained by core logging and the permeability of fault zones, hydrogeological assessments have been conducted, and are based on results from laboratory experiments using core samples of matrices and fault rocks. In such studies, the fault core(s) and damage-zones are identified in drill core and the fault-parallel/normal bulk permeability is modeled (e.g., Evans et al., 1997; Lockner et al., 2009; Walker et al., 2013). However, to assess flow velocity in rock masses, the permeability of each conducting fracture in fault zones is important. The highest transmissivity of fractures in fault zones is significant for conservative assessments of groundwater flow velocity, which is key for safety assessments of geological sites for radioactive waste disposal. To evaluate the permeability of conducting fractures, detailed in situ hydraulic
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measurements are needed, such as short interval packer tests (e.g., Hamm et al., 2007), FFEC logging, and/or PFL measurements. These measurements have been performed at site investigations, including hydrogeological studies of fault zones. For example, Gutmanis et al. (1998) compared core logging data and permeability data obtained by short interval packer tests at the Sellafield site (Ordovician volcaniclastic rocks, United Kingdom), where fault zones are major potential conducting features (Gutmanis et al., 1998; Ishii, 2015), and indicated that the permeability increases with increasing frequency of fractures or potential flowing features of drill cores. Mazurek (1998, 2000) and Mazurek et al. (1998) compared core logging data and locations of flow anomalies detected by FFEC logging at the Wellenberg site (Cretaceous argillaceous marl, Switzerland) and the Northern Switzerland site (granite/gneiss), and reported no apparent systematic difference in the core logging data between fault zones that have or do not have flow anomalies, and found that marked hydrogeological heterogeneities (e.g., channeling) may exist within the fault zones. However, the relationship between the failure mode and the highest potential transmissivity of fractures in fault zones has not been well constrained quantitatively, although studies have shown, for example, that hybrid fractures tend to form the dilational portion (pull-aparts) of fractures in fault zones (Ferrill et al., 2012). Major fluid pathways can form if these open structures are then connected (Sibson, 1996; Holland et al., 2011). Previous studies have suggested that brecciated zones originating from numerous tensile fractures are potentially highly permeable features (Dholakia et al., 1998; Aydin, 2000, 2014). Recently, Ishii (2015) proposed a rock mechanical indicator, ductility index (DI), which is defined by the effective mean stress of a rock mass normalized to the tensile strength of intact host rock, and showed that the highest potential transmissivity of fractures in fault zones can be uniformly predicted from DI, based on data from sedimentary rocks and crystalline rocks at six sites (Fig. 1). The studied fractures include all fractures in fault zones (e.g., damage-zone fractures and slip surfaces bounding fault rocks). The highest transmissivities of fractures in fault zones correspond to the transmissivities of flow anomalies detected by FFEC logging and/or PFL measurements. The correlation between the highest potential transmissivity and DI is given by the following equation: logT ¼ −3:5 logDI−6:54 ðstandarderror ¼ 1:25 in logT Þ:
ð1Þ
DI was calculated using the tensile strengths of intact rocks determined from laboratory indirect tension (Brazilian) tests, and the mean stress and pore pressure of the rock mass were determined from in situ measurements in boreholes. Although the rock strengths in fault zones may be partly reduced by fault deformation, the DI model uses the strengths of intact host rocks due to the following reasons: (1) as
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a harder rock is likely to allow a larger dilation, the strength of intact rock is a more important factor when considering the prediction of the highest potential transmissivities; and (2) the strengths of intact rocks can be more consistently defined and measured (Ishii, 2015). Although the mean stresses around faults can be perturbed by failure within the damage zone (e.g., Segall and Pollard, 1980), the DI model uses the mean stresses calculated from the far-field stresses (Ishii, 2015). Furthermore, Ishii (2016) indicated that DI is closely related to the failure mode of damage-zone fractures in fault zones, as shown by theoretical analysis based on the Griffith-Coulomb failure criterion, laboratory experiments, and core logging. Ishii (2016) reported that: (1) only tensile/hybrid damage-zone fractures form when faulting occurs at DI b 2; and (2) shear damage-zone fractures can form when faulting occurs at DI N 2, which suppresses the formation of tensile/hybrid damage-zone fractures. These damage-zone fractures result from stress concentrations at the asperities/tips of slip surfaces in fault zones (Ishii, 2016), and the tensile/hybrid damage-zone fractures are typically wing/splay cracks (e.g., Kim et al., 2004) that propagate from the slip surfaces (Fig. 2a). The shear damage-zone fractures are typically synthetic/antithetic shear fractures (e.g., Kim et al., 2004) that branch from the slip surfaces (Fig. 2b), and they strictly initiate from other preceding/preexisting structures such as micro shear bands (e.g., Ishii, 2012, 2016) or tensile microcracks (e.g., Petit and Barquins, 1988; Healy et al., 2006). Shear fractures/slip surfaces do not nucleate and self-propagate alone (e.g., Crider and Peacock, 2004; Aydin et al., 2006). During the evolution of fault zones, the initial slip surfaces form along other preexisting structures such as deformation bands (e.g., Aydin and Johnson, 1978; Shipton and Cowie, 2001; Schultz and Siddharthan, 2005; Ishii, 2012) or joints (e.g., Martel and Pollard, 1989; Crider and Peacock, 2004; Myers and Aydin, 2004; Walker et al., 2012). During the evolution of the fault zone, all fractures can be reactivated (sheared) (e.g., Crider and Peacock, 2004; Aydin, 2014), and reactivated fractures may newly develop their own damagezone fractures (e.g., Davatzes et al., 2003; Flodin and Aydin, 2004; Myers and Aydin, 2004; Aydin, 2014; Ishii, 2016). If faulting occurs at DI b 2, these new damage-zone fractures would be tensile/hybrid fractures, according to the model of Ishii (2016). Thus, integrating the above-mentioned findings of Ishii (2015, 2016), it is inferred that fault zones in which tensile/hybrid fractures do not develop are unlikely to experience faulting at DI b 2, and the highest potential transmissivities of fractures in such fault zones are likely to be ≤ 10− 8 m2 /s (Fig. 2). This range of transmissivity corresponds to a range of the highest potential transmissivity of fractures in fault zones at DI N 2, according to Eq. (1). To verify the above-mentioned inference, this study investigates the relationship between the failure mode (tensile/hybrid or shear) and the highest potential transmissivity of fractures in fault zones, based on the
Fig. 1. Transmissivities of the flow anomalies detected in fault zones, and the corresponding DI for the six sites of interest (Ishii, 2015). Horizontal error bars show estimation errors due to uncertainties in tensile strength. Also shown is the best fit line (solid line) with the standard error (=1.25 in logT: broken lines) from regression analysis, using the data for all the sites.
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Fig. 2. A proposed model of the relationship between the failure mode of damage-zone fractures and fracture transmissivity, modified from Ishii (2016). (a) The depicted fault developed in an environment with a high ductility index (DI ≤ 2). Numerous tensile/hybrid fractures developed in the fault damage-zone. (b) The depicted fault did not develop in an environment with a high ductility index (DI ≤ 2). Only a few tensile/hybrid fractures develop along faults.
results of core logging and depth profiles of transmissivity estimated by FFEC logging in boreholes penetrating a low-permeability rock. The FFEC logging allows identification of locations and transmissivities of flow anomalies, as well as locations of no-flow zones, where no flow
anomaly is detected, and is a suitable technique for evaluating the highest transmissivities of fractures (Tsang et al., 1990; Doughty et al., 2013). The conceptual model for fault zone architecture in this study follows that of Caine et al. (1996) and Faulkner et al. (2010), in which
Fig. 3. Geological map and cross-section of the Horonobe area (after Ishii, 2012), showing the locations of boreholes and the Horonobe Underground Research Facility.
