The relationships among brittleness ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, B09206, doi:10.1029/2011JB008279, 2011

The relationships among brittleness, deformation behavior, and transport properties in mudstones: An example from the Horonobe Underground Research Laboratory, Japan Eiichi Ishii,1,2 Hiroyuki Sanada,3 Hironori Funaki,1 Yutaka Sugita,1 and Hiroshi Kurikami4 Received 3 February 2011; revised 21 June 2011; accepted 6 July 2011; published 24 September 2011.

[1] Mudstones are low‐permeability sedimentary rocks; however, when shear stresses

induced by tectonic movement or nonhydrostatic stresses exceed the shear strength of the rock, brittle or ductile deformation occurs. The nature of this deformation is controlled by the brittleness of the mudstone. If brittle deformation occurs, the resulting dilatant structures may increase the permeability and change the transport properties of the strata. This paper addresses the relationships among brittleness, deformation behavior, and transport properties in mudstones at the Horonobe Underground Research Laboratory, Japan. Geological, mechanical, and hydrogeological data from borehole investigations and laboratory tests were systematically interpreted using a brittleness index (BRI), which is the ratio of the unconfined compressive strength to the effective vertical stress. For mudstones under natural strain rates and low temperatures, ductile deformation occurs when BRI 8, although semibrittle behavior may also occur at the brittle‐ductile boundary. When BRI >8 and faulting is well developed, the mudstone behaves hydrogeologically as a fractured medium at the mesoscopic scale, whereas for BRI 10−14 m2 or >10−7 m/s) are limited to shallower depths in the Wakkanai Formation [Ishii et al., 2010]. 5.2. Laboratory Hydraulic Tests [21] To obtain the permeability of intact rock, hydraulic tests were performed in the laboratory on core samples from the boreholes [Kurikami et al., 2008]. Because the permeability of the intact rock was known to be low, transient pulse tests were selected for laboratory testing. Test specimens were less than 10 cm long. The confining pressure and pore water pressure were controlled to simulate the overburden and hydrostatic pressures, respectively, according to the depth at which the specimen was sampled. The results are in the range of 6 × 10−20 to 6 × 10−17 m2 (5 × 10−13 to 6 × 10−10 m/s) (Figure 11).

6. Analysis and Discussion Figure 3. (a) Close‐up view of a core section from diatomaceous sandy mudstone. A compactional shear band, only 0.1 mm thick, cuts a trace fossil and causes a displacement of about 10 mm. (b) A fault plane on a drill core from the diatomaceous mudstone. The darkish plane indicates the presence of a very thin compactional shear band along the fault plane. Slickensides and slickenlines are also evident. residual shear stress. In the semibrittle type, a nonlinear curve, which indicates strain hardening, is observed after shear stress reaches yield strength. A gentle curve is observed at the moderately defined peak strength, and then a stress drop occurs to a residual shear stress. The ductile type is characterized by a very gentle, nonlinear, stress‐strain curve near the poorly defined peak strength, followed by strain softening without a sharp stress drop. Faults eventually formed in all three types (Figure 10). [19] The pore pressure reached a maximum at peak strength, and then rapidly decreases in the brittle and semibrittle types (Figure 9). In contrast, the ductile response type shows pore pressure continuing to increase with increasing axial strain after peak strength, though the rate of increase is reduced (Figure 9). The rapid decrease in pore pressure in the brittle and semibrittle types indicates strong dilatancy, while the response of the ductile type indicates weak, or nonexistent, dilatancy (Table 2).

6.1. Tectonic History of the Mudstones [22] The timing of the initial formation of faults, compactional shear bands, and joints in the mudstones was analyzed to estimate the rock’s brittleness when deformation occurred. [23] The chronology of fault formation can be estimated from their orientations. The most common orientations of faults are the same in both the Wakkanai and Koetoi formations, nearly perpendicular to bedding planes in the

5. Hydrogeological Data 5.1. In Situ Hydraulic Packer Tests [20] Hydraulic packer tests were performed to determine the permeability of the Wakkanai, Koetoi, and Yuchi formations [Kurikami, 2007; Kurikami et al., 2008; Yabuuchi et al., 2008]. The packer testing included a pulse test, a slug test, and a pumping test (in sequence), followed by a recovery test. The test sequence may vary, but enables the measurement of a wide range of permeability, and is

Figure 4. Thin section photomicrograph across compactional shear bands (plane‐polarized light). (a) Compactional shear band in siliceous mudstone. (b) Compactional shear band in diatomaceous mudstone.

