Width Of Damage Zone Research Articles

Seismic and outcrop-based 3D characterization of fault damage zones in sandstones, Rio do Peixe Basin, Brazil

Fault damage zones, composed of sub-seismic deformation structures, are difficult to detect using seismic data. Still, they can be related to fault throw, which is widely measured in the subsurface. This research employs a multiscale approach that integrates outcrop studies with seismic reflection data to investigate the attributes of fault damage zones affecting porous sandstones. We provide an in-depth understanding of the 3D fault zone volume, investigating correlations between geometric attributes of faults (width of damage zone, frequency of subsidiaries structures, and fault termination) in the outcrop and the fault throw in the subsurface, with insights from deformation mechanisms within the fault zone. The methods encompass 1D scanlines to constrain the damage zone width on the outcrop. At the same time, the subsurface analysis uses 3D seismic data, seismic attributes, and deep learning neural network (DNN) fault volumes to interpret fault geometries and quantify fault throw at different depths. Results show that the area presents a complex fault zone with multiple fault sets, deformation bands, and fracture corridors trending mainly NE-SW, NW-SE, and E-W. The integration of surface and subsurface fault data enabled the identification of two portions in each fault set outcropping at the fault tip for E-W/ESE-WNW-striking faults and the central part of the fault for NE-SW-striking faults. Faults with the greatest length do not outcrop with the largest damage zone width since they are outcropping the tip of the fault. The parallel faults overlap their damage zones, increasing the deformation zones in the affected sandstones. Fault throw and damage zone width present a positive correlation. This relation is affected by fault segments and subsidiary faults.

Faulted karst reservoir spaces in Middle-Lower Triassic carbonates, Qingjiang Region, Yangtze block, China

The complexity and heterogeneity of ultra-deep faulted karst reservoirs pose significant challenges for hydrocarbon exploration. In this study, we integrated remote sensing image analysis, field measurements, and core sample testing to evaluate the characteristics and controlling factors of carbonate reservoir spaces within the Middle and Lower Triassic formations along a regional strike-slip fault in the Qingjiang region of China. The strike-slip fault zone is composed of multiple fault cores and damage zones. Based on differences in damage zone width and linear fault density, the fault zone was subdivided into transtensional and transpressional segmentations. The carbonate reservoirs, primarily developed within the damage zones, consist of fractures, fracture clusters, and cavities. Detailed measurements of the carbonate outcrops were conducted to obtain geometric parameters of the reservoir spaces. Quantitative results indicate that in the transtensional segmentation, the reservoir is dominated by tensile fracture-cavity systems, characterized by larger fracture apertures (0.13–1.25 m), higher linear fracture density (0.38–8.37 m⁻1), and well-developed cavities (0.03–4.84 m2), which contribute to better fluid connectivity and storage capacity. In contrast, the transpressional segmentation is dominated by compressional fracture-fracture cluster systems, with longer fractures (0.11–12.52 m), smaller fracture apertures (0.01–0.94 m), and extensive fracture clusters development (0.18–17.87 m2), but with lower fluid connectivity and limited storage capacity. Mechanical testing results show that the average compressive strength in the transtensional segmentation (133.95 MPa) is significantly higher than that in the transpressional segmentation (70.28 MPa). In terms of mineral composition, the transtensional segmentation has a higher calcite content, whereas the transpressional segmentation is richer in dolomite and quartz. Based on the observed differences in reservoir space characteristics across the strike-slip fault zone, we discussed the combined effects of structural segmentation, formation thickness, rock mechanics, and brittle mineral content on reservoir space development. The study emphasizes that stress conditions (primary factor) and material properties (secondary factor) jointly control fluid migration and storage efficiency in the reservoirs. Additionally, we suggest that the outcrop studies in the Qingjiang region provide valuable geological analogs for faulted karst reservoirs, offering critical insights for improving the precision of carbonate reservoir exploration and optimizing production efficiency.

