Fault Damage Zone Research Articles

Analysis of the influence of joint direction on production optimization in enhanced geothermal systems

Heat extraction from geothermal reservoir by circulating cold water into a hot rock requires an amount of fluid pressure, which is capable of inducing fault opening. Although stress change promotes the potential of fault failure and reactivation, the rate at which fluid pressurization within the fault zone generates variations in pore pressure as fault geometry changes during geothermal energy production have not been thoroughly addressed to include the effects of joint orientation. This study examines how different fault/joint models result in different tendency of injection-induced shear failure, and how this could influence the production rate. Here, a numerical simulation method is adopted to investigate the thermo-hydro-mechanical (THM) response of the various fault/joint models during production in a geothermal reservoir. The results indicate that pore pressure evolution has a direct relationship with the evolution of production rate for the three joint models examined, and the stress sensitivity of the individual fault/joint model also produced an effect on the production rate. Changing the position of the injection well revealed that the magnitude of shear failure on the fault plane could be controlled by the hydraulic diffusivity of fluid pressure, and the production rate is also influenced by the magnitude of stress change at the injection and production wells. Overall, the location of the injection well along with the fault damage zone significantly influenced the resulting production rate, but a more dominating factor is the joint orientation with respect to the maximum principal stress direction. Thus, the rate of thermal drawdown is affected by pore pressure elevation and stress change while the fault permeability and the production rate are enhanced when the joint’s frictional resistance is low.

Denoising Surface Waves Extracted From Ambient Noise Recorded by 1‐D Linear Array Using Three‐Station Interferometry of Direct Waves

AbstractWe develop an automatic workflow for enhancing surface wave signals in ambient noise cross correlations (ANCs) calculated for a one‐dimensional (1‐D) linear array. The proposed array‐based method is applied to a 1.6 km‐long dense linear nodal array crossing surface traces of the San Jacinto fault near Anza, California. Fundamental and higher modes of surface waves are observed in ANCs of the nodal array. After attenuating the surface wave overtones by applying a frequency‐dependent tapering window to the ANCs, signals dominated by the fundamental mode surface wave are then enhanced through a denoising process based on three‐station interferometry of direct waves. The signal‐to‐noise ratio is significantly increased at high frequencies (>2 Hz) after denoising. Phase travel times are extracted reliably in the frequency domain for the period ranges of 0.3–1.2 s and 0.3–1.6 s for Rayleigh and Love waves, respectively. The corresponding period‐dependent phase velocity profiles derived from the eikonal equation reveal high‐resolution details of fault zone internal structures beneath the array. A broad (500–1,000 m) low‐velocity zone that narrows with increasing period is observed, illuminating a flower‐shaped structure of the San Jacinto fault damage zone.

Asymmetric Brittle Deformation at the Pāpaku Fault, Hikurangi Subduction Margin, NZ, IODP Expedition 375

AbstractQuantifying fault damage zones provides a window into stress distribution and rheology around faults. International Ocean Discovery Program (IODP) Expeditions 372/375 drilled an active thrust splay fault within the Hikurangi subduction margin. The fault, which is hosted in Pleistocene clastic sediments, is surrounded by brittle fractures and faults as well as ductile deformation features. We find that fracture density in the damage zone enveloping the fault is asymmetric, with the hanging wall showing greater overall fracture density and at greater distances from the fault than the footwall. Furthermore, the peak in fracture density occurs within an area of mesoscale folding and localized slip in the hanging wall rather than adjacent to the main fault zone. We attribute the asymmetry in damage to disparate deformation histories between the hanging wall and footwall, greater ductile deformation within the footwall, and/or dynamic stress asymmetry around a propagating rupture. Damage asymmetry is common at shallow depths in subduction zones and influences the mechanical and hydrological properties of the fault, such as channelized fluid flow and fault stability. Finally, we demonstrate that subduction zone faults show similar damage‐displacement scaling as continental faults.

