Opening-mode Fractures Research Articles

The nature and origins of decametre-scale porosity in Ordovician carbonate rocks, Halahatang oilfield, Tarim Basin, China

At >7 km depths in the Tarim Basin, hydrocarbon reservoirs in Ordovician rocks of the Yijianfang Formation contain large cavities ( c . 10 m or more), vugs, fractures and porous fault rocks. Although some Yijianfang Formation outcrops contain shallow (formed near surface) palaeokarst features, cores from the Halahatang oilfield lack penetrative palaeokarst evidence. Outcrop palaeokarst cavities and opening-mode fractures are mostly mineral filled but some show evidence of secondary dissolution and fault rocks are locally highly ( c . 30%) porous. Cores contain textural evidence of repeated formation of dissolution cavities and subsequent filling by cement. Calcite isotopic analyses indicate depths between c . 220 and 2000 m. Correlation of core and image logs shows abundant cement-filled vugs associated with decametre-scale fractured zones with open cavities that host hydrocarbons. A Sm–Nd isochron age of 400 ± 37 Ma for fracture-filling fluorite indicates that cavities in core formed and were partially cemented prior to the Carboniferous, predating Permian oil emplacement. Repeated creation and filling of vugs, timing constraints and the association of vugs with large cavities suggest dissolution related to fractures and faults. In the current high-strain-rate regime, corroborated by velocity gradient tensor analysis of global positioning system (GPS) data, rapid horizontal extension could promote connection of porous and/or solution-enlarged fault rock, fractures and cavities. Supplementary material: Stable isotopic analyses and the velocity gradient tensor and principal direction and magnitude calculation are available at https://doi.org/10.6084/m9.figshare.c.4946046 Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs

Natural fractures in metamorphic basement reservoirs in the Liaohe Basin, China

The Archaean metamorphic basement reservoirs that are characterized by the development of natural fractures are the primary target for oil and gas exploration in the Liaohe Basin. Based on the analyses of outcrops, cores, thin sections, imaging logs, and laboratory measurements, tectonic fractures are the dominant type of natural fractures, and their development is mainly influenced by petrology and faulting. Tectonic fractures are more developed in metamorphic rocks with a higher content of brittle minerals, while fracture orientations are primarily parallel or almost parallel to the fault strikes. Opening-mode fractures, which account for more than 65% of the total reservoir porosity, provide the primary storage space for metamorphic basement reservoirs. Besides, opening-mode fractures serve as the major pathways for fluid flow throughout the reservoirs and ultimately determine the quality of these reservoirs. Therefore, the discrepancies in fracture development are speculated to be the primary reason leading to the vertical stratification of reservoirs. According to the degree of fracture development, various rock types in metamorphic basement reservoirs can be arranged in order as leptite, granulite, migmatitic granite, gneiss, lamprophyre, diabase, and amphibolite. In this sequence, the former rocks are more likely to become effective reservoirs, while the latter ones are usually inner interlayers. However, the lithology boundary of reservoir rocks and inner interlayers is not definite in this sequence. In a specific metamorphic basement reservoir, the rocks that can become reservoirs have a lower limit in the sequence. But for numerous metamorphic basement reservoirs in the Liaohe Basin, there is no lower limit for rocks to become hydrocarbon-bearing reservoirs.

Effects of bedding planes on fracture behavior of sandstone under semi-circular bending test

The mechanical behavior of layered rock is seriously influenced by the bedding planes that have lower tensile strength and shear strength than rock matrix. We conducted experimental and numerical three-point-bending tests on semi-circular sandstone specimens with an opening mode fracture to study the influences of bedding parameters on their fracture behavior. Experimental study mainly focuses on the influences of the inclination angle between loading direction and bedding plane on fracture load, fracture patterns and failure process. The numerical simulations based on cohesive element method aim to explain how the shear strength, tensile strength and spacing of bedding planes influence the fracture load, crack propagation and fracture patterns. Results show that stronger bedding strength, larger bedding spacing or larger inclination angle will induce a larger fracture load. The anisotropy of sandstone becomes stronger for the weaker bedding strength and larger bedding spacing (≤5.5 mm). The fracture patterns of sandstone are greatly governed by the inclination angle and partly controlled by bedding strength and bedding spacing, which is exceptional for the specimen whose inclination angle is 0°. In addition, the length percentage of fracture along the bedding plane and the proportion of tensile or shear cracks are analyzed to further address the effects of bedding parameters on the fracture properties.

Fluid-driven fractures in granular media: Insights from numerical investigations.