Extracting fault-zone structures using the virtual seismometer method: from theoretical to synthetic test

Summary Rocks near a fault plane are commonly damaged by multiple earthquake ruptures, forming damage zones. The damage zone is important structures controlling various properties of a fault, yet its fine scale (tens to hundreds of meters) structure is difficult to resolve with surface seismic observations. We propose to use earthquakes that occur at depth within a fault zone as virtual seismometers (VSs) and use surface observations to extract Green's function (GFs) between VS pairs (VSGFs) . This method resembles that of ambient noise tomography and the retrieved VSGFs are related to the structures between event pairs. In this study, we develop the theory about how to extract VSGFs using surface stations deployed across a fault zone. Firstly, we use a half-space model and Fresnel zone analysis to determine the upper and lower limits of the GF frequency band, which is controlled by the station spacing and aperture of a given seismic array. Then, for VS in a fault zone, we demonstrate that the VSGF can be retrieved by linear seismic arrays deployed across the fault, and that the VSGF is equivalent to waves emitted simultaneously from an array of mirror sources of one event and received by the other. Secondly, the half-space result is directly adopted to determine the corresponding frequency band in the damage zone situation. Thirdly, we analyze different combinations of VS pair geometry and conclude that a relatively larger VS distance (much larger than the damage zone width) is more effective to recover damage zone structures for the available frequency bands. In this situation, VSGFs are trapped waves, that is represented by the interference of mirror sources. In such a case, the trapped waves are equivalent to surface waves, which have dispersion features to extract damage zone structures. Finally, we adopt the VSGF method to the Ridgecrest earthquake aftershock monitoring array and use a profile of aftershocks to extract 6 pairs of VSGFs. The spatial variation of VSGFs may reflect the depth-dependent variation of damaged zone. Our analysis shows a promising direction to use VSGFs to extract spatial variations of fault damaged zones.

Quantifying damage zones width in strike-slip faults: Insights from a two-dimensional finite-element modeling approach

We use a two-dimensional finite-element modeling approach to investigate the structural evolution of strike-slip faults under five different confining pressures and determine how their damage zone widths vary depending on the kinematic. Three representative fault segments are modeled, accounting for movement obliquity forming in pure strike-slip, oblique convergence and oblique divergence scenarios. The elastoplastic constitutive model (Drucker-Prager) was employed to couple an associated flow rule into the initial stress state of a geostatic stage. We applied two empirical methods for determining the width of the damage zone from progressive model response. The first method involves using the plastic strain distribution curve's inflection points, to mathematically fixate the width. The second method uses the standard deviation obtained from a trend Gaussian/Normal probability distribution, with over 98% fit to the plastic strain curve data, simplifying the inference of the strain propagation along the model. The simulation results indicate inverse proportional relationships between confining pressure and both porosity and plastic strain intensity, while showing a direct proportional relationship with the damage zone width. For fault zone width quantification, results show convergence between the methods, revealing widths of 2.46–2.65 m for oblique divergence (displacement of 0.20 m), 2.65–3.55 m for pure strike-slip (displacement of 0.20 m), and 4.36–4.46 m for oblique convergence zones (displacement of 0.70 m). The pure strike-slip and the oblique convergence damage zone width results align well with outcrop observations of faults with similar kinematics from the literature, underscoring the significance of numerical modeling as a valuable tool for measure and study the mechanics of characterizing the nucleation of fault zones and quantifying the width of the damage zone.

