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The effect of fault slip on permeability and permeability anisotropy in quartz gouge
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Tectonophysics 295 (1998) 41–52
The effect of fault slip on permeability and permeability anisotropy in quartz gouge Shuqing Zhang 1 , Terry E. Tullis * Dept. Geological Sciences, Brown University, Providence, RI 02912, USA Received 12 November 1996; accepted 28 May 1998
Abstract The effects of fault slip on permeability and permeability anisotropy have been investigated during large displacement shearing of quartz bare surfaces and 1 mm thick artificial quartz gouge at a normal stress of 25 MPa. Within the first 10 mm of shear displacement, grain size reduction occurred nearly uniformly within the fault zone. Permeability decreased by 2–3 orders of magnitude and the permeabilities perpendicular and parallel to the fault plane remained approximately equal. With further increasing shear displacement up to 200 mm, deformation was localized within a band of Y shears where strong grain comminution occurred. Permeability decreased further by 1–2 orders of magnitude and the permeability perpendicular to the fault showed larger reduction than parallel to it. A permeability anisotropy of about one order of magnitude developed. The permeability anisotropy is a result of heterogeneous deformation within the fault zone; flow perpendicular to the fault zone is impeded by the fine-grained band of Y shears, but flow parallel to the fault zone can occur in the less-deformed adjacent gouge. Our results suggest that fluid flow in natural faults depends on the degree of shear localization and the permeability contrast between the localized zone and the rest of the fault zone. 1998 Elsevier Science B.V. All rights reserved. Keywords: fault slip; permeability; permeability anisotropy
1. Introduction Pore fluid pressure in fault zones and its variation during earthquake cycles can have profound effects on fault strength and fault stability (Sibson et al., 1988; Byerlee, 1990, 1993; Blanpied et al., 1992; Rice, 1992; Sibson, 1992; Segall and Rice, 1995). Fluid pressure and fluid flow in fault zones depend on permeability and permeability anisotropy which can be significantly modified by fault slip. Ł Corresponding
author. E-mail: [email protected] Present address: Research School of Earth Sciences, The Australian National University, Canberra, Australia. 1
During fault slip mechanical shearing may produce drastic changes in the texture of fault gouge materials (Mandl et al., 1977; Beeler et al., 1996). First, substantial porosity reduction (compaction) is usually associated with fault slip due to rearrangement of packing among particles and to grain size reduction (Biegel et al., 1989; Marone and Scholz, 1989; Morrow and Byerlee, 1989; Marone et al., 1990). The nature and extent of porosity reduction depend on mineral composition, initial particle size distribution and physical conditions such as pressure, slip velocity, and displacement. It has been shown that the compaction during shearing is much more efficient than during compressive loading and
0040-1951/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 1 1 4 - 0
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is particularly effective in granular materials such as quartz-feldspathic aggregates (Mandl et al., 1977; Biegel et al., 1989; Yund et al., 1990; Wong and Shah, 1995). Naturally, one would expect a large impact of shear-enhanced compaction on fluid permeability in quartz-feldspathic fault zones (Zhu and Wong, 1997). Second, mechanical shearing can produce particle shape fabrics and discrete shear fractures such as R shears, P shears and Y shears (Mandl et al., 1977; Logan et al., 1992). The shear fractures represent localized deformation bands within the fault. Severe comminution usually occurs along shear fractures, in particular R1 shears and Y shears (Marone and Scholz, 1989; Beeler et al., 1996). Because of the preferred grain comminution in shear fractures, porosity structure and therefore, fluid permeability may become anisotropic within the fault. Laboratory frictional sliding experiments also show that the initiation and growth of shear fractures are largely a function of cumulative slip displacement (Mandl et al., 1977; Logan et al., 1992; Gu and Wong, 1994; Scruggs and Tullis, 1995). As a result, the permeability anisotropy may depend strongly on shear displacement. Despite the significant progress in identifying the effects of fault slip on the texture of fault gouge, the correlations between shear textures and fluid permeability and permeability anisotropy remain poorly understood. The few available measurements of permeability and permeability anisotropy during fault slip were focused on clay minerals (Morrow et al., 1984; Arch and Maltman, 1990; Brown and Moore, 1993). In the present study, we have directly measured the fluid permeability both perpendicular and parallel to the fault zone during large displacement (up to 200 mm) shearing of a quartz sandstone bare surface and an artificial quartz gouge. Our experiments show a strong correlation between shear localization along Y shears and the development of permeability anisotropy.
2. Materials and experimental techniques We have conducted experiments on (1) bare surfaces of Fontainebleau sandstone and (2) an artificial quartz gouge. The Fontainebleau sandstone used in this study is ultra pure (99% quartz) and has an
average grain size of about 125 µm. It has an intergranular porosity of about 7% and a permeability of about 10 13.5 m2 . Synthetic quartz gouge of grain size <90 µm was prepared by mechanically crushing hand-picked silica sand (ASTM, US Silica Co.) and by passing the fragments through a 170 mesh sieve. Sliding experiments were conducted at room temperature and a normal stress of 25 MPa using a direct rotary shear apparatus (Tullis and Weeks, 1986; Beeler et al., 1996). The normal stress is made up of 21 MPa effective confining pressure (confining pressure minus pore fluid pressure) and 4 MPa axial stress. Doubly deionized water was used as pore fluid and pore fluid pressure ranged from 1 to 4 MPa. During sliding, we alternated remote loading velocity by 10 fold steps within the range of 0.1 to 10 µm=s. Shear displacement up to 200 mm was achieved in our experiments. Fig. 1 shows a simplified drawing of our sample assembly. Shearing occurs either along the contacting asperities of the upper and lower forcing block rings in bare surface experiments or within a layer of artificial gouge initially 1 mm thick placed between the upper and lower forcing block rings. After pressurization, the gouge layer thickness is about 0.85 mm. The outer diameter of the sliding surface is 53.98 mm, the inner diameter is 44.45 mm and the area of contact is 735 mm2 . The upper sample grip is held stationary while the lower sample grip rotates. A full revolution of the lower sample rotation corresponds to 153.6 mm of slip. Axial load and torque were applied to the specimen independently by a hydraulic servo system and a high resolution electrohydraulic rotary servo system, respectively. Axial load and torque were measured with a combined load and torque cell located within the pressure vessel. Shear displacement was determined by a high resolution resolver whose body is mounted on the upper sample grip and whose rotating shaft is attached to the lower sample grip. The axial length change of the specimen was measured by an internal LVDT. Each of the sample grips (upper and lower, see Fig. 1) is connected with a pressure transducer and a volumometer. A volumometer is composed of a cylinder and piston, driven by a hydraulic actuator and an electronic control unit. The piston position is measured with a LVDT attached to the shaft of the
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Fig. 1. An illustration of half of the specimen assembly for rotary shear deformation. An experimental fault with or without artificial gouge lies between the upper and lower forcing block rings. The pore fluid pressure system is separated from the confining pressure system by two teflon rings and two O-rings outside and inside of the forcing block rings. Fluid permeability perpendicular and parallel to the fault is measured with permeable and impermeable forcing blocks respectively. The flow direction (arrows), area (A), path length (L), and inlet and outlet pore pressures (Pf1 and Pf2 ) for permeability measurements are shown in (a) and (b).