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Figure 5. (a) Exposure of diatomaceous mudstone in an underground excavation. Although a natural compactional shear band is seen in the center of the exposure, the faults associated with it are discontinuous (continuity is poor). (b) Close‐up of the area marked “b” in Figure 5a. The compactional shear band (dark band) is continuous, but the fault is not. The thickness of the compactional shear band not associated with the fault is about 10 mm at its widest, but the shear band not associated with the fault is thinner. (c) Close‐up of the area marked “c” in Figure 5a. The fault plane is darkish, showing the existence of the very thin compactional shear band along the fault plane. Slickenlines, which indicate strike‐slip displacement, are also present. Wakkanai Formation [Funaki et al., 2009]. Because bedding planes have different orientations from borehole to borehole, they were rotated into the horizontal plane to enable an analysis of the fault orientations. This analysis showed that the most common orientation of faults in the boreholes was nearly vertical, with a WNW–ESE to NW–SE strike [Hatanaka et al., 2010; Lim et al., 2010]. Typically, where fracture orientations are widely distributed in folded sedimentary rocks, but concentrated in a specific orientation in unfolded strata, such fractures are likely to have been formed by regional tectonic stress prior to folding [e.g., Lorenz et al., 1991; Silliphant et al., 2002; Mynatt et al., 2009]. Consequently, the characteristic orientation of faults in both the Wakkanai and Koetoi formations strongly suggests they formed as vertical faults just before the area was folded. In addition, because folding was initiated slightly prior to, or at a similar time to maximum burial [Ishii et al., 2008], it is likely that most of the faults in the Wakkanai and Koetoi formations formed close to the time of maximum burial. This hypothesis is consistent with the stress field at the time. When the faults formed around the time of maximum burial, their major orientation was vertical, and their strike ranged from WNW–ESE to NW–SE, oblique to the direction of the regional EW compression [Ishii et al., 2008]. This observation implies that the faults formed under a stress state where s1 was nearly E–W and s2 was nearly vertical; i.e., conditions favorable for strike‐slip displacement. [24] On the basis of previous analyses of striations on the faults [Ishii and Fukushima, 2006; Tokiwa et al., 2009], using the multiple inverse method [Yamaji, 2000], s1 was nearly E–W and s2 was nearly vertical (or N–S), when the bedding planes were horizontal (i.e., at the time of maximum burial). Such a stress state is consistent with the above relationship between the orientations of the faults and the regional E–W compression. While the initial stress state was

favorable for normal faulting (i.e., s1 vertical) at the time of maximum burial, the horizontal compressive stresses would have started to increase in response to the E–W compression related to eastward movement of the Amurian Plate. This may have resulted in a stress state that favors strike‐slip displacement (s2 vertical), preceding the stress conditions favoring reverse faulting, which is amenable to folding and in which s3 is vertical. [25] The numerous compactional shear bands present in the Koetoi Formation are commonly associated with faults (e.g., Figures 3b and 5). Such faults are thought to result from strain softening following strain hardening, according to the model of Schultz and Siddharthan [2005]. Hence, the formation of the compactional shear bands is also thought to be contemporaneous with fault development, near the time of maximum burial. [26] The joints in the Wakkanai Formation are believed to have formed as secondary splay joints propagating from strike‐slip faults associated with fault rocks, accompanying the reactivation and growth of faults, which occurred during or after the uplift and denudation that followed folding [Iwatsuki et al., 2009; Ishii et al., 2010]. 6.2. Brittleness of the Mudstones [27] The traditional index used for assessing the likelihood of brittle deformation is the overconsolidation ratio (OCR) (see reviews by Ingram and Urai [1999] or Nygård et al. [2006]). This is the ratio of the effective vertical stress at present‐day burial depths (s′V, MPa) to the past maximum effective vertical stress (s′V max), at the maximum depth of burial, as follows:

