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$Drilling-induced tensile wall-fractures - Stanford University$

Although the Aztec sandstone experienced less than 1% total shortening as the result of CB formation, the low-porosity, low-permeability band fabrics developed have been shown capable of exerting profound fluid flow effects at scales of practical interest. The analysis presented here of CBs as indicators of paleostress and tectonic structure in this exhumed analog aquifer/reservoir also applies in reverse. That is, given knowledge of the stress and material history of a subsurface sandstone, predictions regarding the possible presence and geometry of CBs can be made. 2. Introduction Compaction banding as a recognized mode of localized brittle failure in porous sandstone has a brief history, having first been described and named by Mollema and Antonellini (1996) based on their work in the Navajo sandstone of south central Utah. Compaction bands (CBs) represent one kinematic end member of the suite of structures known collectively as deformation bands, which includes shear and dilation bands (Du Bernard et al., 2002; Borja and Aydin, 2004). Mollema and Antonellini (1996) described CBs as long (meters), thin (centimeters) tabular zones of mechanical porosity loss formed within the compressional quadrants of shear band style faults (Aydin, 1978), interpreting them as “a structural analog for anti-mode I cracks in æolian sandstone...that accommodate pure compaction,” following the conceptual anticrack model for pressure solution surfaces of Fletcher and Pollard (1981). The recognition of CBs, and their obvious potential to act as low-porosity, low- permeability impediments to fluid flow in otherwise highly transmissive sandstones, has excited an ongoing flurry of work both experimental (e.g. Olsson and Holcomb, 2000; Klein et al., 2001; Vajdova and Wong, 2003; Haimson, 2003; Baud et al., 2004; Tembe et al., 2006) and theoretical (e.g. Olsson, 1999; Issen and Rudnicki, 2000, 2001; Detournay et al., 2003; Bésuelle and Rudnicki, 2004; Rudnicki, 2002, 2003, 2004; Katsman et al., 2005; Rudnicki and Sternlof, 2005; Sternlof et al., 2005; Katsman and Aharonov, 2006) all of it aimed at comprehending the mechanics of compaction localization in porous, granular materials. To date, however, the only other field-based study published on CB occurrence and geology also focused on an æolian Jurassic sandstone of the southwestern U.S.—the Navajo-equivalent Aztec sandstone of southeastern Nevada (Sternlof et al., 2005). That paper presents an anticrack-inclusion mechanical model for CBs that is based 10
on detailed outcrop and thin-section observations of the Aztec as extensively exposed in the Valley of Fire State Park (Figure 1.1). Stepping back from the wide variety of patterns visible in individual outcrops, the two most striking aspects of CBs in the Aztec as a whole are their ubiquity and the general consistency of their planar orientation: north-northwest trending, steeply east dipping. Considering the Aztec as an exhumed analog for active sandstone aquifers and reservoirs, the potential for such an anastomosing CB fabric to restrict and channel fluid flow is substantial (Sternlof et al., 2004; Sternlof et al., 2006). The practical goal behind all CB investigations is to develop the capability to predict, or at least forecast their occurrence in the subsurface such that their hydraulic effects can be mitigated. Achieving this goal will require understanding how, in what kinds of materials and under what loading conditions CBs form. As a contribution to this endeavor, we present a broad look at the thin-section to regional scale structural geology of CBs in the Aztec sandstone at the Valley of Fire, and consider their implications for the tectonic framework and history of southern Nevada. 3. Setting and history of study area The following synopsis of the geological setting and history of the Aztec sandstone was synthesized from data and interpretations published in a wide variety of sources over the past 85 years as cited below. Given that the methods and perspectives of structural geology have evolved a great deal since the first research on the area was published during the Wilson administration (Longwell, 1921), long before any modern notion of tectonics, we emphasize here more recent work and interpretations. 3.1. Deposition The Aztec sandstone was deposited during latest Triassic to middle Jurassic time in a back-arc basin associated with uplift of the ancestral Sierra Nevadan magmatic arc (Marzolf, 1986; Burchfiel et al., 1992). It comprises the southwestern edge of a much larger æolian erg system, which includes the Navajo and Nugget sandstones and blanketed much of the intermountain southwest (Blakey, 1989; Marzolf, 1986). Prevailing winds during deposition of the Aztec are interpreted to have blown out of the northeast and over the shallow sea occupying what is now the continental interior (Marzolf, 1982, 1986). 11
Page 1 and 2: STRUCTURAL GEOLOGY, PROPAGATION MEC
Page 4 and 5: Abstract Low-porosity, low-permeabi
Page 6 and 7: delivered with fortitude, humor, su
Page 8 and 9: Chapter 3—Energy-release model of
Page 10 and 11: List of Illustrations Figure A. Cov
Page 12 and 13: Figure A. Cover photo that accompan
Page 14 and 15: likely present in subsurface sandst
Page 16 and 17: hooking-tip interactions—using th
Page 18 and 19: and sandstone, my co-authors—Moha
Page 20 and 21: the hard data from which accurate p
Page 24 and 25: Waterpocket Fault 36 o 26‘ N 0 1
Page 26 and 27: Cenozoic Mesozoic Paleozoic Quatern
Page 28 and 29: 3.3. Deformation The Aztec also has
Page 30 and 31: There also are relatively high-angl
Page 32 and 33: (e) (f) (c) 500µm compaction band
Page 34 and 35: dihedral angle of 80° or more, and
Page 36 and 37: 5. Compaction band orientations Ori
Page 38 and 39: n = 20 M n = 20 P n = 22 B n = 20 R
Page 40 and 41: were encountered, giving the dihedr
Page 42 and 43: Eichhubl et al., 2004) did not lend
Page 44 and 45: P (a) (c) S P S Figure 1.10. Stereo
Page 46 and 47: possible, and use these to better c
Page 48 and 49: 1.5(ρgz) (b) WEST Willow Tank Uppe
Page 50 and 51: 9. Acknowledgements My sincere than
Page 52 and 53: The term compaction band (CB) was c
Page 54 and 55: Pollard, 2002). The particular util
Page 56 and 57: Hue-based image analysis using MATL
Page 58 and 59: a direct genetic relationship (Hill
Page 60 and 61: Compaction band fin Depositional be
Page 62 and 63: suggests—that to first approximat
Page 64 and 65: Porosity Porosity 0.3 0.25 0.2 0.15
Page 66 and 67: pore-clogging clay—due presumably
Page 68 and 69: 500 µm Figure 2.9. Electron backsc