volumometer piston. A full stroke of the volumometer delivers about 32 cm3 of fluid. Pore fluid pressure or piston position can be servo-controlled independently upstream and downstream of the specimen. When pore fluid pressures upstream and downstream of the specimen are servo-controlled at different values, the rate of fluid flow through the specimen is measured by monitoring the changing piston positions of the volumometers. A constant flow rate across the specimen can be produced when one volumometer is on pore pressure control and the other volumometer servo-controls on the computer-generated command signals for piston positions. We measured permeability anisotropy during shearing of the artificial quartz gouge. Fluid permeabilities perpendicular and parallel to the experimental fault were determined in two separate experiments in which permeable Fontainebleau sandstone and impermeable silica glass were used as forcing block rings, respectively. When permeable sandstone was used as forcing block rings, water flowed first to the bottom surface of the lower forcing block through a circular channel of about 1 mm deep and 1 mm wide on the steel sample grip. Fluid was then driven upward first through the lower forcing block ring uniformly, then across the artificial gouge layer,
and finally through the upper forcing block ring and the outlet port (Fig. 1). By applying either constant fluid flow rates ranging from 10 5 to 10 2 cm3 =s or constant pressure differences of 0.1–2 MPa between fluid inlet and outlet port, the fluid permeability for the whole system was determined using Darcy’s law. Because most parts of the forcing block rings are not subject to confining pressure and the deformation of the forcing block is negligible during shearing of quartz gouge, we measured the permeability for the forcing block rings at ambient conditions. By assuming that the two forcing block rings and the gouge layer are connected in series, the perpendicular permeability of the gouge layer .k? / was derived from the permeability measured for the whole system .ke / and that for the forcing block rings .kc / by: ke D
.2`c C `? /kc k? 2`c k? C `? kc
(1)
where `c is the height of each of the forcing block rings and `? is the gouge thickness. Fluid permeability parallel to the fault was measured by using impermeable silica glass as forcing block rings and by controlling fluid flowing in and out (Fig. 1). In one experiment (#158qz), we first introduced fluid to the bottom of the gouge layer
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through a hole of about 1.2 mm in diameter in the lower forcing block. Fluid was then forced to flow along the gouge layer and finally out through a hole in the upper forcing block ring. In another experiment (#177qz), to check reproducibility, we used permeable ceramics slots of 2.5 mm width for fluid inlet and outlet. To ensure that fluid flows through the whole gouge layer and to minimize the effect of the permeability perpendicular to the fault, the time for fluid flowing across the gouge layer needs to be significantly shorter than along the gouge layer. We have found that for our specimen assembly using a 1 mm thick layer the two holes or two slots need to be at least 10 mm apart if the permeability across the gouge layer is about one order of magnitude lower than along the gouge layer. In other words, the flow path length along the gouge layer needs to be longer than 10 mm. This requirement can be easily met by our rotary shear apparatus. The data reduction takes into account the fact that, except for the special case where the two holes=two slots are 180 degrees apart, fluid flow takes place along two paths of unequal length. This is done by assuming that the two flow paths are connected in parallel. The fluid permeability parallel to the gouge layer .kk / is given by: q¼ kk D (2) 1 1 A∆P C `a `b Where q is the measured flow rate, ¼ is the dynamic viscosity, A is the cross-sectional area (see Fig. 1), ∆P is the pressure difference between fluid inlet and outlet, and `a and `b are the lengths of the two flow paths. Fluid permeability was measured intermittently during slide-holds. Because of the long flow path, each permeability measurement typically took 5–24 hours.
3. Results Changes in specimen axial length, porosity , and permeability k with increasing shear displacement for three experiments are shown in Fig. 2. Fig. 2a shows that the specimen axial length in the three runs decreases sharply within the first 10 mm shear displacement and then levels off with further increasing shear displacement. Good reproducibility
of gouge thickness change is observed for the two experiments on artificial quartz gouge using different forcing blocks. The specimen axial length change in the bare surface experiment reflects first, the crushing of contacting asperities and generation of wear products and second, compaction of the wear products (Fig. 3). Thus, the observed decrease in specimen axial length change at large displacement primarily indicates slowing down of gouge production. In the experiments on artificial gouge, the specimen axial length change is a measure of gouge compaction for the whole gouge layer. Therefore, relatively smaller amount of gouge layer thickness change at large displacements indicates (1) an overall decrease of grain size reduction in the gouge layer, or more likely, (2) localization of gouge compaction to a small layerparallel deformation band within the gouge layer. Evolution of total porosity with shear displacement is shown in Fig. 2b. For the experiment on bare surfaces, the total porosity within the deformation band surrounding the contacting asperities was calculated retrospectively from final shear texture and specimen axial length change. We assumed that wearing at the onset of shearing occurred within a 150 µm thick zone (deformation band) on the basis of two observations: (1) the final wear product was about 50 µm thick and porosity within it was about 4–5% from SEM analysis; and (2) the accumulated specimen axial length change was about 100 µm (Fig. 2a). The porosity within the 150 µm thick zone was about 70% before shear deformation started (Fig. 3a). This analysis is supported by our surface roughness measurement. In our experiments, the forcing block surfaces were roughened with 120 mesh silica carbide powder before shear deformation. We found that the average asperity height on the roughened surface is about 75 µm. Thus, shear deformation on bare surfaces basically crushed all asperities of the two contacting surfaces. For experiments on artificial gouge, total porosity was calculated from the starting porosity before shear deformation and the gouge thickness changes during shear deformation as shown in Fig. 2a. The starting porosity was calculated from gouge weight and gouge layer thickness before pressurization and the gouge thickness change during pressurization. For our thin layer ring geometry, gouge thickness change is approximately equal to porosity change. Again, the calculated porosity is an average for the whole gouge
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Fig. 2. Experimental results. (a) Primary data of specimen axial length change with increasing shear displacement. (b) Porosity data derived from specimen axial length change during deformation and the initial porosity and the initial specimen thickness before shear deformation. (c) Primary data of fluid permeability. Note good reproducibility between experiment 158qz and experiment 177qz for fluid flow parallel to the fault. In addition, the dashed line .k2 / and the solid line .k1 / are the permeabilities calculated respectively for the strongly comminuted area of Y shears and the relatively less deformed gouge region within a sheared, artificial quartz gouge layer. The calculation is discussed in the text (see Fig. 3 and Eqs. 7–9). (d) Permeability as a unique function of porosity: k / Þ . Þ should be equal to 3 if pore aperture does not change with porosity.
layer although porosity may decrease substantially more in localized bands as revealed by SEM observation. Corresponding to the large porosity reduction at the beginning of shearing, permeability decreased drastically (Fig. 2c). Calculation of permeability takes into account the changing of the layer thickness shown in Fig. 2a. Fluid permeability is a strong function of cumulative slip displacement. Sliding velocity jumps within the range of 0.1 to 10 µm=s have little effect on the fluid permeability because the
transient dilatation associated with velocity jumps is small and is rapidly overcome by the overall porosity reduction with increasing displacement. Within the first 10 mm of shear displacement, the perpendicular permeability within the wear product of sandstone bare surfaces decreased by about three orders of magnitude. Over the same range of shear displacement, permeability decreased by about two orders of magnitude within the artificial gouge layer. Moreover, fluid permeabilities perpendicular and parallel to the fault remain virtually the same. As shear
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Fig. 3. Schematic diagrams illustrating the nature of porosity and permeability reductions during shear deformation of sandstone bare surfaces (a) and an artificial quartz gouge (b). `1 , k1 and `2 , k2 in (b) are the thickness and the permeability for the less deformed region and the band of Y shears, respectively.
displacement increased up to about 200 mm, permeability decreased further by another one to two orders of magnitude in the three experiments. More importantly, permeability anisotropy developed in the artificial gouge layer, with permeability parallel to the fault being higher than normal to it by about one order of magnitude. The relationship between permeability and porosity is shown in Fig. 2d. If we assume a commonly used relationship between k and : k / Þ , we find that the exponent (Þ) decreases from about 9 at high porosities to about 2–3 at low porosities. Shear textures were examined in thin sections using optical microscopy and in fracture surfaces using SEM. A layer of fine wear product of about 50 µm thickness was present between the upper and lower forcing blocks in the bare surface experiment (Fig. 4a). Many Y shears which are parallel to the artificial fault plane are discernible. Under SEM, the wear product is composed of round aggregates of 1–2 µm in diameter which were sintered together (Fig. 4b). The microstructure of artificial quartz gouge after pressurization is shown in Fig. 4c. Particles are angular and shows some alignment of
particle long-axes parallel to the fault plane. After about 8 mm of shear displacement, most particles are more equant in shape (Fig. 4d). Small fragments, produced possibly by spalling of large particles increase significantly. Numerous R1 shears which intersect at 20–25º to the fault boundaries are present (not shown in Fig. 4). In specimens sheared more than 10–15 mm, the microstructure is characterized by a band of fault boundary parallel Y shears within which extensive comminution occurred (Fig. 4e). The average band thickness is about 115 µm and varies slightly from one location to another. The microstructure outside of the shear band is roughly similar to that seen at short shear displacements, but it is difficult to identify individual R1 shears.