We investigate the mechanisms of opening-mode fracture initiation in granular media. The study is based on a simulation of grain-scale fluid-grain interactions through a coupled numerical approach in which the discrete element method is used to solve for the mechanics of a solid granular medium, and computational fluid dynamics is used to model fluid flow and drag forces. We present benchmark problems with analytical solutions and validate this numerical model against experiments on a viscous-drag-driven cavity in the literature. Additional simulation results show fracture initiation mechanisms in a random granular packing subjected to constant boundary stresses and to fluid injection with a localized source. The dimensionless variable F_{s}/F_{sk} (ratio of seepage force F_{s} and skeletal force F_{sk}) incorporates the impacts of physical properties and injection parameters including fluid viscosity, injection velocity, grain size, and effective stresses, and it has been used as a criterion separating regimes of fluid invasion and drag-driven fracture opening. Our simulation results show that F_{s}/F_{sk} in combination with τ_{1} (ratio of diffusion time from hydromechanical coupling and injection time) serves as a prediction of fracture opening within granular packing. We suggest a simple criterion (F_{s}/F_{sk}>1 or τ_{1}>0.17) that is valid for various types of granular media and injection conditions to determine if fracture opening will occur. Among other applications, this study is useful to predict the initiation and propagation of fractures in natural sediments.

Mechanical control on patterns of overburden deformation induced by dyke-fed sandstone intrusions: Insights from numerical experiments

Conical sandstone intrusions, as a distinct type of hydrocarbon reservoir, remain poorly understood regarding their emplacement mechanics. Here, we report a numerical modelling study of conical sandstone intrusions using the two-dimensional discrete element method. We built numerical models that contain bonded elastic particles with predefined mechanical properties in an open box as the overburden. A thin tube filled with unbonded particles, and connected to the upper box, was used to model the feeder dyke. The dynamic behavior of the assembly was enabled by the displacement of a driving wall that defines the lower boundary of the feeder dyke. The results show that the model composed of soft materials produced a pair of conical, opening-mode fractures in the host sediments as the result of tensile stress concentrations in the fracture tip zones. The overburden deformation was largely localised within the sediments adjacent to the sandbody, and without the formation of a forced fold and significant uplift of the surface. In contrast, the model composed of stiffer materials produced conical fractures that have closed lower segments and opening upper segments with a reverse sense of shear. The intrusion also caused a forced fold in the overburden, with a vertical opening-mode fracture generated in the fold hinge. The modelling results demonstrate that dyke-fed sand intrusions can significantly distort the local stress field and the overburden can be subjected to fracturing and/or folding due to differential compaction during gradual inflation of the intrusive sandbody. Importantly, deformation patterns of the overburden in response to sandstone intrusion largely depend on mechanical properties of the host sediments.

Natural fractures at depth in the Lower Cretaceous Kuqa Depression tight sandstones: identification and characteristics

AbstractThe Kuqa Depression in the northern Tarim Basin, NW China, is characterized by fault-controlled anticlines where natural fractures may influence production. Natural fractures in the Lower Cretaceous tight sandstones in the depression have been studied using seismic profiles, borehole images, cores and thin-sections. Results show that thrust faults, two types of opening-mode macrofractures and two types of microfractures are present. Thrust faults were generated during Cenozoic N–S-directed tectonic shortening and have hydraulically linked Jurassic source rocks and Cretaceous sandstones. Opening-mode fractures can be subdivided on the basis of sizes, filling characteristics and distribution patterns. Type 1 macrofractures are barren or mainly calcite-lined. They have straight traces with widths (opening displacements) that are of the order of magnitude of 10 μm, suggesting that their primary role is that of migration channels. Type 2 macrofractures are calcite-filled opening-mode fractures. They have an elliptical or tabular shape with sharply tapering tips. Transgranular microfractures are lens-shaped and open or filled mostly by calcite; maximum widths range between 0.01 mm and 0.1 mm. Intragranular microfractures are the most common microfracture type. They are filled by calcite, feldspar or quartz. The macrofractures and transgranular microfractures have regular distributions, while most intragranular microfractures are irregularly distributed owing to their inherited origin. The results imply that natural fractures in the tight sandstones were formed as tectonic, diagenetic and natural hydraulic origins. In situ stress and cementation analyses suggest that Type 1 macrofractures and their genesis-related microfractures have controlled the present flow system of the tight sandstones.