Multi-stages of Paleozoic deformation of the fault system in the Tazhong Uplift, Tarim Basin, NW China: Implications for hydrocarbon accumulation

The Tazhong Uplift is an important petroliferous secondary structure unit in the Tarim Basin and has undergone multiple structural deformation. In the current study, we use 3D and 2D seismic data to investigate the nature of the fault system in the Tazhong Uplift. The deformation of the Cambrian sedimentary rocks in the Tazhong Uplift is generally neglected but noted in this study. We found that the Lower–Middle Cambrian rocks are involved in thrust faults. NE-trending weak zones acted as accommodation faults formed at the connective positions and bends of the deep thrust faults. The Middle Cambrian compressional stress stemmed from the northeastern margin of the Tarim Block, pointed SW, and caused the uprising of the Tabei Uplift. The second deformation occurred in the Late Ordovician. In this instance, the compressional stress stemmed from the SW direction of the Tazhong Uplift, which originated from the amalgamation of the Western Kunlun Terrane and Tarim Block. The main strike–slip fault surfaces and subordinate faults developed along the weak zones generated in the Middle Cambrian. The collision that occurred in the southeast part of the Tarim Block in the Middle Silurian to Middle Devonian resulted in the third deformation. The representative structures that occurred during this compressive deformation were the intensive NEE-trending thrust faults in the southeast of the Tazhong Uplift. The three stages of deformation played different and important roles in hydrocarbon accumulation. These strike–slip faults holding the high-productivity hydrocarbon wells generally experienced early-stage Cambrian deformation, had wide fracture damage zones in the Ordovician carbonate rock strata, and had weak en echelon faults in the Silurian to Middle Devonian clastic rocks.

Correlation Between the Effects of CO2 Laser and Histopathological Analysis in Vocal Cord Lesions: An Observational Study.

Lasers are based on the principle of light amplification by empowering atoms to store and emit light in a coherent form. Through their effect on tissues, lasers reduce hemorrhage allowing the surgeon to work in a clear field with precise removal of the tissues. Irradiation of the soft tissues by lasers produces thermal effects on the surrounding healthy tissues which can make histopathological examination difficult. Hence this study was done to find a correlation between adjustable parameters of CO2 laser and the extent of collateral thermal damage in the excised vocal cord lesions on histopathological examination and diagnosis. In this study, we enrolled 80 patients who were divided into 4 groups with different combinations of laser power and mode, used during transoral laser micro laryngeal surgery for the excision of vocal cord lesions and subsequent histopathological analysis to objectively measure the extent of thermal damage zone and subjectively assess histo-morphological effects of thermal damage in terms of grade of carbonization. The extent of the thermal damage zone is directly related to the power of the laser, but the mode of the laser had no relation with the thermal damage zone in our study. On subjective histo-morphological examination of excised lesions showed that both power and mode of laser have significant effects on tissue morphology. Continuous mode causes a significantly higher grade of carbonization as compared to the superpulsed mode of the laser. However, in our study it was seen that charring in no way affected the diagnosis in any of the biopsies examined whatever the power or mode of the laser used. The depth and width of the tissue thermal damage zone are mainly dependent upon the laser parameters (power and mode). Although considering the limitations of this study carried out in terms of sample size, it would be pertinent to mention here that further studies with larger cohorts need to be done to authenticate these results.

Along- and across-strike variation of damage zone parameters in the Kornos-Aghios Ioannis normal fault, Lemnos Island, Greece

The Kornos-Aghios Ioannis Fault (KAIF) is an extensional fault system that deformed Lower Miocene volcanic rocks and Middle Eocene to Lower Miocene turbidite sandstones at shallow depth (<1 km). The fault system is exposed for about 10 km and consists of two major NW-SE-striking segments connected by an E-W-striking one. This structural architecture provides the opportunity to study the variation of damage zone parameters (i.e., damage zone width, fracture density, attitude of deformation structures, and fracture connectivity) in wall, tip and intersecting damage zones. The >122 m wide tip damage zone has a width greater than the wall and intersecting damage zones (72.6 and 50.9m, respectively) and exhibits the highest average fracture density recognized along the entire KAIF. In tip and intersecting damage zones, damage structures exhibit a larger azimuthal variability and hence a greater fracture connectivity (average number of connections per branch (CB) 1.60 and 1.53, respectively) compared to wall damage zones (average CB value 0.88). Accordingly, significant along-strike variations of damage zone parameters occur in the absence of a clear dependence on fault displacement. Particularly, fault intersections and tip regions represent areas of enhanced secondary permeability potentially controlling fluid circulation in the subsurface.