Mesoscale structures in the Rough Creek Graben, U.S. Midcontinent craton: An integrated DEM analysis, structural modeling, and field-based approach

The Rough Creek Graben (RCG) within the U.S. Midcontinent craton possesses some of the world's longest cave and karst landforms such as the Mammoth Cave. Despite the graben's position at the junction of two active seismic zones, the New Madrid Seismic Zone (NMSZ) and the Wabash Valley Seismic Zone (WVSZ), the RCG is seismically inactive. Here, we map mesoscale geologic structures within the RCG for understanding structural deformation and styles. We integrated DEM (Digital Elevation Model)-based structural analysis, field-based structural documentation, 3-d models, and GIS-based spatial analysis. Results of this integrated study reveal the presence of dominantly E-W-striking mesoscale structures with secondary NW-SE- and NE-SW-strikes. Many of these structures are documented in the field as high-angle normal faults with distinct fault cores, displacements, and damage zones. Most of the mapped structures are likely originated from extensional stresses associated with the development of the RCG. The presence of less frequent contractional structures (reverse and thrust faults, and folds) suggests subsequent strain localization within this graben. Although most of these structures are seismically inactive, understanding their deformation and kinematics is crucial for synthesizing any reactivation in response to a large magnitude earthquake originating from either the NMSZ or the WVSZ. The findings of this study not only advance our knowledge of the mesoscale structures in a midcontinent cratonic rift but also help evaluate potential earthquake hazards and risks. Moreover, positive correlations between these mesoscale structures and the karst systems suggest a relationship between these structures and the ultimate morphology of conduit systems that assisted in developing these karst landforms.

Down-dip circulation at the united downs deep geothermal power project maximizes heat recovery and minimizes seismicity

Fault damage zones potentially represent native permeable channels within otherwise ultra-tight igneous formations that may be used to promote fluid circulation for convective heat recovery. The United Downs Deep Geothermal Power (UDDGP) project aims to recover geothermal energy by directly injecting, circulating then recovering fluids from such a fault. The UDDGP project injects fluid into the fault at 2500 m where rock temperature is ~75 °C – 80 °C and recovers the injectate from 4500 m, where the bottom hole temperature is predicted to be 190 °C. We explore such down-dip circulation through numerical modeling to determine the anticipated temperature and longevity of the thermal recovery and the potential for induced seismicity (fault reactivation) by contrasting response for up-dip circulation. The results reveal that down-dip circulation not only maximizes water temperature and flow rate at the outlet but also simultaneously suppresses fault reactivation over the long term. In down-dip circulation, fault permeability in the critically stressed shallow fault damage zone is more greatly enhanced than that of the deep fault as a result of the strong injection-induced thermal stress. Fault sealing is breached as a result of reactivation in the shallow fault, prompting transverse fluid penetration and subsequent fluid circulation in the footwall. Switching the injection scheme from down-dip circulation to up-dip circulation, while maintaining an identical injection rate, leads to a drastic increase in the cumulative number of seismic events, with the average magnitude increasing from 1 to 2. This reversed well configuration (circulation bottom to top) also reduces power output from 6 to 7 MWthermal to 2 MWthermal, due to reduced enthalpy and flow rate in the production well. The outcomes from this study confirm the preferred design of the well pattern as circulating top to bottom, to both maximize heat recovery and limit induced seismicity. A lower production pressure is necessary to avoid pressure build-up inside the fault . Production pressure at 15–20 MPa could ensure both high cumulative power generation and a stable fault state.

Numerical Modeling of Complex Stress State in a Fault Damage Zone and Its Implication on Near‐Fault Seismic Activity

AbstractNear‐fault severe seismic activity is frequently observed when excavating deep underground during various engineering projects. The present study elaborates a method to simulate the heterogeneous, complex stress state inside fractured rock mass in a fault damage zone, based on the concept of equivalent elastic compliance tensor and boundary traction method. After verifying the method with the discrete element method, the stress state of the fault damage zone composed of millions of fractures was analyzed. It is then demonstrated that the method is capable of simulating stress anomalies that could be the cause for severe seismic activity. The quantitative analysis indicates that the discrepancy between local and regional fracture densities is one of the dominant factors that produce an intensely stressed region, suggesting the use of local‐to‐regional fracture density ratio as an index to evaluate the potential for stress anomalies and its degree. Furthermore, a model parametric study on fracture properties was carried out to provide insight into the discrepancy in the likelihood of abnormal stress state between jointed host rock and fault damage zone. Lastly, b‐value was computed from thousands of possible seismic events identified in the model, by assuming seismic efficiency and scaled energy. The computed b‐value was found to fall within a reasonable range estimated from induced seismicity in deep underground mines. These results imply that the proposed method may be used to obtain a first‐order approximation of the complex rock mass stress state in fault damage zones and to evaluate the severity of seismic activity.

