Fault DAMAGE Zones

Modeling fault damage zone evolution

By Sylvain Mayolle

Home Page, ResearchGate, Ph.D. thesis

Faults are macroscopic permeability heterogeneities that substantially affect the circulation of fluids in the earth’s crust. It is commonly accepted that the faults damage zone can constitute an efficient drain for the fluids (or reservoir if top-seal) when it consists of open fractures. Understanding the geometry of these damage zones, their constitution, and establishing empirical rules, can help to better understand fluid migrations. This study aims to determine the scaling laws and mechanisms responsible for the structure of damage zones and to understand their role in the storage and fluid flows around faults.

Figure 1 – Schematic fault zone model commonly used for strike-slip fault. The block-diagram model shows fault-zone architectural components and main structural features, after Choi et al., 2016.

Deformed rock volumes in fault zones are usually classified into two distinct zones, the Core Zone (CZ) and the Damage Zone (DZ). The core zone is generally characterized by the presence of fault rocks, generally acting as macroscopic permeability structures in geological reservoirs. The damage zone develops in response to different processes such as initiation, propagation/interaction of faults, or seismic ruptures. Depending on the nature of the deformed rock and the applied state of stress, a wide diversity of subseismic damaged structures can be found such as tensile fractures, layer folding, stylolites, dilation, shear, and compaction bands. These structures may produce significant and variable impacts on permeability anisotropy in and adjacent to faults and therefore on fault-related fluid flow. The understanding of the fracture network complexity of Naturally Fractured Reservoirs (NFR) represents a major issue for the exploration/production of georesources, especially in carbonate rocks. However, the fault damage zones, their scaling properties, and implications for geological reservoirs are poorly constrained for this lithology.

Figure 2 – a) Normal fault with 1.5m displacement. b) D-T data from this study were reported with data from the literature. c) D-T data from this study on a linear axis, best fit in black and best fit without the two largest faults in red. Modified from Mayolle et al., (2019).

Damage zone width is a key parameter controlling the hydraulic behavior of the fault, energy dissipation during earthquake ruptures, and strain distribution. Several studies over the last decade have evidenced a linear scaling between fault displacement (D) and damage zone thickness (T) up to a scaling limit of a few hundred meters, defining a threshold in the scaling. Although discussed in terms of seismogenic depth for earthquake-related damage, the existence of such a threshold for faults is not yet supported by a physical explanation. To investigate the existence of such a threshold in D-T scaling and to identify the main controlling parameters, we developed a rheologically consistent and scaled analog model of the Earth’s brittle crust. With these aims, we measure the strain field evolution, D-T scaling, and volumetric strain using high-resolution image correlation, for different layer thickness models sharing common rheology.

Figure 3 – Experimental setup. At the base of the model, an elastic foam is uncompacted by a computerized motor, imposing extensional deformation in the upper visco-plastic kinetic sand and frictional layer. Vertical displacement and YY strain field derived from the sub-pixel image correlation are shown on the right.

The experiments are performed in a sandbox device 500 mm long, 300 mm high, and 100 mm wide, equipped with two lateral glass walls, containing two layers of granular materials. The upper layer is composed of 97.5% dry sand of 150 μm mean grain size and 2.5% pumice powder of similar packing to ensure reproducibility. The static friction angle and the cohesion are 38° +/-1° and 100 Pa +/-20 Pa, respectively. These physical properties are analogous to upper crustal rocks in that friction, dilation angle with increasing plastic strain, and cohesion are scaled to satisfy a model-to-nature scaling of 1 cm equivalent to 0.5-1 km in nature. Such materials consistently reproduce the main geometries of faults, their segmentation, interaction, displacement profiles, and gradients, but also displacement-length and size distribution scaling laws. The lower layer is a mixture of 98% dry sand of 150 μm mean grain size, and 2% PDMS silicone (kinetic sand) inducing a visco-plastic mechanical behavior simulating the lower crust or ductile shales. These two layers lie on a pre-compressed elastic polyurethane foam which, once relaxed by moving a lateral backstop, induces distributed extensional strain on the layers above. The extension velocity (30 mm/h) is regulated by a computer-controlled motor and allows a total extension of 100 mm (20%). For all experiments, the lateral side of the model is monitored using a SONY α7RII camera taking 1 picture every 30 seconds with a resolution of 1 pixel = 54 microns. These pictures are used to provide movies of the model evolution, measure the strain field, D-T scaling, deformation tensor components. We also measured the incremental volumetric strain (dεv) along the faults to investigate its evolution in relation to the scaling behavior. To quantify its evolution relative to the incremental shear displacement during fault growth, we also calculate its derivative in the form of dεv/dD (Figure 3).

Figure 4 – Advanced stage of model evolution showing incremental kinematics and mechanics derived from high-resolution digital image processing

Our models show similar fault dip angles, conjugate geometries, segmentation, and also D-T trend to natural data. The ratio of layer thickness to mean threshold thickness found is close to the ratio expected for natural faults in the seismogenic crust (15 km / 0.3 km). Also, the observed volumetric strain can be considered, to a certain extent, analogous to fracture dilation and compaction occurring within fault zones (mode I fracture, dilatant breccias vs. cataclasites, gouge smearing, S-C structures, cleavage, and pressure solution seams). In addition, it is well known that Mode I fractures generally develop early in the growth history of a fault in natural rocks. Larger fault zones generally show a combination of preferentially dilational-shear damage zones and compactional-shear core zones, with an average spatial arrangement consistent with the volumetric strain distribution presented in this study.

Example of analog experiment documenting, in side-view (cross-section), the growth of a normal fault network. The rheology of the model is composed of a thick brittle upper layer made of a low cohesive granular material resting on a visco-plastique layer deposited on top of a pre-compressed PU foam plate.

Finally, as the overall mechanical controls on the failure mode transition govern the threshold of damage zone growth, this threshold is likely to be specific to the geological context of anyone’s fault, probably accounting for the large scatter observed in the natural data. However, our study suggests that mechanical layering is probably one of the most important parameters governing this threshold and scattering in nature, and such layering exists at several scales, with the ultimate scale limit of layer thickness controlling fault damage zone width being the thickness of the brittle upper crust. Other parameters like lithology, tectonic setting, and the physico-chemical behavior of fluids in the fault zone are probably also important for scattering. For this reason, the specific context of anyone’s fault must be taken into account when comparing different damage zone datasets for establishing overall scaling laws.

Learn more:

-> Mayolle, S., Roger, S., Dominguez, S., Wibberley, C., and Caniven, Y., 2021. Non-linear fault damage zone scaling revealed through analog modeling, Geology, 49 (8): 968–972. doi: https://doi.org/10.1130/G48760.1 -> PDF

-> Mayolle, S., Soliva, R., Caniven, Y., Wibberley, C., Ballas, G., Milesi, G., Dominguez, S., 2019. Scaling of fault damage zones in carbonate rocks. Journal of Structural Geology, 124, 35–50. https://doi.org/10.1016/j.jsg.2019.03.007.

See also:

-> Choi, J.-H., Edwards, P., Ko, K., Kim, Y.-S., 2016. Definition and classification of fault damage zones: A review and a new methodological approach. Earth-Science Reviews, 152, 70–87. https://doi.org/10.1016/j.earscirev.2015.11.006.

-> Schlagenhauf, A., I. Manighetti, J. Malavieille, and S. Dominguez, 2008. Incremental growth of normal faults : Insights from a laser-equipped analog experiment : Earth And Planetary Science Letters, v. 273, p. 299-311, https://doi: 10.1016/j.epsl.2008.06.042.