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Fig. 4. Schematic stratigraphic section of the Horonobe area (Ishii et al., 2010). Sb: Sarabetsu Formation; Yc: Yuchi Formation; Kt: Koetoi Formation; Wk: Wakkanai Formation.
a fault zone consists of strain-localized fault cores that comprise fault rocks bounded on one or two sides by slip surfaces, and damage zones that record the development of damage-zone fractures surrounding the fault cores. Fault zones in Neogene siliceous mudstone (the Wakkanai Formation; Figs. 3 and 4), northern Hokkaido, Japan, were selected as the target of this study for the following reasons: • In the Wakkanai Formation, a variety of previous investigations and the underground construction of the Horonobe Underground Research Facility (Fig. 3) have been conducted to develop techniques for geological disposal of radioactive waste and numerous datasets are available; including data from core logging and FFEC logging (e.g., Ishii, 2015, 2016). • In cases where multiple conducting features (e.g., fault zones, joint zones, and dykes/veins) coexist in a formation/rock mass, the cause of flow anomalies cannot always be directly linked to fault zones (e.g., Mazurek, 1998; Mazurek et al., 1998). However, in the Wakkanai Formation, fault zones have been considered the only major conducting features (Ishii et al., 2010, 2011; Ishii, 2012, 2015; see the following section), and locations of fault rocks (fault cores) and flow anomalies detected by FFEC logging show a very close relationship (Fig. 5). This suggests that the major conducting features occur exclusively in fault zones, and flow anomalies may be directly linked to fault zones. • Fractures in the Wakkanai Formation are generally not sealed by infilling minerals (Ishii, 2015). In addition, fracture transmissivity depends on the effective normal stress (Mattila and Tammisto, 2012; Follin and Stigsson, 2014) or the effective normal stress and shear stress (Barton et al., 1995; Rogers, 2003) acting on the fractures, which are determined from the orientations of the fractures and the principal effective stress. However, the transmissivity of fractures in fault zones may depend mainly on shear-induced dilation generated in the fault zone, which may have a close relationship with DI and fracture orientation (Ishii, 2015). The smaller the DI, the greater the potential for shear-induced dilation. A fault zone with a fault core(s) that is favorably oriented for frictional failure in the present stress field and that has damage-zone fractures oriented at a high angle to the minimum principal stress or at a low angle to the maximum principal stress is more susceptible to shear-induced dilation in terms of fracture orientation (Ishii, 2015; Fig. 6). The fault zones related to flow anomalies in the Wakkanai Formation are potentially the most permeable in terms of orientation, and the transmissivities of such flow anomalies may be the highest potential transmissivities of fractures in fault zones for a given DI (Ishii, 2015).
Fig. 5. (a) Frequency of distances (meters along borehole) from a flow anomaly to the nearest fault rock for the borehole intervals where the FFEC logging was conducted at the Horonobe site. Seventy-nine percent of flow anomalies occur within 10 m of fault rocks. (b) Frequency of distances (meters along borehole) from a fault rock to the nearest flow anomaly for the borehole intervals where the FFEC logging was conducted at the Horonobe site. Eighty-seven percent of fault rocks occur within 10 m of flow anomalies. Data are from Bruines et al. (2013), Doughty et al. (2008, 2013), Ishii et al. (2010, 2011), and Suko et al. (2014).
2. Geological setting In the Wakkanai Formation, faults occur in outcrop and in drill cores (Ishii et al., 2010). The faults exhibit slickensides, slickenlines, slickensteps (secondary fractures), contain fault rocks (thickness ≤ decimeters), and are commonly accompanied by damage-zone fractures (tensile, hybrid, and/or shear fractures), forming fault zones that are up to a few meters wide (Ishii et al., 2010; Tokiwa et al., 2013; Ishii, 2015, 2016). The tensile/hybrid damage-zone fractures are typically wing/splay cracks that propagate from the slip surfaces (Ishii et al., 2010; Ishii, 2016), and some of these cracks are reactivated (sheared) (Ishii et al., 2010). Very thin compactional shear bands are observed along shear fracture surfaces and beyond shear fracture tips in the Wakkanai Formation, and the initial slip surfaces and shear damagezone fractures are interpreted to have formed along pre-existing compactional shear bands (Ishii, 2012). Fractures in the Wakkanai Formation are mainly shear fractures and rare joints, based on the results of core logging (Ishii et al., 2010, 2011) and geological mapping in
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Fig. 6. Schematic diagrams of (a) opening of secondary fractures caused by shear-induced tensile stresses in a damage zone, and (b) pore structures formed by mismatch of the shearfracture walls. (c) Sketch of a sinistral strike-slip fault zone exposed at a horizontal outcrop in the Wakkanai Formation at the Horonobe site, and the directions of the maximum and minimum principal stresses. After Ishii (2015).
underground outcrops (Tokiwa et al., 2013). Mineral fillings (carbonates) are rarely observed in the fault zones (Ishii, 2015). The fault zones initiated from compactional shear bands, developed during and/ or after uplift and denudation, following maximum burial, and formed in response to regional tectonic shortening (Ishii et al., 2010, 2011; Ishii, 2012). During uplift and denudation, far-field effective stress loading in the siliceous mudstone would have decreased with reduced thickness of overburden (Ishii et al., 2011; Ishii, 2012), indicating that DI values at the present burial depths are lower than those at any time in the burial history of the mudstone (Ishii, 2016). 3. Methods This study analyzed core logging data and FFEC logging data. During analysis, fault zones were classified according to the failure mode of fractures in the fault zones, and the highest transmissivities of fractures
in fault zones were determined from the transmissivities of flow anomalies detected within the fault zones. The studied boreholes and sections are from depths of 460–775 m in borehole HDB-11, 237–506 m in borehole PB-V01, and 100–710 m in borehole SAB-2 (Fig. 3), where the locations and transmissivities of the flow anomalies have already been determined by FFEC logging and packer tests (Ishii, 2015).
3.1. Core logging During previous core logging, tensile fractures (e.g., Fig. 7a), hybrid fractures (e.g., Fig. 7b), and shear fractures (e.g., Fig. 7c) were detected in drill cores (Suko et al., 2014; Ishii, 2016). However, some of the hybrid fractures might have been sheared tensile fractures (Ishii, 2016). These types of fractures were defined by visual characteristics on fracture surfaces, as described by Ishii (2016):
Fig. 7. Representative drill core photographs. (a) A tensile fracture surface at a depth of 346.18–246.34 m in borehole PB-V01. (b) Hybrid fracture surfaces at a depth of 885.54–885.80 m in borehole HDB-11. (c) A shear fracture surface at a depth of 469.53–469.65 m in borehole PB-V01. (d) A fault rock at a depth of 469.10–469.19 m in borehole PB-V01. (e) A disintegrated core zone neighboring a fault rock at a depth of 287.0–287.7 m in borehole PB-V01. (f) A tensile fracture surface on a fragment from within the disintegrated core zone in (e).
