Evolution of structure and permeability of normal faults with clay smear: Insights from water‐saturated sandbox models and numerical simulations

Abstract

Clay smear is difficult to predict for subsurface flow applications and would benefit from an improved understanding of the processes controlling clay smear. We present water‐saturated sandbox experiments with large clay smear surfaces (~500 cm2) that couple cross‐fault fluid flow measurements with structural analysis of excavated clay smears. We compare measured flow data to numerical flow simulations to develop a tool to evaluate the evolving fault structure. Results show diagnostic relationships between fault structures and cross‐fault flow. In experiments with one or two clay layers and a cumulative thickness of 10 mm at 100 mm displacement, normally consolidated clay in a structural domain of graben faulting initially yields hybrid brittle/ductile failure with early breaching of the clay layer and increased cross‐fault flux. This is followed by fault backstepping, formation of clay smears, and reworking of clay fragments within the fault. Early formed holes remain open during the evolution of the faults. Fault zones are segmented by fault lenses, breached relays, and clay smears in which sand and clay mix by deformation. Experiments with two clay layers show that holes rarely form at the same position on the fault plane, producing a layered sand‐clay fault rock with greater flow path tortuosity and lower permeability than in one‐layer experiments. We compare our results with observations of faults in nature and discuss progress toward models with sufficient detail and understanding to allow prediction of flow across evolving faults, first in laboratory models and then in the subsurface.

Supplemental Material: Fault permeability from stochastic modeling of clay smears

“…The main goal of this work is providing new insight on the impact of stratigraphically-consistent fault material arrangements with clay smears on fault permeability. In § 3 in the main text we validate PREDICT with cm-scale experiments by Kettermann et al (2017), and we also compare our results to those from Sperrevik et al (2002)'s permeability predictor and other laboratory and field measurements in similar geology. A multi-scale, 3-component validation of PREDICT's output requires quantifying the mismatch between the permeability distributions and measurements of the macroscale fault permeability in m-to km-scale faults.…”

“…• Along-strike smear segment number (s s ): This parameter defines the segmentation of smears in the strikeparallel direction, and is based on the hole mappings in excavated fault surfaces by Kettermann et al (2016Kettermann et al ( , 2017; Noorsalehi-Garakani et al (2013). We counted the number of holes along strike-parallel, horizontal lines of length equal to f D to define the maximum and minimum number of holes (N h ).…”

“…greater effective stress, leading to higher SSFc (C ¸iftc ¸i et al, 2013;Clausen and Gabrielsen, 2002;Giger et al, 2013;Sperrevik et al, 2000). The endpoints can be further modified to account for hybrid failure in very shallow faults (e.g., Kettermann et al, 2017), such that discontinuous smears may appear at any value of f D . Finally, the endpoints are adjusted to account for the scale factor, which leads to smaller faults displaying larger SSFc (Faerseth, 2006).…”

Analyzing the influence of correlation length in permeability on convective systems in heterogeneous aquifers using entropy production

“…For instance, fault-related processes, such as clay smear (Vrolijk et al 2016), can drastically influence permeability of a fault, e.g., by acting as a barrier to fluid flow. The existence of clay smear alone, however, is not a defining indicator on the permeability of the fault, as potential breaching of the clay layer coincides with increased flow across the fault (Kettermann et al 2017). Furthermore, experimental data showed that the influence of clay smear on permeability changes with fault activity, i.e., rock deformation (Takahashi 2003).…”

An overview of fault zone permeabilities and groundwater level steps in the Roer Valley Rift System

“…in faulted deposits, and by sedimentary structures as shown by Lapperre et al (1996) who studied heterogeneity using hydraulic conductivity measurements on 271 undisturbed core samples taken in horizontal, vertical and 'structure' direction. Based on water-saturated model experiments (Schmatz et al, 2010) and multiple experiments using a water-saturated sandbox to simulate clay smear, Kettermann et al (2017) concluded that flow patterns under these circumstances are complex and three-dimensional. Vertical temperature profiles in nests of piezometers in unconsolidated sedimentary aquifers in the German opencast lignite mining area also indicate fault zone heterogeneity.…”