Architectural characteristics and evolution sequences of different types of faults in extensional basins—evidence collected from cores in the Dongying Depression

Drilling cores taken from faults in the Bohai Bay extensional basin are investigated to identify the architectural characteristics and tectonic evolution of different types of faults in the clastic strata. Growth and normal faults divided by the diagenetic stage and fault throw exhibit varied structural characteristics. Regardless of the size of growth faults, the fault cores and damage zones are visible. Meanwhile, small normal faults primarily comprise damage zones with slip surfaces (including fractures and deformation bands). The breccias in the fault core of the growth fault rotate under the lubrication of an unconsolidated mudstone, resulting in a mudstone-wrapped state. However, the normal fault is formed in the consolidated stage of the mudstone layer, and the number of its structural breccias are mainly reduced by small faults or fractures lacking mudstone lubrication. The deformation mode directly determines the type of fault architecture and the propagation mode of the fault core. These normal faults (including vertical, flower, and conjugate fault zones) are the products of pure shear, wherein the σ1 principal stress axis is basically vertical, and they constitute a series of fractures or deformation bands without a unified main slip surface. These growth faults can form a complete fault architecture under simple shear, in which the σ1 principal stress axis is gradually inclined to a certain extent with an increase in the fault width. The difference between the two types of faults is essentially attributed to the fault core being easier to propagate under simple shear than under pure shear. In terms of structural evolution, the growth fault evolves from the original fault core with hanging-wall damage zone into a complete fault structure with a fault core and two damage zones. The normal fault is composed of the large conjugate fracture system or multiple single slip surfaces, which may form the fault core when pure shear changes to simple shear or when the displacement is large. The differences in the structural characteristics, deformation modes, and tectonic evolution of growth faults and normal faults would inevitably affect later fluid migration.

Fault controls on spatial variation of fracture density and rock mass strength within the Yarlung Tsangpo Fault damage zone (southeastern Tibet)

Recent technological developments including Unmanned Aerial Vehicles, terrestrial laser scanning, photogrammetry and point cloud analysis software tools greatly enhance our ability to investigate the relationship between faulting and the spatial geometrical and mechanical characteristics of a rock mass controlled by faulting. Using the Yarlung Tsangpo (YLTP) Fault of southeastern Tibet as a case study, we propose the procedures, investigation approaches, evidence and criteria for quantitatively defining the threshold distance for damage zones combining the spatial variations of fracture density, rock mass strength and rockfall inventory in this study. Based on the study, the extent of threshold distance of damage zone of the YLTP Fault is estimated as 5.9 ± 0.6 km, which has been compared with published data from the evidence of thermal effects related to exactly the same fault and shows a good match. Five predominant joint sets were identified in the study area. Influenced by tectonics, the relationship between mean spacing of the joint sets with distance from the fault core shows a strong positive power relationship. It is consistent with the result of the exponential relation between the mean size of fallen blocks and distance from the fault core that is obtained independently. We conclude that fracture density (the joint volumetric count, Jv) and cohesion of the rock mass show power curve relations with distance within the damage zone. The fault first affects the characteristics of rock mass structures, and then the characteristics of the rock structures influence the stability of slope leading to rockfall.