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• The tensile fracture has fracture surfaces exhibiting a natural plumose structure (e.g., Pollard and Aydin, 1988; Kulander et al., 1990) without evidence of shear; • The hybrid fracture (or sheared tensile fracture) is defined by the coexistence of characteristics of both tensile and shear fractures; • The shear fracture has fracture surfaces exhibiting slickensides (polished surfaces), slickenlines (striations on fracture surfaces that result from friction along the surfaces), and/or slickensteps (secondary fractures). Hybrid fractures form under mixed tensile and compressive stress states (e.g., Engelder, 1999; Ramsey and Chester, 2004). Although hybrid fractures can be recognized by the co-occurrence of characteristics of both tensile and shear fractures on the same fracture surface (e.g., Ferrill et al., 2012), these characteristics can be dependent on the scale of observation (Ishii, 2016). A similar occurrence may also be observed on fracture surfaces of sheared tensile fractures, which originated as tensile fractures and were then reactivated by shearing (e.g., Davatzes et al., 2003; Myers and Aydin, 2004). Although numerous researchers have reported natural hybrid fractures and sheared tensile fractures (e.g., Davatzes et al., 2003; Ferrill and Morris, 2003; Myers and Aydin, 2004; Ferrill et al., 2012), few instances of natural hybrid fractures and sheared tensile fractures have been reported in drill core. This study focuses on whether the initial failure mode of damage-zone fractures that formed during fault movement is tensile/hybrid or shear. Although sheared tensile fractures may be grouped into shear fractures as they are sheared, all tensile, hybrid, and sheared tensile fractures are
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grouped together as “tensile/hybrid fractures” in this study because their initial failure mode is tensile/hybrid. Natural plumose structures and artificially induced plumose structures caused by drilling/coring/ handling can be distinguished using the criteria of Kulander et al. (1990). Natural plumose structures are typically oversized with respect to the size of the core, and do not show symmetrical correlations with core geometry (Kulander et al., 1990). Conversely, artificially induced plumose structures commonly show that the fracture originated within the core and/or show unique geometrical relationships with core parameters (Kulander et al., 1990). Although fractures may occur in drill core as a result of stress relief or desiccation after coring, such fractures can be distinguished from tensile fractures with natural plumose structures. In this study, fault zones where natural tensile/hybrid fractures are observed in drill core were defined as dilatant fault zones, and fault zones without these fractures were defined as non-dilatant fault zones. Furthermore, although fault rocks may typically be bounded by slip surfaces (Fig. 7d), drill cores neighboring fault rocks may be disintegrated over tens of centimeters or more, likely due to the artificial effects of drilling and coring (Fig. 7e). Through detailed observation of these disintegrated core zones, natural tensile/hybrid fracture surfaces may be confirmed on disintegrated rock fragments (Fig. 7f). In such disintegrated core zones, open (tensile/hybrid) fractures probably preexisted drilling, and their open spaces allowed shaking of cored rocks in a drilling rod during drilling/coring, which led to fracturing and disintegration of core material. Because these open (tensile/hybrid) fractures inherently tend to develop near fault rocks, fault zones in
Fig. 8. Depth profiles of fault rocks, tensile/hybrid fractures, shear fractures, and disintegrated core zones in drill cores and transmissivities of flow anomalies detected by FFEC logging in boreholes (a) HDB-11 (460–775 m), (b) PB-V01 (237–506 m), and (c) SAB-2 (100–710 m).
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which disintegrated core zones occur adjacent to fault rocks were also classified as dilatant fault zones. Fault zone widths are defined as the sum of fault core thickness (i.e., fault rock thickness) and damage zone widths in both sides of the fault core. In the Wakkanai Formation, the fault rock thicknesses (≤decimeters) are typically thin compared with the damage-zone widths (≤meters), meaning that the fault zone widths are approximately equal to the damage-zone widths. As the damage-zone width may be difficult to determine by core logging alone (Ishii, 2015), the damage-zone widths in the Wakkanai Formation were assumed to be uniformly 10 m, following Ishii (2015). This is because the locations of flow anomalies detected by FFEC logging exclusively occur within 10 m of fault rocks (Fig. 5), and the damage-zone widths observed through geological mapping at surface outcrops and shaft/tunnel excavation faces are typically on the scale of a few meters or less (Ishii et al., 2010; Tokiwa et al., 2013; Ishii, 2015). The damage-zone width of 10 m is the maximum value of damage-zone widths in the Wakkanai Formation, which is sufficient for confidently detecting non-dilatant fault zones. This width is within the globally reported range of damage-zone widths (i.e., 10−2 to 102 m; Faulkner et al., 2010; Torabi and Berg, 2011).
procedure while using two or more pumping rates (multi-rate FFEC logging) allows the determination of transmissivities and hydraulic heads of individual fractures (Tsang et al., 1990; Doughty et al., 2013). The locations and transmissivities of flow anomalies in boreholes HDB-11, PBV01, and SAB-2 have already been determined (Doughty et al., 2008; Bruines et al., 2013; Doughty et al., 2013; Ishii, 2015). In this study, flow anomalies detected within the fault zones were defined as the highest transmissivities of fractures in the fault zones. The highest transmissivities of fractures in fault zones where no flow anomalies were detected were assumed to be less than the practical detection limit of FFEC logging. Because flow anomalies of 10−10 m2/s or more can be practically detected by the FFEC logging (Doughty et al., 2008, 2013; Bruines et al., 2013; Ishii, 2015), the detection limit was assumed to be 10−10 m2/s. Hydraulic conductivity of intact rocks in the Wakkanai Formation is approximately 10−12 m/s (Ishii et al., 2011). 4. Results and discussion
3.2. FFEC logging
Locations of fault rocks, tensile/hybrid fractures, shear fractures, and disintegrated core zones are shown in Fig. 8, as well as the locations and transmissivities of flow anomalies detected by FFEC logging. Analyses based on these data reveal the following:
In FFEC logging, borehole fluids were first replaced with a fluid of contrasting electric conductivity, and FFEC logging was then repeated while the borehole was pumped. When peaks in the FFEC logs occur, fluids that flow into the boreholes are analyzed to enable calculation of inflow rates and salinity within individual fractures. Conducting the
• In borehole HDB-11, the highest transmissivities of 83% (5/6) of nondilatant fault zones are ≤10−8 m2/s, whereas the highest transmissivities of 89% (33/37) of dilatant fault zones are ≥10−8 m2/s (Fig. 9a); • In borehole PB-V01, the highest transmissivities of 100% (2/2) of nondilatant fault zones are ≤ 10−8 m2/s, whereas the highest
Fig. 9. Frequency of the highest transmissivity of fractures in fault zones of non-dilatant fault zones and dilatant fault zones in boreholes (a) HDB-11 (460–775 m), (b) PB-V01 (237– 506 m), (c) SAB-2 (100–710 m), and (d) in all the boreholes.