On the importance of setting 3-D stress field in simulations of on- and off-fault deformation

SUMMARYDuring the last decades, many numerical models have been developed to investigate the conditions for seismic and aseismic slip. Those models explore the behaviour of frictional faults, embedded in either elastic or inelastic media and submitted to a far field loading (seismic cycle models), or initial stresses (single dynamic rupture models). Those initial conditions impact both on-fault and off-fault dynamics. Because of the sparsity of direct measurements of fault stresses, modellers have to make assumptions about these initial conditions. To this day, Anderson’s theory is the only framework that can be used to link fault generation and reactivation to the 3-D stress field. In this work, we look at the role of the 3-D stress field in modelling a 2-D strike-slip fault under plane-strain conditions. We show that setting up an incorrect initial stress field, based on Anderson’s theory, can lead to underestimation of the damage zone width by up to a factor of six, for the studied cases. Moreover, because of the interactions between fault slip and off-fault deformation, initial stress field influences the rupture propagation. Our study emphasizes the need to set up the correct initial 3-D stress field, even in 2-D numerical simulations.

Determination of fault damage zones in sandstone rocks using numerical models and statistical analyses

The width of fault damage zones is usually assessed through scaling relationships using fault displacements measured from outcrops. However, field data show a large dispersion, indicating that other variables, such as rock properties, can affect the development of damage zones. This article proposes a novel, practical and efficient methodology to predict the width of fault damage zones in sandstone reservoirs, including geomechanical properties’ impact. Firstly, a deterministic model based on elastoplastic relationships and the Finite Element Method (FEM) is defined to assess the damage zone width. Next, empirical correlations between the porosity and the geomechanical parameters are established. Then, planned numerical experiments considering specific domains for each input parameter are generated using the Design of Experiments (DOE) technique. The corresponding results are used to build response surfaces. Finally, a statistical analysis is carried out to define a regression model to express the relationship between the damage zone width, the rock porosity, and the fault displacement. The fit quality is adjusted considering the ANOVA table, Pareto charts, and regression analysis. The results from the regression models indicate a perfect fit with the information predicted by the deterministic model and field data.

Assessment of fault damage zones in carbonate rocks based on numerical and sensitivity analyses

Fault damage zones are structural geological domains that can impact a reservoir's fluid flow and geomechanical behavior during the exploration and production phases in the oil and gas industry. Several methodologies have been proposed in the literature to characterize fault damage zones considering earthquake data, geophysical methods and outcrop observations. Based on those data, statistical relationships have been proposed to predict and constrain the width of fault damage zones in the subsurface. Unfortunately, those relationships show significant data dispersion and raise uncertainties regarding the extension of damage zones. A possible explanation for such distribution is the lack of knowledge of other parameters such as lithology and associated diagenesis, depth of faulting, tectonic environment, fault propagation, fault segment linkage and deformation mechanisms. Numerical modeling has emerged as a promising alternative for understanding geological problems. However, few studies have focused on characterizing fault damage zones at depth. This study presents a novel methodology based on the finite element method (FEM) combined with elastoplastic models and fault displacement data to forecast damage zone widths around geological faults on carbonate rocks. Despite the assumptions and simplifications related to the fault shape and displacement profile, this study shows that the proposed models provide a better understanding of the deformation mechanisms that trigger damage zones. Moreover, a sensitivity study considering the proposed methodology shows that in addition to the fault displacements and the carbonate rocks' geomechanical properties, the location of measured damage zone with respect to the fault can significantly impact the assessment of damage zone widths.