The role of mechanical stratigraphy on CO2 migration along faults – examples from Entrada Sandstone, Humbug Flats, Utah, USA

Bleaching patterns along a faulted succession provides new insight into fluid migration pathways controlled by the fault zone and mechanical stratigraphy. In order to investigate this effect, we logged stratigraphic layers in Humbug Flats, Utah, USA both in host rocks and faulted rocks, where a 40 m offset normal fault cuts through the Entrada Sandstone. We measured petrophysical and mechanical properties using TinyPerm II, Schmidt Hammer and laboratory core testing for selected layers. Bleaching has been identified as a proxy for fluid migration parallel to bedding, which fades out with distance from faults. A new observation is that fracturing of bed interfaces contributes to horizontal flow out from the fault zone. Longer-distance comprehensive bleaching dominates the most permeable beds (units), hosting porosity-controlled deformation band associated to fault zones. Fractures prevail in low permeability and stiffer units, where isolated fracture parallel bleaching haloes are observed within fault damage and process zones. Strong correlation is observed between permeability/porosity and stiffness/strength properties for the tested samples of different layers along the stratigraphic logs. For the studied fault, fractures provide important conduits for vertical flow across low permeability and stiff layers, whereas the more porous layers provide bedding-parallel flow paths out of the fault zone.

Analysis of Seismic Damage Zones: A Case Study of the Ordovician Formation in the Shunbei 5 Fault Zone, Tarim Basin, China

Fault damage zone has an important influence on subsurface fluid flow and petrophysical properties. Therefore, it is of great significance to study the characteristics of fault damage zone for oil and gas development of ultra-deep carbonate formation. This study uses seismic data and the derived variance attribute to identify two types of damage zones and analyze the spatial geometric characteristics of the damage zones. The results show that the type 1 damage zone is wider than the type 2 damage zone. The width of damage zones distributed on both sides of the Shunbei 5 fault core shows obvious asymmetry, and the damage zone width and throw conforms to the typical power-law distribution on the log-log plot. We discuss the factors affecting the width of the damage zone and its formation process. Finally, we discuss the influence of the damage zones on oil and gas exploration. It seems that the seismic variance attribute is a useful technique for characterizing the ultra-deep strike-slip fault damage zones.

Pure Void Space and Fracture Pore Space in Fault-Fractured Zones

Fault-fractured pore space is complex and difficult to predict and evaluate. For a single independent ramp-flat fault-bend fold structure, the pure void space between two fault walls equals the integrated fracture pore spaces within the fault damage zone if it were concentrated on the fault plane. Using an area balancing technique and geometrical relationship, we have developed a two-dimensional (2D) model to calculate the pore space of fractures associated with fault development. The development and distribution of fault detachment voids or fault fracture pore space are controlled by the physical properties of the deforming medium, mechanics of deformation, and geometry of a fault-ramp structure. We demonstrate how concordant or discordant folding of the fault wall rock affects the nature of fault-fracture pore space. The pure void space and fracture pores in the fault zone can be quantitatively described by the following parameters: initial ramp angle and height, overlap ramp length, throw and slipping displacement, stack thickness, curvature and derivation of the angle between bed and fault plane (Rθ), and dip isogons.Rθreflects the conformity of two opposite fault sections and the folding accordance of two walls, and it is a key element for the development and distribution of fracture pore space in a fault zone. Furthermore, we observed natural outcrops supporting and validating our model assumptions in the foreland fault system, Central China.

Effect of Stress Interactions on Effective Elasticity and Fracture Parameters in the Damage Zones

Elastic interactions between fractures will greatly affect the effective elasticity, which, in turn, will reshape the effective fracture parameters. The disturbance will be more complex in the fault zone due to the complicated fracture distributions. This problem is addressed by the comparison of two types of solutions: one containing the stress interaction while the other one excluding the stress interaction. The gap between the two solutions allows the quantitative estimation of stress interactions on elasticity. Furthermore, based on the orthorhombic assumption for fracture clustering in the damage zone, the effect of stress interaction on the equivalent fracture parameter is estimated. We first characterize the fracture parameters in the fault damage zone considering more realistic distributions of fractures. Then, a series of numerical simulations are conducted to study the effective parameters of the fractured model. Finally, assuming the orthorhombic system of the fracture clustering, we invert the crack density and validate the accuracy of the inversion through the incidence angle seismic velocities. Our numerical results suggest that the size of fractures will determine the dominant type of stress interactions, and thus significantly reshape the effective properties of the models regardless of the spatial distribution of the fracture. Furthermore, the stress interactions tend to underestimate the fracture density for models containing long fractures but generate a relatively satisfactory inverted fracture density for short fractures.