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transmissivities of 89% (33/37) of dilatant fault zones are ≥10−8 m2/s (Fig. 9b); • In borehole SAB-2, the highest transmissivities of 93% (26/28) of nondilatant fault zones are ≤10−8 m2/s, whereas the highest transmissivities of 93% (54/58) of dilatant fault zones are ≥10−8 m2/s (Fig. 9c); • In all the boreholes, the highest transmissivities of 97% (35/36) of non-dilatant fault zones are ≤10−8 m2/s, whereas the highest transmissivities of 95% (145/153) of dilatant fault zones are ≥ 10−8 m2/s (Fig. 9d). These results support the above-mentioned hypothesis that the highest potential transmissivity of fractures in fault zones where tensile/hybrid fractures do not develop is likely ≤10−8 m2/s (Fig. 2). Ishii (2015) pointed out that the highest potential transmissivity of fractures in possible fault zones can be predicted from the distribution of the rock mechanical indicator, DI. However, in a given site of interest, rock strength data and initial stress data are not always fully available, particularly during the early stages of investigation. In addition, rock strength and stress state may significantly vary in the same host rocks if lithological heterogeneity is strong, and such a condition requires more abundant data for mapping DI in detail. The core logging method that was applied in this study proved useful for the hydrogeological classification of fault zones into potentially high-permeability fault zones (dilatant fault zones: T ≥ 10−8 m2/s) and low-permeability fault zones (non-dilatant fault zones: T ≤ 10−8 m2/s). The transmissivity of ≤10−8 m2/s almost corresponds to the range of fracture transmissivity distribution (log-normal distribution; logarithmic mean value = −8.99; logarithmic standard deviation = 1.07) given for discrete fractured network models (100 m3 block) of the conservative case in the safety assessment for a generic fractured rock mass by the Japan Nuclear Cycle Development Institute (2000). Although quantitative estimation of fault zone permeability through core logging has been considered difficult, core observations on the failure mode of fractures may increase the efficiency of hydrogeological borehole investigations of fault zones. 5. Conclusions Based on previous hydromechanical studies of fault zones (Ishii, 2015, 2016), it is inferred that the highest potential transmissivity of fractures in fault zones where tensile/hybrid fractures do not develop is likely ≤10−8 m2/s. In order to verify this inference, this study investigated the relationship between the failure mode and the highest transmissivity of fractures in fault zones, using core logging data and FFEC logging data derived from a Neogene siliceous mudstone (the Wakkanai Formation). The data show that in 96% (35/36) of fault zones where tensile/hybrid fractures are not observed, the highest transmissivities are ≤10−8 m2/s, whereas in 95% (145/153) of fault zones where tensile/hybrid fractures occur, the highest transmissivities are ≥10−8 m2/s. This result supports the above-mentioned inference, suggesting that core observations of the failure mode of fractures in fault zones may allow assessment of the highest potential transmissivity of fractures in fault zones in terms of whether it is potentially ≥ 10− 8 m2/s or likely ≤10−8 m2/s. Acknowledgments The author thanks geoengineers of Mitsubishi Materials Techno Corporation for conducting core logging, anonymous reviewers for constructive reviews and helpful comments, and A. Stallard for improving the grammar and for helpful suggestions regarding the manuscript. References Aydin, A., 2000. Fractures, faults, and hydrocarbon entrapment, migration and flow. Mar. Pet. Geol. 17, 797–814.
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Representation of fault zone permeability in reservoir flow models. Stanford Rock Fracture Project 12, pp. D1–D10. Follin, S., Stigsson, M., 2014. A transmissivity model for deformation zones in fractured crystalline rock and its possible correlation to in situ stress at the proposed highlevel nuclear waste repository site at Forsmark, Sweden. Hydrogeol. J. 22, 299–311. Genter, A., Castaing, C., Dezayes, C., Tenzer, H., Traineau, H., Villemin, T., 1997. Comparative analysis of direct (core) and indirect (borehole imaging tools) collection of fracture data in the Hot Dry Rock Soultz reservoir (France). J. Geophys. Res. 102, 15,419–15,431. Gutmanis, J.C., Lanyon, G.W., Wynn, T.J., Watson, C.R., 1998. Fluid flow in faults: a study of fault hydrogeology in Triassic sandstone and Ordovician volcaniclastic rocks at Sellafield, north-west England. Proc. Yorks. Geol. Soc. 52, 159–175. Hamm, S., Kim, M., Cheong, J., Kim, J., Son, M., Kim, T., 2007. 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Far-field stress dependency of the failure mode of damage-zone fractures in fault zones: results from laboratory tests and field observations of siliceous mudstone. J. Geophys. Res. 121, 70–91. Ishii, E., Funaki, H., Tokiwa, T., Ota, K., 2010. Relationship between fault growth mechanism and permeability variations with depth of siliceous mudstones in northern Hokkaido, Japan. J. Struct. Geol. 32, 1792–1805. Ishii, E., Sanada, H., Funaki, H., Sugita, Y., Kurikami, H., 2011. The relationships among brittleness, deformation behavior, and transport properties in mudstones: an example from the Horonobe Underground Research Laboratory, Japan. J. Geophys. Res. 116, B09206. Japan Nuclear Cycle Development Institute, 2000. Second Progress Report on Research and Development for the Geological Disposal of HLW in Japan: Supporting Report 3 Safety Assessment of the Geological Disposal System. Japan Nuclear Cycle Development Institute, Tokai-mura, Japan. 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Schultz, R.A., Siddharthan, R., 2005. A general framework for the occurrence and faulting of deformation bands in porous granular rocks. Tectonophysics 411, 1–18. Segall, P., Pollard, D.D., 1980. Mechanics of discontinuous faults. J. Geophys. Res. 85, 4337–4350. Shipton, Z.K., Cowie, P.A., 2001. Damage zone and slip-surface evolution over μm to km scales in high-porosity Navajo sandstone, Utah. J. Struct. Geol. 23, 1825–1844. Sibson, R.H., 1996. Structural permeability of fluid-driven fault-fracture meshes. J. Struct. Geol. 18, 1031–1042. Suko, T., Takano, H., Uchida, M., Seki, Y., Ito, K., Watanabe, Y., Munakata, M., Tanaka, T., Amano, K., 2014. Research on Validation of the Groundwater Flow Evaluation Methods Based on the Information of Geological Environment In and Around Horonobe Underground Research Area. Japan Nuclear Energy Safety Organization, Tokyo. Taylor, W.L., Pollard, D.D., Aydin, A., 1999. Fluid flow in discrete joint sets: field observations and numerical simulations. J. Geophys. Res. 104, 28,983–29,006. Tokiwa, T., Sawada, S., Ochiai, S., Miyakawa, K., 2013. Occurrence of high-permeability fracture estimated by grouting in Horonobe URL of Japan. Proceedings of the 13th Japan Symposium on Rock Mechanics & the 6th Japan-Korea Joint Symposium on Rock Engineering. Japanese Committee for Rock Mechanics, Okinawa, pp. 1021–1025. Torabi, A., Berg, S.S., 2011. Scaling of fault attributes: a review. Mar. Pet. Geol. 28, 1444–1460. Tsang, C.-F., Hufschmied, P., Hale, F.V., 1990. Determination of fracture inflow parameters with a borehole fluid conductivity logging method. Water Resour. Res. 26, 561–578. Tsang, C.-F., Neretnieks, I., Tsang, Y., 2015. Hydrologic issues associated with nuclear waste repositories. Water Resour. Res. 51, 6923–6972. Walker, R.J., Holdsworth, R.E., Imber, J., Ellis, D., 2012. Fault-zone evolution in layered basalt sequences: a case study from the Faroe Islands, NE Atlantic margin. Geol. Soc. Am. Bull. 124, 1382–1393. Walker, R.J., Holdsworth, R.E., Armitage, P.J., Faulkner, D.R., 2013. Fault zone permeability structure evolution in basalts. Geology 41, 59–62. Williams, J.H., Johnson, C.D., 2004. Acoustic and optical borehole-wall imaging for fractured-rock aquifer studies. J. Appl. Geophys. 55, 151–159.