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OCR ¼

′V max ′V

ð1Þ

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Figure 6. Frequency distributions of compactional shear bands not associated with faults, minor faults, major faults, and joints in boreholes. Frequency is expressed as number per 10 m. Also shown is the estimated brittleness index (BRI) at the maximum burial depth and at the present depth. BRI is shown with its associated estimation errors, reflecting the deviation of the unconfined compressive strength (UCS) of the Wakkanai Formation. For BRI at the maximum burial depth, the possible increase in pore pressure is also included in the errors. During burial in which mechanical compression is the only consolidation mechanism, OCR = 1 (i.e., the normal consolidation state). OCR may increase because of decreasing s′V during uplift/denudation (reduced loading) and/or increasing pore pressure. Ductile deformation generally occurs if OCR = 1, whereas brittle deformation tends to occur with increasing OCR. However, even when OCR = 1, brittle deformation may still occur if the rock is strongly indurated

Figure 7. UCS versus maximum burial depth. For the Wakkanai Formation, only UCS not influenced by bedding planes is shown.

Figure 8. Classification of deformation type based on the stress‐strain curve obtained from the Horonobe mudstones.

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Table 2. Definitions of the Brittle, Semibrittle, and Ductile Types Observed in the Undrained Triaxial Tests in This Study According to Dehandschutter et al. [2004, 2005] and Nygård et al. [2006] Brittle Peak strength Strain hardening Strain softening Stress drop at failure Dilatancy

Semibrittle

Ductile

Strongly defined Moderately defined Poorly defined No Yes Yes Yes Yes Yes Strong Strong Weak or absent Strong

Strong

Weak or absent

or cemented by mineral diagenesis [Ingram and Urai, 1999]. Thus, OCR is not always a reliable indicator of brittleness. [28] Ingram and Urai [1999] proposed a brittleness index (BRI) derived by dividing the unconfined compressive strength (UCS) by the unconfined compressive strength of a normally consolidated rock in nonoverpressurized domains (UCSNC; MPa), expressed as follows: BRI ¼

UCS UCSNC

ð2Þ

BRI is useful because it can express both the effects of mechanical compaction due to loading, and the effects of induration/cementation due to mineral diagenesis. However, few case studies have used BRI, and even fewer systematic studies that characterize the hydromechancial behavior of mudstones. [29] The BRI concept is applicable to all mudstones, including siliceous mudstones indurated/cemented by silica diagenesis. Here, we consider BRI at the maximum burial depth, where most of the faults and compactional shear bands are likely to have formed, and then consider it at the present‐day depth. 6.2.1. BRI at the Maximum Burial Depth [30] BRI at the maximum burial depth can be calculated from UCSNC at the maximum burial depth and present UCS using equation (2). In this section, we calculate UCSNC and the representative values of UCS needed for calculating a reliable BRI for the mudstones. [31] The value of UCSNC can be estimated from s′V (equation (3)), if no further data on UCSNC are available [Ingram and Urai, 1999]: UCSNC ¼ 0:5′V

ð3Þ

Biot‐Terzaghi’s effective stress principle can be used to determine s′V [Biot, 1941]: ′V ¼ V PP

ð4Þ

where sV is the total vertical stress (MPa), a is the Biot poroelastic constant, and Pp is pore pressure (MPa). The total vertical stress sV is calculated as follows: Z V ¼

0

z

ðzÞgdz

ð5Þ

where r(z) is the density (g/cm3) as a function of depth (km), and g is gravitational acceleration.

Figure 9. Stress‐strain curves and pore pressures of the brittle, semibrittle, and ductile types observed in undrained triaxial tests of siliceous and diatomaceous mudstones. The shear stresses are normalized by the past (apparent) maximum effective vertical stress.

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s′V max (MPa), and the maximum burial depth, Zmax (km), is as follows: ′V max ¼ 8:6Zmax

ð6Þ

UCSNC at the maximum burial depth is as follows: UCSNC ¼ 4:3Zmax

Figure 10. Photographs of deformed samples after triaxial loading tests. (a) Diatomaceous mudstone from 150 m depth in HDB‐6. (b, c) Diatomaceous mudstone from 250 m depth in HDB‐6. Strain rates of Figures 10a, 10b, and 10c are 0.002%, 0.002%, and 1% per minute, respectively. Effective confining pressures of Figures 10a, 10b, and 10c are 0.5, 0.5, and 2.0 times s′V at present‐day depths, respectively. [32] On the basis of the results of laboratory tests and density logging in boreholes, the densities of the Wakkanai, Koetoi, Yuchi, and Sarabetsu formations are uniform, at approximately 1.9 g/cm3 [Funaki et al., 2005; Niunoya and Matsui, 2005, 2007; Sanada et al., 2009]. The Biot constants for the Wakkanai and Koetoi formations are approximately 1, as determined experimentally [Miyazawa et al., 2011]. In addition, although the actual constant for the Yuchi Formation is not available, it is assumed to be 1, as this is usual for mudstones [e.g., Wang, 2000]. According to the above parameterizations and equations (3)–(5), when pore pressure is assumed to be hydrostatic, the relationship between the past maximum effective vertical stress,