Progressive Damage Behaviours of Triaxially Confined Rocks under Multiple Dynamic Loads

Investigation of rock progressive damage under static confinement and strain rates facilitates the generation mechanism of natural fault damage zones. A triaxial Hopkinson bar apparatus is used to perform dynamic triaxial compression tests to examine the damage and degradation process of rocks subjected to multiple impacts. Dynamic mechanical properties are determined under a static triaxial pre-stress of (30, 20, 10) MPa and multiple dynamic loadings, with the repetitive impact velocity of 27 m/s and strain rates from 50 to 150/s. The acoustic characteristics are identified by ultrasonic measurement to qualify the damage values. The micro-crack parameters, including crack area and volumes are detected using synchrotron X-ray micro-computed tomography (μCT) to characterize the progressive damage. In addition, the microcrack orientation, density and fractal dimension are analysed from thin section. Experimental results show that dynamic stress-strain curves can be divided to elastic, nonlinear deformation and unloading phases. Dynamic peak stress, Young’s modulus and ultrasonic wave velocity decrease with increasing impact times. The high frequency of ultrasonic wave is filtered by the induced microcracks. The progressive damage and evolution of fracture networks are associated highly with microcrack initiation, propagation, branching and coalescence. Shear bands are commonly generated in granite, and tensile cracks are dominant in marble, while sandstone is mainly failed by compaction and deformation band. The absorbed energy of rock increases nonlinearly with increasing crack surface and volume. Besides, microcracks propagate primarily along the maximum principal stress; the density and fractal dimension exhibit an anisotropic distribution controlled by true triaxial confinement and dynamic impacts.

Investigation on Two Mw 3.6 and Mw 4.1 Earthquakes Triggered by Poroelastic Effects of Hydraulic Fracturing Operations Near Crooked Lake, Alberta

AbstractA coupled approach of fluid flow and geomechanics is proposed in this study to quantitatively understand the hydraulic fracturing‐induced poroelastic effects that activate pre‐existing faults and trigger earthquake swarms near Crooked Lake, Alberta. The 3D poroelastic simulation of the seismogenic fault zone is then conducted to characterize the pressure diffusion and stress perturbation that led to fault activation. Results show that the poroelastic effects on the high‐permeable damage zones of a conductive‐barrier fault triggered the sequential activation of the seismogenic fault in the basement and Winterburn Formation. In addition, the high‐permeable damage zones act as conduits for pore pressure diffusion along the fault, whereas the fault core functions as a barrier to prevent crossing flow. The poroelastic effects in terms of pressure diffusion and stress perturbation in response to fluid injection facilitated the fault slip and hence triggered the Mw 3.6 earthquake in the basement formation 40 days after the fracturing operations. Moreover, the mainshock created negative Coulomb failure stress changes, inhibiting further fault slip in the basement formation. Subsequently, the stimulation of another well facilitated poroelastic effects on the fault damage zone in top Winterburn Formation, reactivating the same fault with an Mw 4.1 earthquake 8 days after the initiation of treatments. It is essential to optimize the injection site selection near the existing faults to reduce risks of the induced earthquakes near Crooked Lake.

On the Effect of Grain Fragmentation on Frictional Instabilities in Faults With Granular Gouge

AbstractThe evolution of frictional strength during stick‐slip dynamics of a fault system is key to understanding earthquake nucleation and rupture patterns. In mature faults, granular gouge is produced by wear, comminution or fragmentation during tectonic movements. In this work, we introduce a fragmentation model in the simulation of a sheared granular fault to explore the influence of grain breakage on the stick‐slip dynamics. With fragmentation of highly stressed particles, the fault frictional strength increases accompanied by many small slip events triggered by particle breakage. The small fragments produced by particle breakage are not only stronger and more difficult to break, but they also change the distribution of contact forces, leading to the strengthening of the fault system. Based on statistical analyses on the size distribution of slip events with different particle strengths, we find that with lower particle strength, slip events become more correlated with particle fragmentation events and that the number of large slip events decreases. In addition, our analyses on the relationship between slip and particle fragmentation events reveal three types of correlations: In the first and second types, particle fragmentation events trigger micro or major slips, respectively. In the third category, large‐scale particle fragmentations take place at the end of large slip events owing to stress localization during post‐slip particle rearrangements. Our results in this work highlight the role of micromechanics of particle fragmentation in the failure of fault damage zones and help in understanding the relation between particle breakage and frictional failures.