4. Discussion and conclusions 4.1. Importance of shear-induced grain crushing for permeability reduction Mechanical shearing of quartz gouge shows two prominent features: first, the permeability decreased
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Fig. 4. Microstructures. (a) Gouge layer produced by shear deformation of Fonetainebleau sandstone bare surface (Experiment 151fs). Note that the gouge layer was opened up during depressurization. Numerous Y shears (horizontal lineations) are present. (b) SEM image of the sliding surface of experiment 151fs. (c) Texture of an artificial quartz gouge after being compacted at 21 MPa confining pressure and about 4 MPa axial stress. The axial stress direction is vertical. (d) Texture of an artificial quartz gouge sheared to 8 mm displacement at a normal stress of 25 MPa. Shear plane is horizontal and shear sense is sinistral. (e) Texture of an artificial quartz gouge sheared to 170 mm displacement at a normal stress of 25 MPa. Shear plane is horizontal and shear sense is sinistral. Note that particle size is much finer within the band of Y shears (lower 45% of the image) than within the rest of the gouge layer (upper 55% of the image).
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drastically with initial shearing (Fig. 2c). Similar sharp decreases in permeability were previously found during triaxial deformation of unconsolidated sand and porous sandstones (Zoback and Byerlee, 1976; Zhu and Wong, 1997; Zhu et al., 1998) and during frictional sliding along pre-fractured surfaces of Coconino sandstone (Teufel, 1987). Second, the slopes in the log k vs. log plot (Fig. 2d) are higher than 3. This feature was also observed by David et al. (1994) during pressurization of sandstones beyond the critical pressures at which grain crushing occurred. These two features point to the importance of grain size reduction for decreasing permeability and they can be interpreted using the equivalent channel or Kozeny–Carman model (Paterson, 1983; Walsh and Brace, 1984; Connolly and Thompson, 1989; Sumita et al., 1996). From the equivalent channel model, permeability .k/ can be expressed in the form (Paterson, 1983): k D C1 R 2 x
(3)
where C1 is a numerical factor depending on pore shape, and R is the hydraulic radius defined as the ratio of the pore volume to the solid–fluid interfacial area. The factor C1 can be calculated for channels of simple uniform cross-sections assuming the Navier– Stokes flow equations with non-slip boundary conditions, giving C1 D 12 for circular cross-section, 35 for equilateral triangular cross-section, and 13 for a slot (Paterson, 1983). For an aggregate of uniform spheres, R D r=6.1 /, where r is the grain size (Paterson, 1983). Assuming an equilateral triangular pore geometry, Eq. 3 can be rewritten as k³
nr 2 60.1 /2
or d.log r / d.log k/ D Þ D nC2 d.log / d.log /
(4) ½ d.log.1 // 2 d.log / (5)
Thus, a plot of log k vs. log will give a true porosity exponent n when grain size r does not change with porosity and when porosity is low so that .1 /2 is close to 1. In the cases where pore aperture and connectivity do not change significantly and where pore aperture can be independently assessed, the true porosity exponent n has been found to be about 3
(Bernabe´ et al., 1982; Walsh and Brace, 1984; Wong et al., 1984; Bourbie´ and Zinzsner, 1985; Zhang et al., 1994; Nakashima, 1995). The term in the square brackets of Eq. 5 is always positive and changes from about 2 at D 0:5 to about 0.1 at D 0:1. This term will reduce the nominal value of porosity exponent Þ, particularly at high porosities. If pore aperture decreases with decreasing porosity, d.log r /=d.log / will be positive, which will increase the value of Þ. Therefore, a value of Þ higher than 3 in a log k vs. log plot is a strong indication of grain size reduction if the connectivity of the pore network is maintained. Calculations using Eq. 4 shows that the model particle size for the whole layer of artificial quartz gouge would have been reduced by a factor of 10 after 170 mm shear displacement. This is an average value based on bulk measurements of k and . Because of localization of shear deformation at large displacements, grain size reduction should be more substantial within the band of Y shears. In modelling crustal pore pressure development, Walder and Nur (1984) assumed a relationship between k and : Þ cÞ (6) k D k0 0Þ cÞ where k0 and 0 are the initial permeability and initial porosity, and c is the threshold porosity for throughflow. As demonstrated by David et al. (1994), higher values of Þ will favor the development of fluid overpressure in fault zones. It has been found that during time-dependent compaction, the value of Þ is typically about 3 at porosities higher than 7–10% and it increases to above 7 at lower porosities due to a loss of connectivity among pores (Bernabe´ et al., 1982; Bourbie´ and Zinzsner, 1985; Zhang et al., 1994). Undoubtedly, in natural fault zones shearenhanced compaction will couple with time-dependent compaction in very complicated ways. Simply judging from the Þ values of 9 associated with comminution, we suggest that mechanical shearing would significantly enhance the generation of fluid overpressures within porous fault zones. 4.2. Effect of shear localization on permeability anisotropy Our study shows that the development of permeability anisotropy in quartz gouge is associated with
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localization of shear deformation along a band of Y shears at shear displacements larger than 15 mm. The band of Y shears tends to be near the lower gouge–rock interface and is continuous along the sliding direction although its thickness could vary slightly from one location to another. R1 shears are prominent during the early stages of deformation (<10–15 mm) but give way to Y shears at larger displacements. Also, R1 shears are never observed to cut the band of Y shears. Therefore, R1 shears appear to have little effect on permeability anisotropy in quartz gouge. The band of Y shears represents a less permeable layer as indicated by its much higher degree of comminution than other parts of the fault zone. For fluid flow perpendicular to the fault zone, the same fluid flux has to flow through both the finely comminuted, low permeability band of Y shears and the relatively less deformed layer. In contrast, for fluid flow parallel to the fault zone, a higher fluid flux goes through the less deformed layer. If we assume a two layer structure within the fault zone (Fig. 3), layer one of thickness `1 and permeability k1 , and layer two of thickness `2 and permeability k2 , the effective fluid permeability of the fault zone can be treated as the two layers in series and parallel, respectively, when fluid flows perpendicular and parallel to the fault zone. The effective permeability normal and parallel to the fault zone is given, respectively, by: .`1 C `2 /k1 k2 (7) k? D `1 k2 C `2 k1 k1 `1 C k2 `2 (8) `1 C `2 If we take `2 =`1 D ½ and k2 =k1 D m, then the permeability anisotropy is given by: .1 C ½m/.½ C m/ kk D (9) k? m.1 C ½/2 Permeability anisotropies as a function of ½ and m are shown in Fig. 5. When the band of Y shears makes up less than about 1=6 of the fault zone .½ < 0:2/, the permeability anisotropy increases rapidly with increasing ½ and m; and when the width of the band of Y shears is larger than 1=6 of the fault zone, the permeability anisotropy depends largely on m only. In other words, propagation or growth of Y shears due to shear band hardening at large shear kk D
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Fig. 5. Permeability as a function of ½ and m. ½ is the thickness ratio of strongly deformed layer with less deformed layer. m is the permeability contrast between strongly deformed layer and less deformed layer.