ð7Þ

[33] Moreover, pore pressure at the maximum burial depth may have increased because of tectonic compression (in addition to dehydration associated with burial diagenesis). However, such an increase in pore pressure is difficult to estimate accurately. In this study, when the stress state changed from that favoring normal faulting to that favoring strike slip, in response to tectonic compression near the time of maximum burial, it is assumed that only the maximum horizontal effective stress (s′H) increased and that the minimum horizontal effective stress (s′h) decreased under the influence of pore pressure increase, according to the classical model of Lorenz et al. [1991]. In addition, the increase of pore pressure (DPp) is assumed to be below s′h, as determined from s′Vmax in equation (6) and the following equation (8), because pore pressure does not usually exceed s′h. DPp < ′h ¼

′V max 1

ð8Þ

where u is Poisson’s ratio and s′V max is the principal stress at the time of maximum burial. assuming hydrostatic conditions. The Poisson ratios of the Wakkanai and Koetoi formations were calculated to be 0.2, based on the results of laboratory tests [Niunoya and Matsui, 2005, 2007]. However, such a pore pressure increase would dissipate once brittle deformation occurred, as shown in Figure 9. [34] Figure 7 shows the present UCS profile of the Koetoi and Yuchi formations, and the UCSNC profile determined using equation (7) versus the maximum burial depth. The

Figure 11. Intrinsic permeability estimated from packers and laboratory hydraulic tests for the Wakkanai, Koetoi, and Yuchi formations. 9 of 15

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Table 3. Interpretation of the Relationship Between the Geological Structures Observed in Core Logs and the Rheological Definitions in Table 2 Geological Structure

Implications for Rheology

Compactional shear band Fault

Strain hardening, no dilatancy Strain softening, weak to strong dilatancy No strain hardening, strong dilatancy

Joint

present UCS profiles of both formations show little scatter and fit the UCSNC profile well (Figure 7). Consequently, the UCS of both formations at the maximum burial depth can be closely approximated by UCSNC as calculated in equation (7). [35] The profile of UCS at maximum burial depth (Figure 7) indicates that the Wakkanai Formation can be divided into three depth zones: 1000–1200 m (UCS ≤ 30 MPa), 1200– 1400 m (UCS ≤ 15 MPa), and 1400–1700 m (UCS ≤ 20 MPa). Mean UCS values (±1s) for each zone are 22.4 ± 5.4, 13.6 ± 2.6, and 17.0 ± 3.7 MPa, respectively. [36] Using the parameters derived above, BRI at the maximum burial depth was calculated, taking into consideration estimation errors related to the deviation of UCS of the Wakkanai Formation, and the possible increase of pore pressure in response to tectonic compression near the time of maximum burial (Figure 6). 6.2.2. BRI at the Present‐Day Burial Depth [37] The present‐day pore pressure in the formations is almost hydrostatic [Kurikami, 2007; Funaki et al., 2009]. Figure 6 shows BRI at the present‐day depth, calculated using equations (2)–(5) and considering estimation errors related to the deviation of UCS in the Wakkanai Formation. 6.3. Relationship Between Deformation Behavior and BRI 6.3.1. Core Logging Data [38] This section considers the relationship between structural data and BRI. Compactional shear bands not associated with faults are strong indicators of ductile deformation, because their compactional/cataclastic fabric (Figure 4) and lack of associated faults indicates strain hardening and no dilatancy (Tables 2 and 3). Core data indicate that compactional shear bands not associated with faults occur frequently in the Koetoi Formation, but rarely in the Wakkanai Formation. Furthermore, BRI at the time of formation of the compactional shear bands was less than 1.3 for the Koetoi Formation, and 1.8–8.8 for the Wakkanai Formation, taking into consideration estimation errors (Figure 6). This finding strongly suggests that ductile deformation occurred in the Koetoi Formation at BRI 8, but are only weakly developed when BRI is 1.8–8 (allowing for estimation errors) (Figure 6). Although the age of all of the joints is not known exactly, the BRI at present‐day depths indicates the maximum possible BRI at the time of formation, since the current BRI represents the absolute maximum BRI of the rock. Therefore, it is highly likely that brittle deformation always occurred when BRI >8, but that it became possible once BRI exceeded 1.8. [41] In summary, the core logs suggest that (1) when BRI 8, brittle deformation occurs, although semibrittle deformation may also occur near the boundary between the brittle and the ductile regimes. 6.3.2. Experimental Data [42] In this subsection, we consider the relationship between the mechanical data and the BRI, integrating the logging and experimental data with the concept of BRI and the tectonic history. [43] Figure 12 shows the shear strength and the effective mean stress at the peak strength, in the undrained triaxial tests. The stresses are normalized (divided) by the (apparent) s′V max so that the results achieved under the different stress conditions can be more easily compared with each other. Here, the shear stress, t, and the effective mean stress, s′m, are defined as follows: ¼