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Technical note
Preliminary assessment of the highest potential transmissivity of fractures in fault zones by core logging Eiichi Ishii Horonobe Underground Research Laboratory, Japan Atomic Energy Agency, Hokushin 432-2, Horonobe-cho, Hokkaido 098-3224, Japan
a r t i c l e
i n f o
Article history: Received 9 November 2016 Received in revised form 21 February 2017 Accepted 25 February 2017 Available online xxxx Keywords: Damage-zone fracture Core logging Failure mode Fault zone Transmissivity
a b s t r a c t Fault zones are representative flow paths in rock masses, and the highest transmissivity of fractures in fault zones is important for conservative assessment of groundwater flow velocity for the purpose of radioactive waste disposal. Based on previous hydromechanical studies of fault zones, fault zones without tensile/hybrid damage-zone fractures are unlikely to have experienced faulting in a highly brittle manner, and the highest potential transmissivity of fractures in such fault zones is inferred to be relatively low (transmissivity (T) ≤ 10−8 m2/s). To verify this inference, this study investigates the relationship between the failure mode (tensile/hybrid/shear) and the highest transmissivity of fractures in fault zones, using results from core logging and flowing-fluids electric conductivity (FFEC) logging in boreholes penetrating a Neogene siliceous mudstone (Wakkanai Formation) of the Horonobe area, northern Hokkaido, Japan. In 96% (35/36) of fault zones where tensile/hybrid fractures were not observed, the transmissivities of flow anomalies detected within the fault zones by FFEC logging are within the range of ≤10−8 m2/s, whereas in 95% (145/153) of fault zones where tensile/hybrid fractures were observed, the transmissivities are within the range of ≥10−8 m2/s. This result supports the above-mentioned inference, suggesting that core observations of whether tensile/hybrid fractures develop in fault zones allow preliminary assessment of the highest potential transmissivity of fractures in fault zones (i.e., potentially T ≥ 10−8 m2/s or likely T ≤ 10−8 m2/s). Such assessment increases the efficiency of hydrogeological borehole investigations of fault zones. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Transmissivity of fault zones, which are representative flow paths in low-permeability rocks (e.g., Gutmanis et al., 1998; Mazurek et al., 1998; Follin and Stigsson, 2014; Ishii, 2015), can be directly measured by in situ hydraulic tests in boreholes that penetrate fault zones (Tsang et al., 2015). Such tests include packer tests, flowing-fluids electric conductivity (FFEC) logging (Tsang et al., 1990), and the Posiva flow log (PFL) measurements (Öhberg and Rouhiainen, 2000). The transmissivity data are important for site selection, design, and safety assessment of radioactive waste disposal areas (NAGRA, 1997; Japan Nuclear Cycle Development Institute, 2000; Nuclear Waste Management Organization of Japan, 2013; Tsang et al., 2015). However, when time and cost are somewhat limited in borehole investigations, abundant hydraulic tests may not be feasible, and hydraulic tests themselves may also be impossible, depending on borehole conditions (e.g., borehole diameter, dissolved gas, pore pressure, etc.). Although the aperture of fractures measured from borehole wall images can also be used to assess the transmissivity (e.g., using the parallel plate model (cubic
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http://dx.doi.org/10.1016/j.enggeo.2017.02.026 0013-7952/© 2017 Elsevier B.V. All rights reserved.
law); Taylor et al., 1999; Flodin and Durlofsky, 2001; Jourde et al., 2002), the application of optical borehole-wall imaging for the precise measurement of fracture aperture requires limited borehole fluid and wall conditions (Williams and Johnson, 2004). Additionally, measurements of fracture aperture by acoustic/resistivity borehole-wall imaging have relatively poor detection limits (Genter et al., 1997). To efficiently conduct hydrogeological investigations of fault zones, even under such conditions, full utilization of core logging is necessary, as it enables geological information to be obtained easily and quickly. To elucidate the relationship between geological information obtained by core logging and the permeability of fault zones, hydrogeological assessments have been conducted, and are based on results from laboratory experiments using core samples of matrices and fault rocks. In such studies, the fault core(s) and damage-zones are identified in drill core and the fault-parallel/normal bulk permeability is modeled (e.g., Evans et al., 1997; Lockner et al., 2009; Walker et al., 2013). However, to assess flow velocity in rock masses, the permeability of each conducting fracture in fault zones is important. The highest transmissivity of fractures in fault zones is significant for conservative assessments of groundwater flow velocity, which is key for safety assessments of geological sites for radioactive waste disposal. To evaluate the permeability of conducting fractures, detailed in situ hydraulic
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measurements are needed, such as short interval packer tests (e.g., Hamm et al., 2007), FFEC logging, and/or PFL measurements. These measurements have been performed at site investigations, including hydrogeological studies of fault zones. For example, Gutmanis et al. (1998) compared core logging data and permeability data obtained by short interval packer tests at the Sellafield site (Ordovician volcaniclastic rocks, United Kingdom), where fault zones are major potential conducting features (Gutmanis et al., 1998; Ishii, 2015), and indicated that the permeability increases with increasing frequency of fractures or potential flowing features of drill cores. Mazurek (1998, 2000) and Mazurek et al. (1998) compared core logging data and locations of flow anomalies detected by FFEC logging at the Wellenberg site (Cretaceous argillaceous marl, Switzerland) and the Northern Switzerland site (granite/gneiss), and reported no apparent systematic difference in the core logging data between fault zones that have or do not have flow anomalies, and found that marked hydrogeological heterogeneities (e.g., channeling) may exist within the fault zones. However, the relationship between the failure mode and the highest potential transmissivity of fractures in fault zones has not been well constrained quantitatively, although studies have shown, for example, that hybrid fractures tend to form the dilational portion (pull-aparts) of fractures in fault zones (Ferrill et al., 2012). Major fluid pathways can form if these open structures are then connected (Sibson, 1996; Holland et al., 2011). Previous studies have suggested that brecciated zones originating from numerous tensile fractures are potentially highly permeable features (Dholakia et al., 1998; Aydin, 2000, 2014). Recently, Ishii (2015) proposed a rock mechanical indicator, ductility index (DI), which is defined by the effective mean stress of a rock mass normalized to the tensile strength of intact host rock, and showed that the highest potential transmissivity of fractures in fault zones can be uniformly predicted from DI, based on data from sedimentary rocks and crystalline rocks at six sites (Fig. 1). The studied fractures include all fractures in fault zones (e.g., damage-zone fractures and slip surfaces bounding fault rocks). The highest transmissivities of fractures in fault zones correspond to the transmissivities of flow anomalies detected by FFEC logging and/or PFL measurements. The correlation between the highest potential transmissivity and DI is given by the following equation: logT ¼ −3:5 logDI−6:54 ðstandarderror ¼ 1:25 in logT Þ:
ð1Þ
DI was calculated using the tensile strengths of intact rocks determined from laboratory indirect tension (Brazilian) tests, and the mean stress and pore pressure of the rock mass were determined from in situ measurements in boreholes. Although the rock strengths in fault zones may be partly reduced by fault deformation, the DI model uses the strengths of intact host rocks due to the following reasons: (1) as