Fault damage zone and its effect on deep shale gas: Insights from 3D seismic interpretation in the southern Sichuan Basin, China

Fault damage zones affect the preservation and enrichment of shale gas. Most fault damage zone studies are based on directly observed outcrops. In this study, we provide an example of fault damage zone analysis using 3D seismic data of deep shales buried more than 3500 m in the Sichuan Basin, China. The 3D seismic variance attribute and the cumulative frequency method were used to define the background value and then estimate the damage zone widths of different sizes of faults. We found that the damage zone widths range from 80 m to 1570 m, and trend to stabilize with the increase in displacements. The scattering caused by the relay zone can be interpreted as segment linkage mode and damage superposition related to subsidiary faults. The damage intensity decays following a power law with distance from the fault core, in agreement with field observations and the distribution of seismic variance. Damage zone widths display a significant asymmetry across the fault, and the hanging wall damage zone is generally wider than the footwall damage zone. We discuss the impact of the fault damage zone on shale gas preservation, which is related to the sealing of the direct caprock. It appears that the seismic variance attribute is an effective tool for subsurface characterization of fault damage zones and can help identify the location of deep unconventional energy.

The spatial characterization of stepovers along deeply-buried strike-slip faults and their influence on reservoir distribution in the central Tarim Basin, NW China

Strike-slip faults imaged on seismic reflection data from the central Tarim Basin clearly illustrate the spatial and geometrical characteristics of deeply buried (>6.8 km depth) stepovers and their influence on reservoir distributions in fractured Middle Ordovician carbonates. Characterization of along- and across-strike seismic profiles demonstrates that restraining and releasing stepovers characteristically produce pop-up and pull-apart structures, respectively, defined by systems of high-angle subsidiary faults with down-dip reverse or normal displacements. Along-strike vertical displacement and damage zone width measurements reveal that restraining stepovers typically exhibit much wider deformation zones and higher densities of fractures compared to releasing stepovers. Core samples and well logs show that the fractured reservoirs in restraining stepovers comprise stylolite meshes and mosaic breccias. Damage zones in narrower releasing stepovers are more prone to the development of cavities, dilational breccias, and high-angle open fractures. Both restraining and releasing stepovers are strongly asymmetric in the across-strike direction, except in the central part of fault overlap zones where the maximum vertical displacements are symmetrical. Restraining stepovers can typically be subdivided into four sectors separated by helicoidal intermediate-scale linking faults: two in the hanging walls and two in the footwalls. Hanging walls are normally wider, intensely fractured and exhibit better well performance compared to footwalls. Strain compartmentalization with localized zones of upward and downward displacement exist both within, and adjacent to releasing stepovers. The localized extensional zones are where cavities and open fractures are most likely best developed. Seismic reflection profiles and well logging show that certain intraclastic grainstone and packstone horizons in the Middle Ordovician carbonate host rock sequences are more favorable for fracture development. This leads to vertical partitioning of local fractured reservoirs due to the development of down-dip stepovers or bends localized close to major lithological boundaries. Thus interactions between lithological layering and tectonic processes cause further three-dimensional complexity in the reservoirs.

Architecture of intraplate strike-slip fault zones in the Yanchang Formation, Southern Ordos Basin, China: Characterization and implications for their control on hydrocarbon enrichment