Shear Zone Development in Serpentinized Mantle: Implications for the Strength of Oceanic Transform Faults

AbstractOceanic transform faults display fewer and smaller‐magnitude earthquakes than expected for their length. Several mechanisms have been inferred to explain this seismic slip deficit, including increased fault zone damage resulting in elevated fluid flow, and the alteration of olivine to serpentine. However, to date, these possible mechanisms are not supported by direct observation. We use micro to kilometer scale observations from an exhumed oceanic transform fault in the Troodos Ophiolite, Cyprus, to determine mineral‐scale deformation mechanisms and infer likely controls on seismic behavior of serpentinized lithospheric mantle in active oceanic transform faults. We document a range of deformation fabrics including massive, scaly and phyllonitic serpentinite, attesting to mixed brittle‐ductile deformation within serpentinite shear zones. The progressive development of a foliation, with cumulative strain, is an efficient weakening mechanism in scaly and phyllonitic serpentinite. Further weakening is promoted by a transition in the serpentine polytype from lizardite‐dominated massive and scaly serpentinites to chrysotile‐dominated phyllonitic serpentinite. The development of a foliation and polytype transition requires dissolution‐precipitation processes. Discrete faults and fractures locally crosscut, but are also deformed by, foliated serpentinites. These brittle structures can be explained by local and transient elevated strain rates, and play a crucial role in strain localization by providing positive feedback for dissolution‐precipitation by increasing permeability. We propose that the evolution in structure and deformation style documented within the serpentinized lithospheric mantle of the Southern Troodos Transform Fault Zone is a viable explanation for the dominantly creeping behavior and long‐term weakness of oceanic transform faults.

Nonlinear fault damage zone scaling revealed through analog modeling

AbstractFault damage zones strongly influence fluid flow and seismogenic behavior of faults and are thought to scale linearly with fault displacement until reaching a threshold thickness. Using analog modeling with different frictional layer thicknesses, we investigate damage zone dynamic evolution during normal fault growth. We show that experimental damage zone growth with displacement is not linear but progressively tends toward a threshold thickness, being larger in the thicker models. This threshold thickness increases significantly at fault segment relay zones. As the thickness threshold is approached, the failure mode progressively transitions from dilational shear to isochoric shear. This process affects the whole layer thickness and develops as a consequence of fault segment linkage as inferred in nature when the fault matures. These findings suggest that fault damage zone widths are limited both by different scales of mechanical unit thickness and the evolution of failure modes, ultimately controlled in nature by lithology and deformation conditions.

Microstructures of epidote-prehnite bearing damaged granitoids (northern Victoria Land, Antarctica): clues for the interaction between faulting and hydrothermal fluids

Exhumed faults in granitoids along the Lanterman Fault-Rennick Graben Fault system (northern Victoria Land, Antarctica) show superposed ductile to brittle deformation and pervasive hydrothermal fluid-rock interaction. These processes triggered multiple brittle slip events producing crosscutting epidote and prehnite-rich fault veins, ultracataclasites and pseudotachylytes of crushing origin. Combined microstructural and minerochemical investigations on fault damage zones show three types of alteration: (i) albitization of K-feldspar and Ca-plagioclase; (ii) crystallization of prehnite and calcite in veins; (iii) alteration of magmatic phases by secondary hydrous minerals (e.g. chlorite, white mica, epidote and prehnite). The fault experienced various episodes of strain weakening and hardening, due to alteration of minerals and precipitation of epidote and prehnite within ultracataclastic intervals, at decreasing temperature conditions (200 < T°C < 450) and varying CO2 fugacity of the fluids. Cyclic crystallization of epidote/prehnite within the fault cores caused cementation and locking of faults, concentration of deformation at weaker horizons and a progressive broadening of the fault zone. Our results indicate that multiple co-seismic slip and syntectonic fluid flow very likely occurred prior to the Cenozoic brittle reactivation of inherited anisotropies in the northern Victoria Land crust along the Lanterman Fault-Rennick Graben Fault system and underlines its high potential for polyphasicity.