displacements will have little effect on permeability anisotropy. In our sheared quartz gouge, ½ is about 1=5 based on microstructural observations. From Eq. 9, we found that if the permeability within the band of Y shears is lower than the rest of the layer by a factor of about 70, the observed anisotropy of about one order of magnitude can be accounted for. Based on our values of ½ and m, we can calculate the permeability in both the Y shear layer .k2 / and the rest of the sample .k1 /. The calculated result is shown in Fig. 2c. The permeability in the zone of highest shear strain is reduced by nearly five orders of magnitude from the starting value. Our simple model implies that the magnitude of permeability anisotropy in natural faults will depend on the degree of shear localization and the permeability contrast between the localized zone and the rest of the fault zone. Studies of the internal structures of natural fault zones show that fault zones are composed of two distinct components, a fault core where most of the shear displacement is accommodated and a less deformed, subsidiary damage zone (Logan, 1991; Chester et al., 1993; Caine et al., 1996). Recently, Caine et al. (1996) developed a qualitative scheme using the width percentage of the damage zone within a fault zone to characterise the hydrological behaviour of natural fault zones. They suggested that fault zones with large percentages of damage zone would act as conduits for fluid flow and fault zones with large percentages of fault core would
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act as barriers for fluid flow. Our experimental results support their conclusion that strongly deformed fault cores are less permeable because of grain size reduction. Furthermore, our study suggests that individual fault zones could play dual roles acting as conduits for parallel flow and as barriers for flow perpendicular to the fault plane if the permeability contrast between fault core and its surrounding damage zone is large. Observations of the internal structures of the San Andreas fault indicate that the fault core of several meters thickness is bounded by a zone of damaged host rock of the order of 100 m thick (Chester et al., 1993). Thus, the value of ½ is larger than 0.01. Permeability anisotropies of 2–3 orders of magnitude could exist in the San Andreas fault if the permeability contrast is about 4–5 orders of magnitude between the fault core and its surrounding fracture zones. The existence of permeability anisotropy and its magnitude are important because it can significantly modify the fluid flow path and fluid pressure fluctuation within fault zones. Large permeability anisotropy would effectively channel fluids along fault zones (Henry and Wang, 1991). As the pores in the zone of localized deformation within a fault will tend to have lower fluid storage capacity, fluid discharge and fluid pressure recovery during an earthquake event will be much slower than in the rest of the fault zone (Mattha¨i and Fischer, 1996), which may lead to heterogeneous fluid pressure distribution within a fault zone. A number of field studies have shown that permeability within natural fault zones and shear bands is anisotropic (Seeburger et al., 1991; Logan, 1991; Antonellini and Aydin, 1994). Permeability anisotropy ranging from one to seven orders of magnitude was reported by Antonellini and Aydin (1994) for shear bands in porous sandstones based on bench measurements using a minipermeameter. High permeability anisotropy was attributed to the existence of discrete slip planes. However, it is also possible that these extremely high permeability anisotropy values are due to the lack of confining pressure during their measurements. At high confining pressures, the aperture of the discrete slip planes should be significantly reduced or even completely closed. Although the possible connection between deformation-induced textures and permeability anisotropy was suggested a long time ago (Zoback and Byer-
lee, 1976; Mandl et al., 1977), direct measurements of permeability anisotropy during shearing of fault gouge material are still scarce. Zoback and Byerlee’s (1976) study on Ottawa sands and the more recent study by Zhu et al. (1998) on porous sandstones are the only two references for deformation-induced permeability anisotropy in quartz-feldspathic materials. In these two studies, cylindrical specimens were deformed in separate triaxial compression and extension tests during which fluid flowed always along the specimen axial direction. Their results show that at the same mean stress, the permeability parallel to the maximum principal stress was one to two orders of magnitude higher than perpendicular to it. The permeability anisotropy observed in these two studies was caused by preferential alignments of stress-induced microcracks parallel to the maximum principal stress direction (Zhu et al., 1998). The present study highlights the importance of mechanical grain crushing and shear localization structures for permeability reduction and the development of permeability anisotropy in fault zones. It has been shown that both the degree of grain crushing and the shear textures vary with mineral composition and experimental conditions, in particular, strain and effective confining pressure (Mandl et al., 1977; Logan et al., 1992; Scruggs and Tullis, 1995). Therefore, the extent of permeability reduction by fault slip and the nature and magnitude of permeability anisotropy will vary markedly in natural fault zones. Further laboratory studies on the development of shear textures and its correlation with permeability anisotropy at crustal conditions will undoubtedly lead to a better understanding of fluid flow and fluid–rock interactions in fault zones.
Acknowledgements We wish to acknowledge our indebtedness to John Logan for the stimulation and inspiration he has provided us by his many experimental and field studies of fault zone fabrics and mechanical properties. We wish to thank John Weeks for his early work of constructing and calibrating the pore fluid system and for his many valuable suggestions during the present study. Constructive comments by David Olgaard, Toshihiko Shimamoto, and an anonymous reviewer
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improved the manuscript greatly. The research was supported by NSF grant EAR-9317038 and USGS grant 1434-94-G-2422 to TET.
References Antonellini, M., Aydin, A., 1994. Effect of faulting on fluid flow in porous sandstones: Petrophysical properties. Bull. Am. Assoc. Pet. Geol. 78, 355–377. Arch, J., Maltman, A., 1990. Anisotropic permeability and tortuosity in deformed wet sediments. J. Geophys. Res. 95, 9035– 9046. Beeler, N.M., Tullis, T.E., Blanpied, M.L., Weeks, J.D., 1996. Frictional behavior of large displacement experimental faults. J. Geophys. Res. 101, 8697–8715. Bernabe´, Y., Brace, W.F., Evans, B., 1982. Permeability, porosity and pore geometry of hot-pressed calcite. Mech. Mater. 1, 173–183. Biegel, R.L., Sammis, C.G., Dieterich, J.H., 1989. The frictional properties of a simulated gouge having a fractal particle distribution. J. Struct. Geol. 11, 827–846. Blanpied, M.L., Lockner, D.A., Byerlee, J.D., 1992. An earthquake mechanism based on rapid sealing of faults. Nature 358, 574–576. Bourbie´, T., Zinzsner, B., 1985. Hydraulic and acoustic properties as a function of porosity in Fontainebleau sandstone. J. Geophys. Res. 90, 11524–11532. Brown, K.M., Moore, J.C., 1993. Comment on “Anisotropic permeability and tortuosity in deformed wet sediments” by J. Arch and A. Maltman. J. Geophys. Res. 98, 17859–17864. Byerlee, J.D., 1990. Friction, overpressure and fault normal compression. Geophys. Res. Lett. 17, 2109–2112. Byerlee, J.D., 1993. Model for episodic flow of high-pressure water in fault zones before earthquakes. Geology 21, 303– 306. Caine, J.S., Evans, J.P., Forster, C.B., 1996. Fault zone architecture and permeability structure. Geology 24, 1025–1028. Chester, F.M., Evans, J.P., Biegel, R.L., 1993. Internal structure and weakening mechanisms of the San Andreas Fault. J. Geophys. Res. 98, 771–786. Connolly, J.A.D., Thompson, A.B., 1989. Fluid and enthalpy production during regional metamorphism. Contrib. Mineral. Petrol. 102, 347–366. David, C., Wong, T.-f., Zhu, W., Zhang, J., 1994. Laboratory measurements of compaction-induced permeability change in porous rocks; implications for the generation and maintenance of pore pressure excess in the crust. Pure Appl. Geophys. 143, 425–456. Gu, Y., Wong, T.-f., 1994. Development of shear localization in simulated quartz gouge: Effect of cumulative slip and gouge particle size. Pure Appl. Geophys. 143, 387–422. Henry, P., Wang, C.-Y., 1991. Modelling of fluid flow and pore pressure at the toe of Oregon and Barbados accretionary wedges. J. Geophys. Res. 96, 20109–20130.