Page 70 and 71: (a) (b) σ 3 σ 1 x 3 x 1 x 1 (c) u
Page 72 and 73:
6. Elastic properties Despite a lon
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Comparison of the two approaches es
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term in (6a) and (6c) begins to dom
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MPa MPa 80 70 60 50 40 30 20 10 0 0
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2002) and use a BEM approach (Crouc
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MPa 10 4 10 3 10 2 10 1 10 −5 10
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concentration of quartz plasticity
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conducted on well-cemented sandston
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~ 62 m compaction band trend 500µm
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Sternlof et al. (2005) have suggest
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2005). It consists of a long (infin
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⎡ ∆ ⎤ ⎧ p 1 ∆ ⎛ M ⎞
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which point the inelastic strain ha
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they can exert significant effects
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interact is inversely proportional
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(a) (b) (c) (d) Figure 4.3. Typical
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Viewed individually, CB traces tend
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(a) (b) (c) (d) (e) (f) Figure 4.7.
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1973; Mardon, 1988; Peck et al., 19
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undamaged host rock. That CBs canno
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0.5 0.4 0.3 0.2 0.1 0.5 0.4 0.3 0.2
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Normalized stress magnitude 3 2.5 2
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helps to explain why the oblique ap
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and ts = G·ds + H·dn (2) where tn
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plastic compaction, as suggested by
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To determine the sensitivity of pro
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segments will self-correct to provi
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6.3. Approaching tip interactions A
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0.2 0.15 0.1 0.05 0 −0.05 −0.1
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0.2 0.15 0.1 0.05 0 −0.05 −0.1
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the hooking patterns commonly obser
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(a) (b) (c) σ1 compaction band max
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σ 3 σ 2 σ 1 Figure 4.24. Schemat
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NV UT CA AZ Park Road Map Detail Pa
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compaction band Figure 5.2. Typical
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3. Computational method The methodo
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compaction band A 5 mm A‘ compact
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4. Application to the Aztec sandsto
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Permeability (mD) 10 4 10 3 10 2 10
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B‘ A‘ A c f m c m c f c f c f c
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equivalent of the Aztec sandstone,
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effective permeability represents a
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NV UT CA AZ Park Road Map Detail Pa
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Commonly from ~1 mm to ~1.5 cm in t
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Although systematic arrays of DBs p
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to 2 m, with both sets in a cross-h
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y identical blocks all subject to t
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contiguous areas. This type of char
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(0,b) n 3 (0,0) f 2 n 2 n 1 (a,0) (
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6.1. Parallel Effective permeabilit
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By contrast, band-parallel effectiv
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Relative Effective Permeability 1.0
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(a) (b) (c) 0 meters 3 = 10-2 kb /k
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Though grossly systematic, anastomo
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168
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100 meters 5 meters NV UT CA AZ Par
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compaction band trend 500µm Figure
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Cover Outcrop Band N 100 meters 20
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(CBs) and the matrix rock in all si
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The finite volume discretization fo
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isotropic) can lead to numerical di
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5. Simulations Three sets of 2-D si
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Normalized ∆P ∆P ratio (n/p) 4.
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P P P P I I P P Case 1 P P Case 2 I
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Case 1 Case 1 Case 1 (a) (b) (c) In
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5.2.2. Discussion The elliptical pa
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5.3.1. Results With the regional gr
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(a) (b) Leak 200 meters Regional gr
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anticipated sustainable pumping rat
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Finally, there is the issue of fore
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200
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Bakke, S., and Øren, P. E., 1997,
Page 216 and 217:
Carpenter, D. G., and Carpenter, J.
Page 218 and 219:
Eichhubl, P., Taylor, W. L., Pollar
Page 220 and 221:
Karimi-Fard, M., Durlofsky, L. J.,
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-, 1987, Fracture from a straight c
Page 224 and 225:
Shipton, Z. K., and Cowie, P. A., 2
Page 226:
Wong, T. F., David, C., and Zhu, W.
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