Commercial discoveries in the Ordos, Tarim, and Sichuan Basins of China have aroused widespread interest in strike-slip fault-controlled reservoirs. The productivity of different wells, however, can vary within one strike-slip fault zone, suggesting that variability in fault zone architecture controls hydrocarbon enrichment. To date, very few studies have finely and quantitatively explored fault zone architecture in the southern Ordos Basin, inhibiting hydrocarbon development. With the goal of understanding the architecture variability and scale of strike-slip fault zones in the subsurface, we carried out a refined and quantitative study of faults in the Jinghe Oilfield in the southern Ordos Basin by integrating outcrops, wellbore cores, well logs, and 3D seismic data. We quantitatively constrained the subsurface damage zone boundaries according to the slope changes in the cumulative CFI (comprehensive fracture index log) curves at logging scale and the cumulative FSI (fault shape index seismic attribute) curves at seismic scale. We proposed a model highlighting the effects of fault segmentation, architectural configuration, and damage zone asymmetry on the variability of fault zone architecture. Results showed that strike-slip faults can be divided into transtensional, pure strike-slip, and transpressional segments along the fault strike, with transtensional and pure strike-slip segments dominant in the Jinghe Oilfield. These segments exhibited six types of geometric structures, forming complex architectures with multi-fault cores and multi-damage zones surrounding the main and secondary faults. Each segment was further complicated by three configurations of gouge, breccia, and damage zones along the fault dip. The damage zone asymmetry further increased the architecture variability, with the fracture density and damage zone width of the hanging wall being greater than those of the footwall. Quantitative analysis showed that the fault zone width was the greatest for transtensional segments, intermediate for transpressional segments, and the lowest for pure strike-slip segments. And there was a positive linear correlation between the fault core width and the fault zone width (including the fault core and damage zone). Fault segmentation and damage zone asymmetry affected the hydrocarbon enrichment along the fault strike and dip within one strike-slip fault zone. We conclude that damage zones in transtensional segments, particularly in hanging walls, are primary targets for hydrocarbon development in the southern Ordos Basin.

Efficient gradient enhancements for plasticity with ductile damage in the logarithmic strain space

We contrast different gradient-enhancements for plasticity-damage material models in the logarithmic strain space and compare them to reference models based on multiplicative plasticity. The models being compared include plasticity - gradient-damage, where the gradient-enhancement is applied on the local damage variable, and gradient-plasticity - damage with a gradient-enhanced plastic hardening variable. Thereby, gradient-plasticity proved to be able to simultaneously regularise plastic and ductile (plasticity-driven) damage localisation as confirmed by numerical localisation analyses. This appears to be especially advantageous for logarithmic strain space plasticity-damage, because of the observed plastic localisation even for the case of plasticity with hardening. Even though gradient-plasticity appears to be numerically more demanding, two numerical examples indicate a good robustness and mesh objectivity in the softening regime. Moreover, the internal length for plasticity is able to adjust the damage zone width, similarly to gradient-damage, however ensuring a priori that damage takes place exclusively inside the plastic zone.

Seismic Description of Deep Strike-Slip Fault Damage Zone by Steerable Pyramid Method in the Sichuan Basin, China

Large quantities of gas resources have been found in the Paleo-Mesozoic carbonate rocks in the Sichuan Basin. However, many wells cannot obtain high production in deep low porosity-permeability reservoirs. For this contribution, we provide a steerable pyramid method for identifying the fault damage zone in the Kaijiang–Liangping platform margin, which is infeasible by conventional seismic methods. The results show that steerable pyramid processing could enhance the seismic fault imaging and a series of NW-trending strike-slip faults are found along the trend of the carbonate platform margin. The steerable pyramid attribute presents distinct vertical and horizontal boundaries of the fault damage zone, and heterogeneous intensity of an un-through-going damage zone. The width of the fault damage zone is generally varied in the range of 100–500 m, and could be increased to more than 1000 m in the fault overlap zone, intersection area, and fault tips. Further, the fault damage zone plays a constructive role in the high gas production in the deep tight carbonate reservoir. The results suggest the steerable pyramid method is favorable for identifying the weak strike-slip faults and their damage zone. The width of the fault damage zone is closely related to fault displacement, and the much wider damage zone is generally influenced by the fault overlapping and interaction. The fractured reservoirs in the fault damage zone could be a new favorable exploitation domain in the Sichuan Basin.