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Logan, J.M., 1991. The influence of fault zones on crustal-scale fluid transport. Bull. Am. Assoc. Pet. Geol. 75, 623. Logan, J.M., Dengo, C.A., Higgs, N.G., Wang, Z.Z., 1992. Fabrics of experimental fault zones: Their development and relationship to mechanical behavior. In: Evans, B., Wong, T.-f. (Eds.), Fault Mechanics and Transport Properties of Rocks. Academic Press, New York, NY, pp. 33–67. Mandl, G., De Jong, L.N.J., Maltha, A., 1977. Shear zones in granular materials. Rock Mech. 9, 95–144. Marone, C., Scholz, C.H., 1989. Particle-size distribution and microstructures within simulated fault gouge. J. Struct. Geol. 11, 799–814. Marone, C., Raleigh, C.B., Scholz, C.H., 1990. Frictional behaviour and constitutive modelling of simulated fault gouge. J. Geophys. Res. 95, 7007–7025. Mattha¨i, S.K., Fischer, G., 1996. Quantitative modelling of faultfluid-discharge and fault-dilation-induced fluid-pressure variations in the seismogenic zone. Geology 24, 183–186. Morrow, C.A., Byerlee, J.D., 1989. Experimental studies of compaction and dilatancy during frictional sliding on faults containing gouge. J. Struct. Geol. 11, 815–825. Morrow, C.A., Shi, L.Q., Byerlee, J.D., 1984. Permeability of fault gouge under confining pressure and shear stress. J. Geophys. Res. 89, 3193–3200. Nakashima, S., 1995. Diffusivity of ions in pore water as a quantitative basis for rock deformation rate estimates. Tectonophysics 245, 185–203. Paterson, M.S., 1983. The equivalent channel model for permeability and resistivity in fluid-saturated rock — A re-appraisal. Mech. Mater. 2, 345–352. Rice, J.R., 1992. Fault stress states, pore pressure distributions, and the weakness of the San Andreas Fault. In: Evans, B. Wong, T.-f. (Eds.), Fault Mechanics and Transport Properties of Rocks. Academic Press, New York, NY, pp. 475–503. Scruggs, V.J., Tullis, T.E., 1995. Localization and mechanical behavior correlation in the frictional deformation of Feldspar and mica gouges. EOS Trans. Am. Geophys. Union 76, 560. Seeburger, D.A., Aydin, A., Warner, J.L., 1991. Structure of fault zones in sandstone and its effect on permeability. Bull. Am. Assoc. Pet. Geol. 75, 669. Segall, P., Rice, J.R., 1995. Dilatancy, compaction, and slip instability of a fluid infiltrated fault. J. Geophys. Res. 100, 22155–22173. Sibson, R.H., 1992. Implications of fault-valve behavior for rupture nucleation and recurrence. Tectonophysics 211, 283– 293. Sibson, R.H., Robert, F., Poulsen, K.H., 1988. High-angle reverse faults, fluid-pressure cycling, and mesothermal gold-quartz deposits. Geology 16, 551–555. Sumita, I., Yoshida, S., Kumazawa, M., Hamano, Y., 1996. A model for sedimentary compaction of a viscous medium and its application to inner-core growth. Geophys. J. Int. 124, 502– 524. Teufel, L., 1987. Permeability changes during shear deformation of fractured rock. In: Farman, I.W., Daemen, J., Desai, C.S., Glass, C.E., Neuman, S.P. (Eds.), Rock Mechanics: Proc. 28th U.S. Symp. on Rock Mechanics 28, 473–480.
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Tullis, T.E., Weeks, J.D., 1986. Constitutive behavior and stability of frictional sliding of granite. Pure Appl. Geophys. 124, 384–414. Walder, J., Nur, A., 1984. Porosity reduction and crustal pore pressure development. J. Geophys. Res. 89, 11539–11548. Walsh, J.B., Brace, W.F., 1984. The effect of pressure on porosity and the transport properties of rock. J. Geophys. Res. 89, 9425–9431. Wong, T.-f., Shah, K.R., 1995. Mechanism of comminution and shear-enhanced compaction in geomaterials. EOS Trans. Am. Geophys. Union 76, 583. Wong, P.-Z., Koplik, J., Tomanic, J.P., 1984. Conductivity and permeability of rocks. Phys. Rev. B30, 6606–6614. Yund, R.A., Blanpied, M.L., Tullis, T.E., Weeks, J.D., 1990.
Observations of experimental fault gouges using transmission electron microscopy. J. Geophys. Res. 95, 15589–15602. Zhang, S., Paterson, M.S., Cox, S.F., 1994. Porosity and permeability evolution during hot isostatic pressing of calcite aggregates. J. Geophys. Res. 99, 15741–15760. Zhu, W., Wong, T.-f., 1997. The transition from brittle faulting to cataclastic flow: Permeability evolution. J. Geophys. Res. 102, 3027–3041. Zhu, W., Montesi, L., Wong, T.-f., 1998. Shear-enhanced compaction and permeability reduction: triaxial extension tests on porous sandstone. Mech. Mater. (in press). Zoback, M.D., Byerlee, J.D., 1976. Effect of high-pressure deformation on permeability of Ottawa sand. Bull. Am. Assoc. Pet. Geol. 60, 1531–1542.
Tectonophysics 295 (1998) 41–52
The effect of fault slip on permeability and permeability anisotropy in quartz gouge Shuqing Zhang 1 , Terry E. Tullis * Dept. Geological Sciences, Brown University, Providence, RI 02912, USA Received 12 November 1996; accepted 28 May 1998
Abstract The effects of fault slip on permeability and permeability anisotropy have been investigated during large displacement shearing of quartz bare surfaces and 1 mm thick artificial quartz gouge at a normal stress of 25 MPa. Within the first 10 mm of shear displacement, grain size reduction occurred nearly uniformly within the fault zone. Permeability decreased by 2–3 orders of magnitude and the permeabilities perpendicular and parallel to the fault plane remained approximately equal. With further increasing shear displacement up to 200 mm, deformation was localized within a band of Y shears where strong grain comminution occurred. Permeability decreased further by 1–2 orders of magnitude and the permeability perpendicular to the fault showed larger reduction than parallel to it. A permeability anisotropy of about one order of magnitude developed. The permeability anisotropy is a result of heterogeneous deformation within the fault zone; flow perpendicular to the fault zone is impeded by the fine-grained band of Y shears, but flow parallel to the fault zone can occur in the less-deformed adjacent gouge. Our results suggest that fluid flow in natural faults depends on the degree of shear localization and the permeability contrast between the localized zone and the rest of the fault zone. 1998 Elsevier Science B.V. All rights reserved. Keywords: fault slip; permeability; permeability anisotropy
1. Introduction Pore fluid pressure in fault zones and its variation during earthquake cycles can have profound effects on fault strength and fault stability (Sibson et al., 1988; Byerlee, 1990, 1993; Blanpied et al., 1992; Rice, 1992; Sibson, 1992; Segall and Rice, 1995). Fluid pressure and fluid flow in fault zones depend on permeability and permeability anisotropy which can be significantly modified by fault slip. Ł Corresponding
author. E-mail: [email protected] Present address: Research School of Earth Sciences, The Australian National University, Canberra, Australia. 1
During fault slip mechanical shearing may produce drastic changes in the texture of fault gouge materials (Mandl et al., 1977; Beeler et al., 1996). First, substantial porosity reduction (compaction) is usually associated with fault slip due to rearrangement of packing among particles and to grain size reduction (Biegel et al., 1989; Marone and Scholz, 1989; Morrow and Byerlee, 1989; Marone et al., 1990). The nature and extent of porosity reduction depend on mineral composition, initial particle size distribution and physical conditions such as pressure, slip velocity, and displacement. It has been shown that the compaction during shearing is much more efficient than during compressive loading and
0040-1951/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 1 1 4 - 0
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is particularly effective in granular materials such as quartz-feldspathic aggregates (Mandl et al., 1977; Biegel et al., 1989; Yund et al., 1990; Wong and Shah, 1995). Naturally, one would expect a large impact of shear-enhanced compaction on fluid permeability in quartz-feldspathic fault zones (Zhu and Wong, 1997). Second, mechanical shearing can produce particle shape fabrics and discrete shear fractures such as R shears, P shears and Y shears (Mandl et al., 1977; Logan et al., 1992). The shear fractures represent localized deformation bands within the fault. Severe comminution usually occurs along shear fractures, in particular R1 shears and Y shears (Marone and Scholz, 1989; Beeler et al., 1996). Because of the preferred grain comminution in shear fractures, porosity structure and therefore, fluid permeability may become anisotropic within the fault. Laboratory frictional sliding experiments also show that the initiation and growth of shear fractures are largely a function of cumulative slip displacement (Mandl et al., 1977; Logan et al., 1992; Gu and Wong, 1994; Scruggs and Tullis, 1995). As a result, the permeability anisotropy may depend strongly on shear displacement. Despite the significant progress in identifying the effects of fault slip on the texture of fault gouge, the correlations between shear textures and fluid permeability and permeability anisotropy remain poorly understood. The few available measurements of permeability and permeability anisotropy during fault slip were focused on clay minerals (Morrow et al., 1984; Arch and Maltman, 1990; Brown and Moore, 1993). In the present study, we have directly measured the fluid permeability both perpendicular and parallel to the fault zone during large displacement (up to 200 mm) shearing of a quartz sandstone bare surface and an artificial quartz gouge. Our experiments show a strong correlation between shear localization along Y shears and the development of permeability anisotropy.