1 3 1 þ 3 ; ′m ¼ 2 2

ð9Þ

In addition, the normalized shear stress, t, and the normalized effective mean stress, s′m, are expressed by the following equations derived from equation (3): ðnormalizedÞ ¼

′m ; ′m ðnormalizedÞ ¼ 2UCS 2UCS

ð10Þ

As shown in Figure 12, in the Koetoi Formation, ductile, semibrittle, and brittle behavior were observed at normalized s′m values of >0.6, 0.3–0.7, and 2. 6.3.3. Deformation Response of the Horonobe Mudstones [46] Integrating the above information, we propose the following deformation responses for the mudstones under naturally occurring strain rates, and at low temperatures: (1) ductile deformation occurs at approximately BRI 8, although semibrittle behavior may also occur near the brittle–ductile boundary. [47] This model of deformation response agrees with Ingram and Urai [1999], who found that when BRI >2, the likelihood of brittle deformation increases with increasing BRI, but when BRI 2, it is predicted that mudstones will behave hydrogeologically as porous media at BRI 2, mudstones may begin to progressively exhibit the transport properties of fractured media with the increasing development of fractures due to brittle deformation. [49] One indicator of whether a rock mass is a porous medium or a fractured medium is the ratio of the intrinsic permeability of the fractures to the intrinsic permeability of the matrices, Ki‐fracture/Ki‐matrix (Ki: intrinsic permeability; m2) [Taylor et al., 1999]. However, the ratio of intrinsic permeability (Ki‐packer), as determined from the packer tests, to intrinsic permeability (Ki‐lab), as determined from laboratory tests, may be more applicable than the Ki‐fracture/Ki‐matrix ratio, even though Ki‐packer/Ki‐lab is usually smaller than Ki‐fracture/Ki‐matrix, because Ki‐packer may be influenced to some extent by the intrinsic permeability of the rock mass bounding the fractures. Ki‐packer/Ki‐lab is usually >1, but may be 8 (Figure 6), and (2) packer sections with a log Ki‐packer/Ki‐lab >3 are restricted to BRI >8 (Figure 13). [56] In summary, for the mudstones at the Horonobe URL site, we propose that if BRI exceeds about eight, splay joints develop pervasively in association with faulting, which facilitates fault linkages and results in larger faults and increased permeability [Ishii et al., 2010]. Under such conditions, these heavily faulted mudstones can behave hydrogeologically as a fractured medium. In contrast, if BRI is less than about eight, splay joints do not develop pervasively in association with faulting, and because fault linkages are not facilitated, permeability does not increase [Ishii et al., 2010], meaning that the mudstones still behave hydrogeologically as a porous medium.

7. Conclusions [57] Geological, mechanical, and hydrogeological data from borehole investigations and laboratory tests on mudstones at the Horonobe URL site were systematically interpreted using the BRI concept, where BRI is the ratio of UCS to s′V. Under naturally occurring strain rates and at

low temperatures, for BRI 8, brittle deformation dominates, although semibrittle behavior may occur near the boundary between the brittle and ductile regimes (Table 4). Furthermore, when BRI >8 the mudstones may behave hydrogeologically as fractured media, when faults associated with fault rocks develop, whereas if BRI