Comparisons of water and argon permeability in natural clay‐bearing fault gouge under high pressure at 20°C

Abstract

Abstract. Quantification of fluid transport through fault zones is critical for the understanding of fault mechanics and prediction of subsurface fluid flow. The permeability of clay-bearing fault gouge has been determined using first argon then water as pore fluids under total confining pressures ranging up to 200 MPa and pore pressures of 40 MPa at room temperature. Use of the two pore fluids allows interactions between the gouge and pore fluids to be examined. Natural clay-bearing fault gouge recovered from surface exposures of the Carboneras fault zone in southeastern Spain was used and was collected in such a way that the in situ microstructure was preserved. Cores were collected in directions relative to the well-developed planar fabrics seen in these types of fault rock. The mineralogy of the gouges included muscovite/illite, chlorite, and quartz, with minor amounts of gypsum, albite, and graphite. Glycolation of the gouge showed no discernible amounts of swelling phases. Grain size analyses revealed a bimodal grain size distribution, with the <2 •xm fraction dominant (>50 wt %). This fraction contained predominantly 17 2 22clay phases. Permeabilities in the range of 10-m to 10-m 2 were measured. Experimental results show that the previous highest in situ effective pressure to which the fault gouge had been subjected (overconsolidation pressure) could not be determined from changes in permeability. Differences between water and argon permeabilities determined on the same sample amounted to -•1 order of magnitude, even if the sample had been pressure cycled (reduction to zero and reapplication of both confining and pore pressure) using argon as pore fluid until asymptotic values for permeability had been attained. Volumetric strain measurements showed no enhanced compaction due to the introduction of water as the pore fluid, leading to the conclusion that the reduction in permeability must be due to physicochemical interactions of the water with the fault gouge. The low permeabilities measured support models invoking high fluid pressure weakening of large faults with minimal fluid loss. The stability of structured water films with varying temperature, water pressure and water chemistrv may produce a heterogeneous permeability profile with depth in fault zones.

Seismically invisible fault zones: Laboratory insights into imaging faults in anisotropic rocks

“…Their measurements through graphitic mica-schist also indicate a similar degree of anisotropy at similar pressures and confirm that strong anisotropy persists within the mica-schist to greater pressures (27% anisotropy at 200 MPa). Measurements of fault rock permeability from numerous localities along the Carboneras fault have shown a remarkable degree of consistency, implying homogeneity of the fault rock in this region [Faulkner and Rutter, 1998, 2000, 2001. Velocities measured in the laboratory for the Carboneras fault zone have also previously been compared to those measured in field seismic surveys by Taylor et al [2015], with good agreement at pressures of about 2 MPa (corresponding to depths of approximately 80 m).…”

Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)

“…We further note that the permeability measurements presented herein were measured using inert gas as the pore fluid. Permeability to water will likely be lower than the gas permeabilities provided herein due to the presence of swelling clays (although we note that only 5-6% of the clays are smectite) (Davy et al 2007;Faulkner and Rutter 2000;Shimamoto 2006, 2009). The relationship between permeability and clay content may also be more pronounced if water was used as the pore fluid.…”

Shear strength of sediments approaching subduction in the Nankai Trough, Japan as constraints on forearc mechanics

“…The basement thrust coincides with the convergence of the frontal thrust and the LSB reflector and thus appears to be disrupting the local lithostratigraphy ( Figure 5), suggesting that this feature causes the plate boundary to be located on the frictionally stronger frontal thrust. Because it has been suggested that rupture propagation in shallow accretionary prisms is facilitated by weak, clay-rich sediment [Faulkner et al, 2011] or high-overall pore pressures , the high-friction, well-drained deposits on the frontal thrust make the toe of the Kumano transect less conducive for earthquake propagation than adjacent areas (e.g., Muroto). This is supported by earlier experiments documenting that friction coefficients on the frontal thrust are $0.1 higher than on the Muroto decollement [Ikari and Saffer, 2011].…”

“…However, our results show that the pelagic clay is also frictionally weak in general (consistently the second-weakest sample) and therefore is the next likely candidate to host the decollement if fluid pressure becomes further elevated relative to Unit V. Although sand-rich lithologies in Unit V exhibit high permeability and could act as fluid conduits [H€ upers and Kopf, 2012], the transition zone is located $25-35 km from the trench so that the path length of fluid escape is large, a condition favorable for maintaining overpressure. Additionally, shearing of sediment can significantly reduce fault-perpendicular permeability [e.g., Brown and Moore, 1993;Faulkner and Rutter, 2000;Kopf, 2001;Ikari and Saffer, 2012], so it is possible that permeability reduction driven by shearing in Unit V could limit vertical flow in Unit VI and build overpressure. Since the strengths of the volcaniclastic and pelagic samples are very similar, only a small amount of pore pressure increase may be necessary for further decollement step-down.…”