2. Materials and experimental techniques We have conducted experiments on (1) bare surfaces of Fontainebleau sandstone and (2) an artificial quartz gouge. The Fontainebleau sandstone used in this study is ultra pure (99% quartz) and has an
average grain size of about 125 µm. It has an intergranular porosity of about 7% and a permeability of about 10 13.5 m2 . Synthetic quartz gouge of grain size <90 µm was prepared by mechanically crushing hand-picked silica sand (ASTM, US Silica Co.) and by passing the fragments through a 170 mesh sieve. Sliding experiments were conducted at room temperature and a normal stress of 25 MPa using a direct rotary shear apparatus (Tullis and Weeks, 1986; Beeler et al., 1996). The normal stress is made up of 21 MPa effective confining pressure (confining pressure minus pore fluid pressure) and 4 MPa axial stress. Doubly deionized water was used as pore fluid and pore fluid pressure ranged from 1 to 4 MPa. During sliding, we alternated remote loading velocity by 10 fold steps within the range of 0.1 to 10 µm=s. Shear displacement up to 200 mm was achieved in our experiments. Fig. 1 shows a simplified drawing of our sample assembly. Shearing occurs either along the contacting asperities of the upper and lower forcing block rings in bare surface experiments or within a layer of artificial gouge initially 1 mm thick placed between the upper and lower forcing block rings. After pressurization, the gouge layer thickness is about 0.85 mm. The outer diameter of the sliding surface is 53.98 mm, the inner diameter is 44.45 mm and the area of contact is 735 mm2 . The upper sample grip is held stationary while the lower sample grip rotates. A full revolution of the lower sample rotation corresponds to 153.6 mm of slip. Axial load and torque were applied to the specimen independently by a hydraulic servo system and a high resolution electrohydraulic rotary servo system, respectively. Axial load and torque were measured with a combined load and torque cell located within the pressure vessel. Shear displacement was determined by a high resolution resolver whose body is mounted on the upper sample grip and whose rotating shaft is attached to the lower sample grip. The axial length change of the specimen was measured by an internal LVDT. Each of the sample grips (upper and lower, see Fig. 1) is connected with a pressure transducer and a volumometer. A volumometer is composed of a cylinder and piston, driven by a hydraulic actuator and an electronic control unit. The piston position is measured with a LVDT attached to the shaft of the
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43
Fig. 1. An illustration of half of the specimen assembly for rotary shear deformation. An experimental fault with or without artificial gouge lies between the upper and lower forcing block rings. The pore fluid pressure system is separated from the confining pressure system by two teflon rings and two O-rings outside and inside of the forcing block rings. Fluid permeability perpendicular and parallel to the fault is measured with permeable and impermeable forcing blocks respectively. The flow direction (arrows), area (A), path length (L), and inlet and outlet pore pressures (Pf1 and Pf2 ) for permeability measurements are shown in (a) and (b).
volumometer piston. A full stroke of the volumometer delivers about 32 cm3 of fluid. Pore fluid pressure or piston position can be servo-controlled independently upstream and downstream of the specimen. When pore fluid pressures upstream and downstream of the specimen are servo-controlled at different values, the rate of fluid flow through the specimen is measured by monitoring the changing piston positions of the volumometers. A constant flow rate across the specimen can be produced when one volumometer is on pore pressure control and the other volumometer servo-controls on the computer-generated command signals for piston positions. We measured permeability anisotropy during shearing of the artificial quartz gouge. Fluid permeabilities perpendicular and parallel to the experimental fault were determined in two separate experiments in which permeable Fontainebleau sandstone and impermeable silica glass were used as forcing block rings, respectively. When permeable sandstone was used as forcing block rings, water flowed first to the bottom surface of the lower forcing block through a circular channel of about 1 mm deep and 1 mm wide on the steel sample grip. Fluid was then driven upward first through the lower forcing block ring uniformly, then across the artificial gouge layer,
and finally through the upper forcing block ring and the outlet port (Fig. 1). By applying either constant fluid flow rates ranging from 10 5 to 10 2 cm3 =s or constant pressure differences of 0.1–2 MPa between fluid inlet and outlet port, the fluid permeability for the whole system was determined using Darcy’s law. Because most parts of the forcing block rings are not subject to confining pressure and the deformation of the forcing block is negligible during shearing of quartz gouge, we measured the permeability for the forcing block rings at ambient conditions. By assuming that the two forcing block rings and the gouge layer are connected in series, the perpendicular permeability of the gouge layer .k? / was derived from the permeability measured for the whole system .ke / and that for the forcing block rings .kc / by: ke D
.2`c C `? /kc k? 2`c k? C `? kc
(1)
where `c is the height of each of the forcing block rings and `? is the gouge thickness. Fluid permeability parallel to the fault was measured by using impermeable silica glass as forcing block rings and by controlling fluid flowing in and out (Fig. 1). In one experiment (#158qz), we first introduced fluid to the bottom of the gouge layer
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through a hole of about 1.2 mm in diameter in the lower forcing block. Fluid was then forced to flow along the gouge layer and finally out through a hole in the upper forcing block ring. In another experiment (#177qz), to check reproducibility, we used permeable ceramics slots of 2.5 mm width for fluid inlet and outlet. To ensure that fluid flows through the whole gouge layer and to minimize the effect of the permeability perpendicular to the fault, the time for fluid flowing across the gouge layer needs to be significantly shorter than along the gouge layer. We have found that for our specimen assembly using a 1 mm thick layer the two holes or two slots need to be at least 10 mm apart if the permeability across the gouge layer is about one order of magnitude lower than along the gouge layer. In other words, the flow path length along the gouge layer needs to be longer than 10 mm. This requirement can be easily met by our rotary shear apparatus. The data reduction takes into account the fact that, except for the special case where the two holes=two slots are 180 degrees apart, fluid flow takes place along two paths of unequal length. This is done by assuming that the two flow paths are connected in parallel. The fluid permeability parallel to the gouge layer .kk / is given by: q¼ kk D (2) 1 1 A∆P C `a `b Where q is the measured flow rate, ¼ is the dynamic viscosity, A is the cross-sectional area (see Fig. 1), ∆P is the pressure difference between fluid inlet and outlet, and `a and `b are the lengths of the two flow paths. Fluid permeability was measured intermittently during slide-holds. Because of the long flow path, each permeability measurement typically took 5–24 hours.
3. Results Changes in specimen axial length, porosity , and permeability k with increasing shear displacement for three experiments are shown in Fig. 2. Fig. 2a shows that the specimen axial length in the three runs decreases sharply within the first 10 mm shear displacement and then levels off with further increasing shear displacement. Good reproducibility
of gouge thickness change is observed for the two experiments on artificial quartz gouge using different forcing blocks. The specimen axial length change in the bare surface experiment reflects first, the crushing of contacting asperities and generation of wear products and second, compaction of the wear products (Fig. 3). Thus, the observed decrease in specimen axial length change at large displacement primarily indicates slowing down of gouge production. In the experiments on artificial gouge, the specimen axial length change is a measure of gouge compaction for the whole gouge layer. Therefore, relatively smaller amount of gouge layer thickness change at large displacements indicates (1) an overall decrease of grain size reduction in the gouge layer, or more likely, (2) localization of gouge compaction to a small layerparallel deformation band within the gouge layer. Evolution of total porosity with shear displacement is shown in Fig. 2b. For the experiment on bare surfaces, the total porosity within the deformation band surrounding the contacting asperities was calculated retrospectively from final shear texture and specimen axial length change. We assumed that wearing at the onset of shearing occurred within a 150 µm thick zone (deformation band) on the basis of two observations: (1) the final wear product was about 50 µm thick and porosity within it was about 4–5% from SEM analysis; and (2) the accumulated specimen axial length change was about 100 µm (Fig. 2a). The porosity within the 150 µm thick zone was about 70% before shear deformation started (Fig. 3a). This analysis is supported by our surface roughness measurement. In our experiments, the forcing block surfaces were roughened with 120 mesh silica carbide powder before shear deformation. We found that the average asperity height on the roughened surface is about 75 µm. Thus, shear deformation on bare surfaces basically crushed all asperities of the two contacting surfaces. For experiments on artificial gouge, total porosity was calculated from the starting porosity before shear deformation and the gouge thickness changes during shear deformation as shown in Fig. 2a. The starting porosity was calculated from gouge weight and gouge layer thickness before pressurization and the gouge thickness change during pressurization. For our thin layer ring geometry, gouge thickness change is approximately equal to porosity change. Again, the calculated porosity is an average for the whole gouge
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Fig. 2. Experimental results. (a) Primary data of specimen axial length change with increasing shear displacement. (b) Porosity data derived from specimen axial length change during deformation and the initial porosity and the initial specimen thickness before shear deformation. (c) Primary data of fluid permeability. Note good reproducibility between experiment 158qz and experiment 177qz for fluid flow parallel to the fault. In addition, the dashed line .k2 / and the solid line .k1 / are the permeabilities calculated respectively for the strongly comminuted area of Y shears and the relatively less deformed gouge region within a sheared, artificial quartz gouge layer. The calculation is discussed in the text (see Fig. 3 and Eqs. 7–9). (d) Permeability as a unique function of porosity: k / Þ . Þ should be equal to 3 if pore aperture does not change with porosity.