The impact of faulting complexity and type on earthquake rupture dynamics

The statistical properties of seismicity are known to be affected by several factors such as the rheological parameters of rocks. We analysed the earthquake double-couple as a function of the faulting type. Here we show that it impacts the moment tensors of earthquakes: thrust-faulting events are characterized by higher double-couple components with respect to strike-slip- and normal-faulting earthquakes. Our results are coherent with the stress dependence of the scaling exponent of the Gutenberg-Richter law, which is anticorrelated to the double-couple. We suggest that the structural and tectonic control of seismicity may have its origin in the complexity of the seismogenic source marked by the width of the cataclastic damage zone and by the slip of different fault planes during the same seismic event; the sharper and concentrated the slip as along faults, the higher the double-couple. This phenomenon may introduce bias in magnitude estimation, with possible impact on seismic forecasting.

Using active source seismology to image the Palos Verdes Fault damage zone as a function of distance, depth, and geology

Fault damage zones provide a window into the non-elastic processes of an earthquake. Geological and seismic tomography methods have been unable to measure damage zones at depth with sufficient spatial sampling to evaluate the relative influence of depth, distance, and lithological variations. Here, we identify and analyze the damage zone of the Palos Verdes Fault offshore southern California using two 3D seismic reflection datasets. We apply a novel algorithm to identify discontinuities attributed to faults and fractures in large seismic volumes and examine the spatial distribution of fault damage in sedimentary rock surrounding the Palos Verdes Fault. Our results show that damage through fracturing is most concentrated around mapped faults and decays exponentially to a distance of ∼2 km, where fracturing reaches a clearly defined and relatively undamaged background for all examined depths and lithologies (450 m to 2.2 km). This decrease in fracturing with distance from the central fault strand exhibits similar functional form to outcrop studies. However, here we extend analysis to distances seldom accessible (∼10 km lateral distance). Separating the data by geologic units we find that the damage decay and background level differs for each unit, with the older and deeper units having higher levels of background fracturing and shallower exponential decays of fracturing with distance from the fault. Surprisingly, these differences in damage decay and background level trade-off result in a consistent damage zone width regardless of lithology or depth. We find that the damage zone has similar decay trends on both sides of the fault. When examining the damage zone at shorter (4 km vs 17 km) along strike distances, the damage zone has a more complex decay trend and at least two strands are resolvable.

Strain partitioning between ductile and brittle stratigraphy

The Sand Wash fault zone is a segmented and discontinuous fault system that strikes northwest to south east in the central part of the Uinta Basin. It is approximately 34 kilometers long with an uncommonly wide damage zone, typically 100 to 200 meters wide. Due to recent, rapid, and large-scale incision by the Green River and its tributaries, the Sand-Wash fault zone is well exposed in several closely spaced canyons. These canyon exposures allow mapping of the lateral relationships through panoramic photographs and surface kinematic descriptions. Most movement on the Sand Wash fault zone occurred in the late Eocene, but minor, more recent movement likely occurred. Evidence for fault timing includes strata-bound, syndepositional movement which occurred during Lake Uinta time (55 to 43 Ma BP) resulting in debris flows, slump blocks, and small (&gt;150 meters diameter) sag basins filled with poorly organized sediments. After lithification, elongate grabens formed with up to 33.5 meters of horizontal extension. Two styles of deformation are present. Brittle rocks, such as sandstone and limestone beds, are intensely fractured and faulted, whereas clay and organic-rich rocks are largely unfractured and unfaulted, with variably folded beds that have experienced some layer-parallel slip. Laterally, deformation is distributed up to 100 meters from the fault core, which is uncommonly large for faults with short lengths and little displacement. Vertically, displacement is concentrated in brittle sandstone and carbonate beds and rare in clay and hydrocarbon-rich units, such as the Mahogany oil-shale zone of the Eocene Green River Formation. The Mahogany oil-shale zone mostly displays ductile flow (granular flow) commonly forming small décollements between overlying and underlying units. Vertical displacement on separate fault segments is generally less than 5 meters and decreases down section, dying out completely around the top of the Mahogany oil-shale zone. In this paper we show evidence for syndepositional deformation along the Sand Wash fault zone, strain partitioning along décollement surfaces, fault surfaces that experience multiple deformational phases, pop-up blocks, and graben development. We also show that deformation on the fault zone is related to extension above a neutral surface of a larger fold. This larger fold is associated with a basement-rooted fault zone that moved during Laramide tectonism as the Uncompahgre uplift developed. The Sand Wash fault zone appears to have many similarities to the larger, and more deeply buried, Duchesne fault zone 25 kilometers to the north, and the more deeply eroded Cedar Ridge fault zone located 30 kilometers to the south. The high-resolution fault model, developed herein, is thus a good proxy for other complex fault zones in the Uinta Basin. Our model will be useful to oil and gas operators as they develop horizontal wells across this and other complex fault zones in the basin.