Quantitative anisotropies of palaeopermeability in a strike-slip fault damage zone: Insights from micro-CT analysis and numerical simulations

Fracturing and damage around faults related to seismogenesis can enhance hydrothermal fluid percolation, causing mineral precipitation. This study uses hydrothermally sealed microfractures across an ancient exhumed fault to unravel the 3D-spatial distribution of fault damage and related anisotropy in permeability. We studied the fault damage zone of the Jorgillo Fault, a left-lateral strike-slip fault, exposed by ca. 20 km in the Atacama Fault System, northern Chile. The study was conducted by addressing the 3D-spatial distribution of the microfracture network through X-ray micro-computed tomography and palaeopermeability modeling using a computational fluid dynamic approach, thus assessing mm-scale fault-related permeability tensors. 3D modeled fault-directed permeability ellipsoids on both sides of the fault core are transverse anisotropic, where palaeopermeability (matrix permeability) in the fault-parallel plane is higher than across-strike of the Jorgillo Fault (2.4 and 1.9 times in the eastern and western block of the fault, respectively). Modeled 3D permeability values (ca. 10−11 to 10−15 m2) show a mean overestimation factor of 8.4 of the estimated 2D permeability (ca. 10−9 to 10−12 m2). Permeability anisotropy distribution in the damage zone is related to off-fault damage generation, and could be explained by tip propagation fault growth and dynamic rupture due to earthquakes under the fault-valve mechanism. Whereas the fault would act as an impermeable seal except for post-failure, when it became highly permeable for fluids.

Constraining fault architecture and fluid flow using crustal noble gases

Deformational features, such as faults, strongly affect the pathways, rates, and length-scales of fluid flow in sedimentary basins. The hydrologic properties of faults vary greatly, allowing them to exhibit hydrologic behaviors spanning the gamut from open and conductive pathways for fluid flow to closed barriers to fluid flow. As a result, determining the role of faults with respect to fluid flow and their impact on geologic fluid migration throughout the life cycle of sedimentary basins remains challenging. Previous studies interrogate fault behavior using structural characteristics of fault cores and damage zones or the geochemistry of fluid inclusions and mineral veins associated with these features. Here, we evaluate the utility of crustal noble gases as tracers of fluid flow in fault systems by examining the composition of a well-constrained fault system in the Northern Appalachian Basin. The Seneca Stone Thrust fault, located near Seneca Falls, NY, USA, displays ~5 m of offset in the Onondaga Limestone and Marcellus Shale. Significant loss of 4He and 21Ne* near the fault appears to occur prior to basin exhumation, likely resulting from compactive dewatering or hydrocarbon generation and migration. Subsequent cooling shifted fault function from acting as a conduit to acting as a barrier, allowing accumulation of crustal noble gases for extended geologic time (~140 Myr) apparently in a closed-system. We identify significantly lower [4He] and 4He/21Ne* in a discrete zone that includes the fault core and extends ~30 cm into the intensely fractured portion of the fault damage zone as compared to the rest of the fault damage zone (~5 m in width) and other nearby samples from the Marcellus Fm. in the quarry pavement. These results imply that the Seneca Stone Thrust fault has reverted to behaving like a localized conduit in recent geological time. We interpret the discrete zone of extreme 4He loss near the fault plane as evidence of recent focused fluid flow within the fault core at temperatures below 94 °C, possibly within the past ~45,000 years. Our data suggest that crustal noble gases can be used to evaluate the timing of crustal isolation and/or hydraulic communication in fault zones locally in the Marcellus Shale and potentially in other geological settings. Future work can further develop this approach to examine the long-term suitability of geological formations for permanent subsurface storage of CO2, other gases (e.g., H2), or nuclear waste, as well as to determine if various subsurface intervals represent zones of hydrocarbon accumulation and/or loss over geological time.