layer although porosity may decrease substantially more in localized bands as revealed by SEM observation. Corresponding to the large porosity reduction at the beginning of shearing, permeability decreased drastically (Fig. 2c). Calculation of permeability takes into account the changing of the layer thickness shown in Fig. 2a. Fluid permeability is a strong function of cumulative slip displacement. Sliding velocity jumps within the range of 0.1 to 10 µm=s have little effect on the fluid permeability because the
transient dilatation associated with velocity jumps is small and is rapidly overcome by the overall porosity reduction with increasing displacement. Within the first 10 mm of shear displacement, the perpendicular permeability within the wear product of sandstone bare surfaces decreased by about three orders of magnitude. Over the same range of shear displacement, permeability decreased by about two orders of magnitude within the artificial gouge layer. Moreover, fluid permeabilities perpendicular and parallel to the fault remain virtually the same. As shear
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Fig. 3. Schematic diagrams illustrating the nature of porosity and permeability reductions during shear deformation of sandstone bare surfaces (a) and an artificial quartz gouge (b). `1 , k1 and `2 , k2 in (b) are the thickness and the permeability for the less deformed region and the band of Y shears, respectively.
displacement increased up to about 200 mm, permeability decreased further by another one to two orders of magnitude in the three experiments. More importantly, permeability anisotropy developed in the artificial gouge layer, with permeability parallel to the fault being higher than normal to it by about one order of magnitude. The relationship between permeability and porosity is shown in Fig. 2d. If we assume a commonly used relationship between k and : k / Þ , we find that the exponent (Þ) decreases from about 9 at high porosities to about 2–3 at low porosities. Shear textures were examined in thin sections using optical microscopy and in fracture surfaces using SEM. A layer of fine wear product of about 50 µm thickness was present between the upper and lower forcing blocks in the bare surface experiment (Fig. 4a). Many Y shears which are parallel to the artificial fault plane are discernible. Under SEM, the wear product is composed of round aggregates of 1–2 µm in diameter which were sintered together (Fig. 4b). The microstructure of artificial quartz gouge after pressurization is shown in Fig. 4c. Particles are angular and shows some alignment of
particle long-axes parallel to the fault plane. After about 8 mm of shear displacement, most particles are more equant in shape (Fig. 4d). Small fragments, produced possibly by spalling of large particles increase significantly. Numerous R1 shears which intersect at 20–25º to the fault boundaries are present (not shown in Fig. 4). In specimens sheared more than 10–15 mm, the microstructure is characterized by a band of fault boundary parallel Y shears within which extensive comminution occurred (Fig. 4e). The average band thickness is about 115 µm and varies slightly from one location to another. The microstructure outside of the shear band is roughly similar to that seen at short shear displacements, but it is difficult to identify individual R1 shears.
4. Discussion and conclusions 4.1. Importance of shear-induced grain crushing for permeability reduction Mechanical shearing of quartz gouge shows two prominent features: first, the permeability decreased
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Fig. 4. Microstructures. (a) Gouge layer produced by shear deformation of Fonetainebleau sandstone bare surface (Experiment 151fs). Note that the gouge layer was opened up during depressurization. Numerous Y shears (horizontal lineations) are present. (b) SEM image of the sliding surface of experiment 151fs. (c) Texture of an artificial quartz gouge after being compacted at 21 MPa confining pressure and about 4 MPa axial stress. The axial stress direction is vertical. (d) Texture of an artificial quartz gouge sheared to 8 mm displacement at a normal stress of 25 MPa. Shear plane is horizontal and shear sense is sinistral. (e) Texture of an artificial quartz gouge sheared to 170 mm displacement at a normal stress of 25 MPa. Shear plane is horizontal and shear sense is sinistral. Note that particle size is much finer within the band of Y shears (lower 45% of the image) than within the rest of the gouge layer (upper 55% of the image).
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drastically with initial shearing (Fig. 2c). Similar sharp decreases in permeability were previously found during triaxial deformation of unconsolidated sand and porous sandstones (Zoback and Byerlee, 1976; Zhu and Wong, 1997; Zhu et al., 1998) and during frictional sliding along pre-fractured surfaces of Coconino sandstone (Teufel, 1987). Second, the slopes in the log k vs. log plot (Fig. 2d) are higher than 3. This feature was also observed by David et al. (1994) during pressurization of sandstones beyond the critical pressures at which grain crushing occurred. These two features point to the importance of grain size reduction for decreasing permeability and they can be interpreted using the equivalent channel or Kozeny–Carman model (Paterson, 1983; Walsh and Brace, 1984; Connolly and Thompson, 1989; Sumita et al., 1996). From the equivalent channel model, permeability .k/ can be expressed in the form (Paterson, 1983): k D C1 R 2 x
(3)
where C1 is a numerical factor depending on pore shape, and R is the hydraulic radius defined as the ratio of the pore volume to the solid–fluid interfacial area. The factor C1 can be calculated for channels of simple uniform cross-sections assuming the Navier– Stokes flow equations with non-slip boundary conditions, giving C1 D 12 for circular cross-section, 35 for equilateral triangular cross-section, and 13 for a slot (Paterson, 1983). For an aggregate of uniform spheres, R D r=6.1 /, where r is the grain size (Paterson, 1983). Assuming an equilateral triangular pore geometry, Eq. 3 can be rewritten as k³
nr 2 60.1 /2
or d.log r / d.log k/ D Þ D nC2 d.log / d.log /
(4) ½ d.log.1 // 2 d.log / (5)
Thus, a plot of log k vs. log will give a true porosity exponent n when grain size r does not change with porosity and when porosity is low so that .1 /2 is close to 1. In the cases where pore aperture and connectivity do not change significantly and where pore aperture can be independently assessed, the true porosity exponent n has been found to be about 3
(Bernabe´ et al., 1982; Walsh and Brace, 1984; Wong et al., 1984; Bourbie´ and Zinzsner, 1985; Zhang et al., 1994; Nakashima, 1995). The term in the square brackets of Eq. 5 is always positive and changes from about 2 at D 0:5 to about 0.1 at D 0:1. This term will reduce the nominal value of porosity exponent Þ, particularly at high porosities. If pore aperture decreases with decreasing porosity, d.log r /=d.log / will be positive, which will increase the value of Þ. Therefore, a value of Þ higher than 3 in a log k vs. log plot is a strong indication of grain size reduction if the connectivity of the pore network is maintained. Calculations using Eq. 4 shows that the model particle size for the whole layer of artificial quartz gouge would have been reduced by a factor of 10 after 170 mm shear displacement. This is an average value based on bulk measurements of k and . Because of localization of shear deformation at large displacements, grain size reduction should be more substantial within the band of Y shears. In modelling crustal pore pressure development, Walder and Nur (1984) assumed a relationship between k and : Þ cÞ (6) k D k0 0Þ cÞ where k0 and 0 are the initial permeability and initial porosity, and c is the threshold porosity for throughflow. As demonstrated by David et al. (1994), higher values of Þ will favor the development of fluid overpressure in fault zones. It has been found that during time-dependent compaction, the value of Þ is typically about 3 at porosities higher than 7–10% and it increases to above 7 at lower porosities due to a loss of connectivity among pores (Bernabe´ et al., 1982; Bourbie´ and Zinzsner, 1985; Zhang et al., 1994). Undoubtedly, in natural fault zones shearenhanced compaction will couple with time-dependent compaction in very complicated ways. Simply judging from the Þ values of 9 associated with comminution, we suggest that mechanical shearing would significantly enhance the generation of fluid overpressures within porous fault zones. 4.2. Effect of shear localization on permeability anisotropy Our study shows that the development of permeability anisotropy in quartz gouge is associated with