Regolith and host rock influences on CO2 leakage: Active source seismic profiling across the Little Grand Wash fault, Utah

Understanding carbon dioxide (CO2) reservoir to surface migration is crucial to successful carbon capture and sequestration approaches; especially fault/reservoir interactions under injection pressure. Through seismic imaging, we explore regolith and shallow stratigraphy across the Little Grand Wash fault. The presence of natural CO2 seeps, travertine and tufa deposits confirm modern and ancient fault-controlled CO2 leakage. We consider this an analogue for a long-failed sequestration site. We estimate bulk porosity and fracture density for host rock, regolith, and fault zone from petrophysical relationships. When combined with existing geochemical and geological data, we characterize a 60 m wide damage zone that represents the primary surface delivery channel for CO2 originating from reservoir depths. Within this damage zone, low seismic velocities suggest sediments have formed through host rock chemical dissolution or mechanical weathering. In contrast, velocities within the adjacent host rock are consistent with low fracture density clastic rocks. We measure anomalously high seismic velocities within the fault zone along one profile that best represents a sealed (cemented/plugged) low permeability, relic flow channel. This suggests that shallow fault zone permeability varies along strike. While regional stress changes may account for decadal- to millennial-scale changes in CO2 pathways, we speculate that the total fluid pressure has locally reduced the fault's minimum horizontal effective stress; thereby producing both low- and high-permeability fault segments that either block or promote fluid migration. Studying CO2 migration in this system can inform potential risks to future sequestration projects and guide monitoring efforts.

Radiated seismic energy and source damage evolution from the analysis of simulated dynamic rupture and far-field seismograms

SUMMARY We present results on radiated seismic energy during simulations of dynamic ruptures in a continuum damage-breakage rheological model incorporating evolution of damage within the seismic source region. The simulations vary in their initial damage zone width and rate of damage diffusion with parameter values constrained by observational data. The radiated energy recorded at various positions around the source is used to calculate seismic potency and moment. We also calculate the normalized radiated energy from the source, in a way that allows comparing between results of different simulations and highlighting aspects related to the dilatational motion during rupture. The results show that at high-frequencies, beyond the dominant frequency of the source ($( {f &amp;gt; 3{f}_d} )$, the damage process produces an additional burst of energy mainly in the Pwaves. This eccess of high-frequency energy is observed by comparing the radiated energy to a standard Brune's model with a decay slope of the radiated energy of n = 2. While the Swaves show good agreement with the n = 2 slope, the Pwaves have a milder slope of n = 1.75 or less depending on the damage evolution at the source. In the used damage-breakage rheology, the rate of damage diffusivity governs the damage evolution perpendicular to the rupture direction and dynamic changes of the damage zone width. For increasing values of damage diffusivity, dilatational energy becomes more prominent during rupture, producing a high-frequency dilatational signature within the radiation pattern. The high-frequency radiation pattern of the Pwaves includes two main lobes perpendicular to the rupture direction, reflecting high-rate local tensile cracking during the overall shear rupture process. Analysing the possible existence and properties of such high-frequency radiation pattern in observed Pwaves could provide important information on earthquake source processes.