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a harder rock is likely to allow a larger dilation, the strength of intact rock is a more important factor when considering the prediction of the highest potential transmissivities; and (2) the strengths of intact rocks can be more consistently defined and measured (Ishii, 2015). Although the mean stresses around faults can be perturbed by failure within the damage zone (e.g., Segall and Pollard, 1980), the DI model uses the mean stresses calculated from the far-field stresses (Ishii, 2015). Furthermore, Ishii (2016) indicated that DI is closely related to the failure mode of damage-zone fractures in fault zones, as shown by theoretical analysis based on the Griffith-Coulomb failure criterion, laboratory experiments, and core logging. Ishii (2016) reported that: (1) only tensile/hybrid damage-zone fractures form when faulting occurs at DI b 2; and (2) shear damage-zone fractures can form when faulting occurs at DI N 2, which suppresses the formation of tensile/hybrid damage-zone fractures. These damage-zone fractures result from stress concentrations at the asperities/tips of slip surfaces in fault zones (Ishii, 2016), and the tensile/hybrid damage-zone fractures are typically wing/splay cracks (e.g., Kim et al., 2004) that propagate from the slip surfaces (Fig. 2a). The shear damage-zone fractures are typically synthetic/antithetic shear fractures (e.g., Kim et al., 2004) that branch from the slip surfaces (Fig. 2b), and they strictly initiate from other preceding/preexisting structures such as micro shear bands (e.g., Ishii, 2012, 2016) or tensile microcracks (e.g., Petit and Barquins, 1988; Healy et al., 2006). Shear fractures/slip surfaces do not nucleate and self-propagate alone (e.g., Crider and Peacock, 2004; Aydin et al., 2006). During the evolution of fault zones, the initial slip surfaces form along other preexisting structures such as deformation bands (e.g., Aydin and Johnson, 1978; Shipton and Cowie, 2001; Schultz and Siddharthan, 2005; Ishii, 2012) or joints (e.g., Martel and Pollard, 1989; Crider and Peacock, 2004; Myers and Aydin, 2004; Walker et al., 2012). During the evolution of the fault zone, all fractures can be reactivated (sheared) (e.g., Crider and Peacock, 2004; Aydin, 2014), and reactivated fractures may newly develop their own damagezone fractures (e.g., Davatzes et al., 2003; Flodin and Aydin, 2004; Myers and Aydin, 2004; Aydin, 2014; Ishii, 2016). If faulting occurs at DI b 2, these new damage-zone fractures would be tensile/hybrid fractures, according to the model of Ishii (2016). Thus, integrating the above-mentioned findings of Ishii (2015, 2016), it is inferred that fault zones in which tensile/hybrid fractures do not develop are unlikely to experience faulting at DI b 2, and the highest potential transmissivities of fractures in such fault zones are likely to be ≤ 10− 8 m2 /s (Fig. 2). This range of transmissivity corresponds to a range of the highest potential transmissivity of fractures in fault zones at DI N 2, according to Eq. (1). To verify the above-mentioned inference, this study investigates the relationship between the failure mode (tensile/hybrid or shear) and the highest potential transmissivity of fractures in fault zones, based on the
Fig. 1. Transmissivities of the flow anomalies detected in fault zones, and the corresponding DI for the six sites of interest (Ishii, 2015). Horizontal error bars show estimation errors due to uncertainties in tensile strength. Also shown is the best fit line (solid line) with the standard error (=1.25 in logT: broken lines) from regression analysis, using the data for all the sites.
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Fig. 2. A proposed model of the relationship between the failure mode of damage-zone fractures and fracture transmissivity, modified from Ishii (2016). (a) The depicted fault developed in an environment with a high ductility index (DI ≤ 2). Numerous tensile/hybrid fractures developed in the fault damage-zone. (b) The depicted fault did not develop in an environment with a high ductility index (DI ≤ 2). Only a few tensile/hybrid fractures develop along faults.
results of core logging and depth profiles of transmissivity estimated by FFEC logging in boreholes penetrating a low-permeability rock. The FFEC logging allows identification of locations and transmissivities of flow anomalies, as well as locations of no-flow zones, where no flow
anomaly is detected, and is a suitable technique for evaluating the highest transmissivities of fractures (Tsang et al., 1990; Doughty et al., 2013). The conceptual model for fault zone architecture in this study follows that of Caine et al. (1996) and Faulkner et al. (2010), in which
Fig. 3. Geological map and cross-section of the Horonobe area (after Ishii, 2012), showing the locations of boreholes and the Horonobe Underground Research Facility.
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Fig. 4. Schematic stratigraphic section of the Horonobe area (Ishii et al., 2010). Sb: Sarabetsu Formation; Yc: Yuchi Formation; Kt: Koetoi Formation; Wk: Wakkanai Formation.
a fault zone consists of strain-localized fault cores that comprise fault rocks bounded on one or two sides by slip surfaces, and damage zones that record the development of damage-zone fractures surrounding the fault cores. Fault zones in Neogene siliceous mudstone (the Wakkanai Formation; Figs. 3 and 4), northern Hokkaido, Japan, were selected as the target of this study for the following reasons: • In the Wakkanai Formation, a variety of previous investigations and the underground construction of the Horonobe Underground Research Facility (Fig. 3) have been conducted to develop techniques for geological disposal of radioactive waste and numerous datasets are available; including data from core logging and FFEC logging (e.g., Ishii, 2015, 2016). • In cases where multiple conducting features (e.g., fault zones, joint zones, and dykes/veins) coexist in a formation/rock mass, the cause of flow anomalies cannot always be directly linked to fault zones (e.g., Mazurek, 1998; Mazurek et al., 1998). However, in the Wakkanai Formation, fault zones have been considered the only major conducting features (Ishii et al., 2010, 2011; Ishii, 2012, 2015; see the following section), and locations of fault rocks (fault cores) and flow anomalies detected by FFEC logging show a very close relationship (Fig. 5). This suggests that the major conducting features occur exclusively in fault zones, and flow anomalies may be directly linked to fault zones. • Fractures in the Wakkanai Formation are generally not sealed by infilling minerals (Ishii, 2015). In addition, fracture transmissivity depends on the effective normal stress (Mattila and Tammisto, 2012; Follin and Stigsson, 2014) or the effective normal stress and shear stress (Barton et al., 1995; Rogers, 2003) acting on the fractures, which are determined from the orientations of the fractures and the principal effective stress. However, the transmissivity of fractures in fault zones may depend mainly on shear-induced dilation generated in the fault zone, which may have a close relationship with DI and fracture orientation (Ishii, 2015). The smaller the DI, the greater the potential for shear-induced dilation. A fault zone with a fault core(s) that is favorably oriented for frictional failure in the present stress field and that has damage-zone fractures oriented at a high angle to the minimum principal stress or at a low angle to the maximum principal stress is more susceptible to shear-induced dilation in terms of fracture orientation (Ishii, 2015; Fig. 6). The fault zones related to flow anomalies in the Wakkanai Formation are potentially the most permeable in terms of orientation, and the transmissivities of such flow anomalies may be the highest potential transmissivities of fractures in fault zones for a given DI (Ishii, 2015).