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localization of shear deformation along a band of Y shears at shear displacements larger than 15 mm. The band of Y shears tends to be near the lower gouge–rock interface and is continuous along the sliding direction although its thickness could vary slightly from one location to another. R1 shears are prominent during the early stages of deformation (<10–15 mm) but give way to Y shears at larger displacements. Also, R1 shears are never observed to cut the band of Y shears. Therefore, R1 shears appear to have little effect on permeability anisotropy in quartz gouge. The band of Y shears represents a less permeable layer as indicated by its much higher degree of comminution than other parts of the fault zone. For fluid flow perpendicular to the fault zone, the same fluid flux has to flow through both the finely comminuted, low permeability band of Y shears and the relatively less deformed layer. In contrast, for fluid flow parallel to the fault zone, a higher fluid flux goes through the less deformed layer. If we assume a two layer structure within the fault zone (Fig. 3), layer one of thickness `1 and permeability k1 , and layer two of thickness `2 and permeability k2 , the effective fluid permeability of the fault zone can be treated as the two layers in series and parallel, respectively, when fluid flows perpendicular and parallel to the fault zone. The effective permeability normal and parallel to the fault zone is given, respectively, by: .`1 C `2 /k1 k2 (7) k? D `1 k2 C `2 k1 k1 `1 C k2 `2 (8) `1 C `2 If we take `2 =`1 D ½ and k2 =k1 D m, then the permeability anisotropy is given by: .1 C ½m/.½ C m/ kk D (9) k? m.1 C ½/2 Permeability anisotropies as a function of ½ and m are shown in Fig. 5. When the band of Y shears makes up less than about 1=6 of the fault zone .½ < 0:2/, the permeability anisotropy increases rapidly with increasing ½ and m; and when the width of the band of Y shears is larger than 1=6 of the fault zone, the permeability anisotropy depends largely on m only. In other words, propagation or growth of Y shears due to shear band hardening at large shear kk D
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Fig. 5. Permeability as a function of ½ and m. ½ is the thickness ratio of strongly deformed layer with less deformed layer. m is the permeability contrast between strongly deformed layer and less deformed layer.
displacements will have little effect on permeability anisotropy. In our sheared quartz gouge, ½ is about 1=5 based on microstructural observations. From Eq. 9, we found that if the permeability within the band of Y shears is lower than the rest of the layer by a factor of about 70, the observed anisotropy of about one order of magnitude can be accounted for. Based on our values of ½ and m, we can calculate the permeability in both the Y shear layer .k2 / and the rest of the sample .k1 /. The calculated result is shown in Fig. 2c. The permeability in the zone of highest shear strain is reduced by nearly five orders of magnitude from the starting value. Our simple model implies that the magnitude of permeability anisotropy in natural faults will depend on the degree of shear localization and the permeability contrast between the localized zone and the rest of the fault zone. Studies of the internal structures of natural fault zones show that fault zones are composed of two distinct components, a fault core where most of the shear displacement is accommodated and a less deformed, subsidiary damage zone (Logan, 1991; Chester et al., 1993; Caine et al., 1996). Recently, Caine et al. (1996) developed a qualitative scheme using the width percentage of the damage zone within a fault zone to characterise the hydrological behaviour of natural fault zones. They suggested that fault zones with large percentages of damage zone would act as conduits for fluid flow and fault zones with large percentages of fault core would
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act as barriers for fluid flow. Our experimental results support their conclusion that strongly deformed fault cores are less permeable because of grain size reduction. Furthermore, our study suggests that individual fault zones could play dual roles acting as conduits for parallel flow and as barriers for flow perpendicular to the fault plane if the permeability contrast between fault core and its surrounding damage zone is large. Observations of the internal structures of the San Andreas fault indicate that the fault core of several meters thickness is bounded by a zone of damaged host rock of the order of 100 m thick (Chester et al., 1993). Thus, the value of ½ is larger than 0.01. Permeability anisotropies of 2–3 orders of magnitude could exist in the San Andreas fault if the permeability contrast is about 4–5 orders of magnitude between the fault core and its surrounding fracture zones. The existence of permeability anisotropy and its magnitude are important because it can significantly modify the fluid flow path and fluid pressure fluctuation within fault zones. Large permeability anisotropy would effectively channel fluids along fault zones (Henry and Wang, 1991). As the pores in the zone of localized deformation within a fault will tend to have lower fluid storage capacity, fluid discharge and fluid pressure recovery during an earthquake event will be much slower than in the rest of the fault zone (Mattha¨i and Fischer, 1996), which may lead to heterogeneous fluid pressure distribution within a fault zone. A number of field studies have shown that permeability within natural fault zones and shear bands is anisotropic (Seeburger et al., 1991; Logan, 1991; Antonellini and Aydin, 1994). Permeability anisotropy ranging from one to seven orders of magnitude was reported by Antonellini and Aydin (1994) for shear bands in porous sandstones based on bench measurements using a minipermeameter. High permeability anisotropy was attributed to the existence of discrete slip planes. However, it is also possible that these extremely high permeability anisotropy values are due to the lack of confining pressure during their measurements. At high confining pressures, the aperture of the discrete slip planes should be significantly reduced or even completely closed. Although the possible connection between deformation-induced textures and permeability anisotropy was suggested a long time ago (Zoback and Byer-
lee, 1976; Mandl et al., 1977), direct measurements of permeability anisotropy during shearing of fault gouge material are still scarce. Zoback and Byerlee’s (1976) study on Ottawa sands and the more recent study by Zhu et al. (1998) on porous sandstones are the only two references for deformation-induced permeability anisotropy in quartz-feldspathic materials. In these two studies, cylindrical specimens were deformed in separate triaxial compression and extension tests during which fluid flowed always along the specimen axial direction. Their results show that at the same mean stress, the permeability parallel to the maximum principal stress was one to two orders of magnitude higher than perpendicular to it. The permeability anisotropy observed in these two studies was caused by preferential alignments of stress-induced microcracks parallel to the maximum principal stress direction (Zhu et al., 1998). The present study highlights the importance of mechanical grain crushing and shear localization structures for permeability reduction and the development of permeability anisotropy in fault zones. It has been shown that both the degree of grain crushing and the shear textures vary with mineral composition and experimental conditions, in particular, strain and effective confining pressure (Mandl et al., 1977; Logan et al., 1992; Scruggs and Tullis, 1995). Therefore, the extent of permeability reduction by fault slip and the nature and magnitude of permeability anisotropy will vary markedly in natural fault zones. Further laboratory studies on the development of shear textures and its correlation with permeability anisotropy at crustal conditions will undoubtedly lead to a better understanding of fluid flow and fluid–rock interactions in fault zones.
Acknowledgements We wish to acknowledge our indebtedness to John Logan for the stimulation and inspiration he has provided us by his many experimental and field studies of fault zone fabrics and mechanical properties. We wish to thank John Weeks for his early work of constructing and calibrating the pore fluid system and for his many valuable suggestions during the present study. Constructive comments by David Olgaard, Toshihiko Shimamoto, and an anonymous reviewer
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improved the manuscript greatly. The research was supported by NSF grant EAR-9317038 and USGS grant 1434-94-G-2422 to TET.
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