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The relationships among brittleness, deformation behavior, and transport properties in mudstones: An example from the Horonobe Underground Research Laboratory, Japan

2011; American Geophysical Union; Volume: 116; Issue: B9 Linguagem: Inglês

10.1029/2011jb008279

2156-2202

Eiichi Ishii, Hiroyuki Sanada, Hironori Funaki, Yutaka Sugita, Hiroshi Kurikami,

Hydraulic Fracturing and Reservoir Analysis

Journal of Geophysical Research: Solid EarthVolume 116, Issue B9 Chemistry and Physics of Minerals and Rocks/VolcanologyFree Access The relationships among brittleness, deformation behavior, and transport properties in mudstones: An example from the Horonobe Underground Research Laboratory, Japan Eiichi Ishii, Eiichi Ishii [email protected] Horonobe Underground Research Unit, Japan Atomic Energy Agency, Horonobe-cho, Japan Currently at Nuclear Waste Management Organization of Japan, Tokyo, Japan.Search for more papers by this authorHiroyuki Sanada, Hiroyuki Sanada Tono Geoscientific Research Unit, Japan Atomic Energy Agency, Mizunami, JapanSearch for more papers by this authorHironori Funaki, Hironori Funaki Horonobe Underground Research Unit, Japan Atomic Energy Agency, Horonobe-cho, JapanSearch for more papers by this authorYutaka Sugita, Yutaka Sugita Horonobe Underground Research Unit, Japan Atomic Energy Agency, Horonobe-cho, JapanSearch for more papers by this authorHiroshi Kurikami, Hiroshi Kurikami Headquarters of Fukushima Partnership Operations, Tokyo, JapanSearch for more papers by this author Eiichi Ishii, Eiichi Ishii [email protected] Horonobe Underground Research Unit, Japan Atomic Energy Agency, Horonobe-cho, Japan Currently at Nuclear Waste Management Organization of Japan, Tokyo, Japan.Search for more papers by this authorHiroyuki Sanada, Hiroyuki Sanada Tono Geoscientific Research Unit, Japan Atomic Energy Agency, Mizunami, JapanSearch for more papers by this authorHironori Funaki, Hironori Funaki Horonobe Underground Research Unit, Japan Atomic Energy Agency, Horonobe-cho, JapanSearch for more papers by this authorYutaka Sugita, Yutaka Sugita Horonobe Underground Research Unit, Japan Atomic Energy Agency, Horonobe-cho, JapanSearch for more papers by this authorHiroshi Kurikami, Hiroshi Kurikami Headquarters of Fukushima Partnership Operations, Tokyo, JapanSearch for more papers by this author First published: 24 September 2011 https://doi.org/10.1029/2011JB008279Citations: 29AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract [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]. Figure 11Open in figure viewerPowerPoint Intrinsic permeability estimated from packers and laboratory hydraulic tests for the Wakkanai, Koetoi, and Yuchi formations. 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 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 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 σ1 was nearly E–W and σ2 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], σ1 was nearly E–W and σ2 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., σ1 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 (σ2 vertical), preceding the stress conditions favoring reverse faulting, which is amenable to folding and in which σ3 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]. 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 (σ′V, MPa) to the past maximum effective vertical stress (σ′V max), at the maximum depth of burial, as follows: 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 σ′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 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 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