Fig. 5. (a) Frequency of distances (meters along borehole) from a flow anomaly to the nearest fault rock for the borehole intervals where the FFEC logging was conducted at the Horonobe site. Seventy-nine percent of flow anomalies occur within 10 m of fault rocks. (b) Frequency of distances (meters along borehole) from a fault rock to the nearest flow anomaly for the borehole intervals where the FFEC logging was conducted at the Horonobe site. Eighty-seven percent of fault rocks occur within 10 m of flow anomalies. Data are from Bruines et al. (2013), Doughty et al. (2008, 2013), Ishii et al. (2010, 2011), and Suko et al. (2014).
2. Geological setting In the Wakkanai Formation, faults occur in outcrop and in drill cores (Ishii et al., 2010). The faults exhibit slickensides, slickenlines, slickensteps (secondary fractures), contain fault rocks (thickness ≤ decimeters), and are commonly accompanied by damage-zone fractures (tensile, hybrid, and/or shear fractures), forming fault zones that are up to a few meters wide (Ishii et al., 2010; Tokiwa et al., 2013; Ishii, 2015, 2016). The tensile/hybrid damage-zone fractures are typically wing/splay cracks that propagate from the slip surfaces (Ishii et al., 2010; Ishii, 2016), and some of these cracks are reactivated (sheared) (Ishii et al., 2010). Very thin compactional shear bands are observed along shear fracture surfaces and beyond shear fracture tips in the Wakkanai Formation, and the initial slip surfaces and shear damagezone fractures are interpreted to have formed along pre-existing compactional shear bands (Ishii, 2012). Fractures in the Wakkanai Formation are mainly shear fractures and rare joints, based on the results of core logging (Ishii et al., 2010, 2011) and geological mapping in
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Fig. 6. Schematic diagrams of (a) opening of secondary fractures caused by shear-induced tensile stresses in a damage zone, and (b) pore structures formed by mismatch of the shearfracture walls. (c) Sketch of a sinistral strike-slip fault zone exposed at a horizontal outcrop in the Wakkanai Formation at the Horonobe site, and the directions of the maximum and minimum principal stresses. After Ishii (2015).
underground outcrops (Tokiwa et al., 2013). Mineral fillings (carbonates) are rarely observed in the fault zones (Ishii, 2015). The fault zones initiated from compactional shear bands, developed during and/ or after uplift and denudation, following maximum burial, and formed in response to regional tectonic shortening (Ishii et al., 2010, 2011; Ishii, 2012). During uplift and denudation, far-field effective stress loading in the siliceous mudstone would have decreased with reduced thickness of overburden (Ishii et al., 2011; Ishii, 2012), indicating that DI values at the present burial depths are lower than those at any time in the burial history of the mudstone (Ishii, 2016). 3. Methods This study analyzed core logging data and FFEC logging data. During analysis, fault zones were classified according to the failure mode of fractures in the fault zones, and the highest transmissivities of fractures
in fault zones were determined from the transmissivities of flow anomalies detected within the fault zones. The studied boreholes and sections are from depths of 460–775 m in borehole HDB-11, 237–506 m in borehole PB-V01, and 100–710 m in borehole SAB-2 (Fig. 3), where the locations and transmissivities of the flow anomalies have already been determined by FFEC logging and packer tests (Ishii, 2015).
3.1. Core logging During previous core logging, tensile fractures (e.g., Fig. 7a), hybrid fractures (e.g., Fig. 7b), and shear fractures (e.g., Fig. 7c) were detected in drill cores (Suko et al., 2014; Ishii, 2016). However, some of the hybrid fractures might have been sheared tensile fractures (Ishii, 2016). These types of fractures were defined by visual characteristics on fracture surfaces, as described by Ishii (2016):
Fig. 7. Representative drill core photographs. (a) A tensile fracture surface at a depth of 346.18–246.34 m in borehole PB-V01. (b) Hybrid fracture surfaces at a depth of 885.54–885.80 m in borehole HDB-11. (c) A shear fracture surface at a depth of 469.53–469.65 m in borehole PB-V01. (d) A fault rock at a depth of 469.10–469.19 m in borehole PB-V01. (e) A disintegrated core zone neighboring a fault rock at a depth of 287.0–287.7 m in borehole PB-V01. (f) A tensile fracture surface on a fragment from within the disintegrated core zone in (e).
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• The tensile fracture has fracture surfaces exhibiting a natural plumose structure (e.g., Pollard and Aydin, 1988; Kulander et al., 1990) without evidence of shear; • The hybrid fracture (or sheared tensile fracture) is defined by the coexistence of characteristics of both tensile and shear fractures; • The shear fracture has fracture surfaces exhibiting slickensides (polished surfaces), slickenlines (striations on fracture surfaces that result from friction along the surfaces), and/or slickensteps (secondary fractures). Hybrid fractures form under mixed tensile and compressive stress states (e.g., Engelder, 1999; Ramsey and Chester, 2004). Although hybrid fractures can be recognized by the co-occurrence of characteristics of both tensile and shear fractures on the same fracture surface (e.g., Ferrill et al., 2012), these characteristics can be dependent on the scale of observation (Ishii, 2016). A similar occurrence may also be observed on fracture surfaces of sheared tensile fractures, which originated as tensile fractures and were then reactivated by shearing (e.g., Davatzes et al., 2003; Myers and Aydin, 2004). Although numerous researchers have reported natural hybrid fractures and sheared tensile fractures (e.g., Davatzes et al., 2003; Ferrill and Morris, 2003; Myers and Aydin, 2004; Ferrill et al., 2012), few instances of natural hybrid fractures and sheared tensile fractures have been reported in drill core. This study focuses on whether the initial failure mode of damage-zone fractures that formed during fault movement is tensile/hybrid or shear. Although sheared tensile fractures may be grouped into shear fractures as they are sheared, all tensile, hybrid, and sheared tensile fractures are
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grouped together as “tensile/hybrid fractures” in this study because their initial failure mode is tensile/hybrid. Natural plumose structures and artificially induced plumose structures caused by drilling/coring/ handling can be distinguished using the criteria of Kulander et al. (1990). Natural plumose structures are typically oversized with respect to the size of the core, and do not show symmetrical correlations with core geometry (Kulander et al., 1990). Conversely, artificially induced plumose structures commonly show that the fracture originated within the core and/or show unique geometrical relationships with core parameters (Kulander et al., 1990). Although fractures may occur in drill core as a result of stress relief or desiccation after coring, such fractures can be distinguished from tensile fractures with natural plumose structures. In this study, fault zones where natural tensile/hybrid fractures are observed in drill core were defined as dilatant fault zones, and fault zones without these fractures were defined as non-dilatant fault zones. Furthermore, although fault rocks may typically be bounded by slip surfaces (Fig. 7d), drill cores neighboring fault rocks may be disintegrated over tens of centimeters or more, likely due to the artificial effects of drilling and coring (Fig. 7e). Through detailed observation of these disintegrated core zones, natural tensile/hybrid fracture surfaces may be confirmed on disintegrated rock fragments (Fig. 7f). In such disintegrated core zones, open (tensile/hybrid) fractures probably preexisted drilling, and their open spaces allowed shaking of cored rocks in a drilling rod during drilling/coring, which led to fracturing and disintegration of core material. Because these open (tensile/hybrid) fractures inherently tend to develop near fault rocks, fault zones in
Fig. 8. Depth profiles of fault rocks, tensile/hybrid fractures, shear fractures, and disintegrated core zones in drill cores and transmissivities of flow anomalies detected by FFEC logging in boreholes (a) HDB-11 (460–775 m), (b) PB-V01 (237–506 m), and (c) SAB-2 (100–710 m).
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which disintegrated core zones occur adjacent to fault rocks were also classified as dilatant fault zones. Fault zone widths are defined as the sum of fault core thickness (i.e., fault rock thickness) and damage zone widths in both sides of the fault core. In the Wakkanai Formation, the fault rock thicknesses (≤decimeters) are typically thin compared with the damage-zone widths (≤meters), meaning that the fault zone widths are approximately equal to the damage-zone widths. As the damage-zone width may be difficult to determine by core logging alone (Ishii, 2015), the damage-zone widths in the Wakkanai Formation were assumed to be uniformly 10 m, following Ishii (2015). This is because the locations of flow anomalies detected by FFEC logging exclusively occur within 10 m of fault rocks (Fig. 5), and the damage-zone widths observed through geological mapping at surface outcrops and shaft/tunnel excavation faces are typically on the scale of a few meters or less (Ishii et al., 2010; Tokiwa et al., 2013; Ishii, 2015). The damage-zone width of 10 m is the maximum value of damage-zone widths in the Wakkanai Formation, which is sufficient for confidently detecting non-dilatant fault zones. This width is within the globally reported range of damage-zone widths (i.e., 10−2 to 102 m; Faulkner et al., 2010; Torabi and Berg, 2011).
procedure while using two or more pumping rates (multi-rate FFEC logging) allows the determination of transmissivities and hydraulic heads of individual fractures (Tsang et al., 1990; Doughty et al., 2013). The locations and transmissivities of flow anomalies in boreholes HDB-11, PBV01, and SAB-2 have already been determined (Doughty et al., 2008; Bruines et al., 2013; Doughty et al., 2013; Ishii, 2015). In this study, flow anomalies detected within the fault zones were defined as the highest transmissivities of fractures in the fault zones. The highest transmissivities of fractures in fault zones where no flow anomalies were detected were assumed to be less than the practical detection limit of FFEC logging. Because flow anomalies of 10−10 m2/s or more can be practically detected by the FFEC logging (Doughty et al., 2008, 2013; Bruines et al., 2013; Ishii, 2015), the detection limit was assumed to be 10−10 m2/s. Hydraulic conductivity of intact rocks in the Wakkanai Formation is approximately 10−12 m/s (Ishii et al., 2011). 4. Results and discussion
3.2. FFEC logging
Locations of fault rocks, tensile/hybrid fractures, shear fractures, and disintegrated core zones are shown in Fig. 8, as well as the locations and transmissivities of flow anomalies detected by FFEC logging. Analyses based on these data reveal the following:
In FFEC logging, borehole fluids were first replaced with a fluid of contrasting electric conductivity, and FFEC logging was then repeated while the borehole was pumped. When peaks in the FFEC logs occur, fluids that flow into the boreholes are analyzed to enable calculation of inflow rates and salinity within individual fractures. Conducting the
• In borehole HDB-11, the highest transmissivities of 83% (5/6) of nondilatant fault zones are ≤10−8 m2/s, whereas the highest transmissivities of 89% (33/37) of dilatant fault zones are ≥10−8 m2/s (Fig. 9a); • In borehole PB-V01, the highest transmissivities of 100% (2/2) of nondilatant fault zones are ≤ 10−8 m2/s, whereas the highest
Fig. 9. Frequency of the highest transmissivity of fractures in fault zones of non-dilatant fault zones and dilatant fault zones in boreholes (a) HDB-11 (460–775 m), (b) PB-V01 (237– 506 m), (c) SAB-2 (100–710 m), and (d) in all the boreholes.
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transmissivities of 89% (33/37) of dilatant fault zones are ≥10−8 m2/s (Fig. 9b); • In borehole SAB-2, the highest transmissivities of 93% (26/28) of nondilatant fault zones are ≤10−8 m2/s, whereas the highest transmissivities of 93% (54/58) of dilatant fault zones are ≥10−8 m2/s (Fig. 9c); • In all the boreholes, the highest transmissivities of 97% (35/36) of non-dilatant fault zones are ≤10−8 m2/s, whereas the highest transmissivities of 95% (145/153) of dilatant fault zones are ≥ 10−8 m2/s (Fig. 9d). These results support the above-mentioned hypothesis that the highest potential transmissivity of fractures in fault zones where tensile/hybrid fractures do not develop is likely ≤10−8 m2/s (Fig. 2). Ishii (2015) pointed out that the highest potential transmissivity of fractures in possible fault zones can be predicted from the distribution of the rock mechanical indicator, DI. However, in a given site of interest, rock strength data and initial stress data are not always fully available, particularly during the early stages of investigation. In addition, rock strength and stress state may significantly vary in the same host rocks if lithological heterogeneity is strong, and such a condition requires more abundant data for mapping DI in detail. The core logging method that was applied in this study proved useful for the hydrogeological classification of fault zones into potentially high-permeability fault zones (dilatant fault zones: T ≥ 10−8 m2/s) and low-permeability fault zones (non-dilatant fault zones: T ≤ 10−8 m2/s). The transmissivity of ≤10−8 m2/s almost corresponds to the range of fracture transmissivity distribution (log-normal distribution; logarithmic mean value = −8.99; logarithmic standard deviation = 1.07) given for discrete fractured network models (100 m3 block) of the conservative case in the safety assessment for a generic fractured rock mass by the Japan Nuclear Cycle Development Institute (2000). Although quantitative estimation of fault zone permeability through core logging has been considered difficult, core observations on the failure mode of fractures may increase the efficiency of hydrogeological borehole investigations of fault zones. 5. Conclusions Based on previous hydromechanical studies of fault zones (Ishii, 2015, 2016), it is inferred that the highest potential transmissivity of fractures in fault zones where tensile/hybrid fractures do not develop is likely ≤10−8 m2/s. In order to verify this inference, this study investigated the relationship between the failure mode and the highest transmissivity of fractures in fault zones, using core logging data and FFEC logging data derived from a Neogene siliceous mudstone (the Wakkanai Formation). The data show that in 96% (35/36) of fault zones where tensile/hybrid fractures are not observed, the highest transmissivities are ≤10−8 m2/s, whereas in 95% (145/153) of fault zones where tensile/hybrid fractures occur, the highest transmissivities are ≥10−8 m2/s. This result supports the above-mentioned inference, suggesting that core observations of the failure mode of fractures in fault zones may allow assessment of the highest potential transmissivity of fractures in fault zones in terms of whether it is potentially ≥ 10− 8 m2/s or likely ≤10−8 m2/s. Acknowledgments The author thanks geoengineers of Mitsubishi Materials Techno Corporation for conducting core logging, anonymous reviewers for constructive reviews and helpful comments, and A. Stallard for improving the grammar and for helpful suggestions regarding the manuscript. References Aydin, A., 2000. Fractures, faults, and hydrocarbon entrapment, migration and flow. Mar. Pet. Geol. 17, 797–814.
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