2D spatial analysis of the natural fractures in the organic-rich Qusaiba Shale outcrop, NW Saudi Arabia

Natural fracture is the main control on production from unconventional shale reservoirs as it serves as a drainage conduit and promotes the efficiency of the hydraulic fracturing technique. The Qusaiba Shale (QS), particularly its basal unit was recently identified as a potential unconventional shale gas reservoir in Saudi Arabia. This study presents a comprehensive and systematic description as well as statistical analysis of the natural fractures that cut through the QS outcrop in the Tabuk Basin, NW Saudi Arabia. These analyses were used to identify the factors that control the development of a fracture system in this organic-rich Shale. Three main approaches including areal sampling, scanline and topology analyses were utilized to collect fracture measurements. A total of 495 fracture traces were measured. The fracture length ranges between 7 cm and 8.2 m. The measured fracture traces are classified into four groups; subvertical to vertical, high-angle, low angle and bedding to subparallel fracture traces. Using twenty-one scan lines, the total number of fracture traces that cut through are 374, with average fracture intensity ranging from 0.9 to 3.7 fracture/meter. Four main fracture sets trending E-W, NE-SW, NW-SE and NNW-SSE were identified. The measured fractures vary from cemented (59%), partially filled (38%) to open-mode (3%). The cement in fractures is either gypsum or calcite. Applying the topology method analysis, the total number of nodes is 811, where the isolated nodes (I-type) account for 522, the termination nodes (Y-type) accounts for 222, and the cross-cutting nodes (X-type) are only 83. Microfractures are sharp with high-angle, closely spaced, open-mode fractures with lithology-induced terminations. The small vertical to sub-vertical fracture traces are terminated against the more resistant silty laminae. Pyrite shows positive correlation with the linear fracture density. The high amount of pyrite in association with high quartz and subordinate feldspars contents in the studied interval might have enhanced the brittleness of the lithologies and facilitated fracture propagation. Tectonically, two main fracture types exist in the studied section; shear and extensional fractures. The strike of the shear fractures in the study area is NE-SW and NW-SE directions. The open-mode, vertical long fractures cutting through the entire outcrop indicate an extension stress regime, which has already shaped the Tabuk Basin with multiple NW-SE rift structures.

Subsidence rings and fracture pattern around dolines in carbonate platforms – Implications for evolution and petrophysical properties of collapse structures

Abstract This work focuses on the study of collapse dolines, which are the most expressive collapse structures in carbonate rocks, and their relations with preexisting and syn-collapse fractures. The study area has two fracture sets that were formed before folding, early N-S/E-W- and late NE-SW/NW-SE-striking sets, which concentrate most of the dissolution in the region and allow the formation of the dolines. We define subsidence rings as the circular and ellipsoidal concentric zones around collapse structures, which are subjected to subsidence due to major collapses and represent locations where new fractures are formed. In these subsidence rings, the downfaulted topography plunges towards the doline center and reaches more than 10 m in relation to unaffected areas away from dolines. The topographic data indicate that the mean radius of the combined rings is ~twice the radius of the collapse, which corresponds to the closed depression due to failure and downfall of blocks. The subsidence process enlarges, links preexisting fractures, and forms a new set of semicircular concentric opening mode fractures, here named collapse fractures. Increases in the apertures and densities of these fractures occur towards the dolines, which increases fracture porosity around collapse structures. Fractures are reactivated as normal faults close to the main collapse at the doline edge. This increase in fracture intensity could represent an indicator of permo-porous quality improvement in these areas. Further, this fracturing increases structural instability, raising the risk of accidents in areas built on soluble carbonate rocks, since the affected area may be much larger than previously predicted. Subsidence rings around collapse dolines could merge with other rings from neighboring collapse structures and potentially increase porosity and permeability, as well as linking areas in carbonate reservoirs.

Fracture Analysis of Uranium-Bearing Rock in Eko-Remaja Exploration Tunnel at Depth 50-200 Meters, Kalan, West Kalimantan

Remaja Sector is one of the potential sectors of U mineralization at Kalan Area, West Kalimantan. Host rock of the mineralization is metasiltstone and schistosed metapelite. The 618 meters long Eko-Remaja Exploration Tunnel was built in 1980 to understand the character of uranium ore in this sector. The U mineralization in Kalan, West Kalimantan is influenced by the host rock type and fractures formed by regional tectonic. Regionally, ductile and brittle deformation occurs at Kalan. The ductile deformation observed from N 70o E fold plunging 30o NE that generate schistocity plane dipping 70-80o relatively to the north. First brittle deformation resulting open-mode fractures and schistocity planes that later filled by U-rich solution forming U veins, veinlets and breccia mineralization. Later, brittle deformation forming fractures that filled with calcite-gypsum solution afterwards, forming centimetric-desimetric vein and veinlet cutting the U vein. The purpose of this study is to determine the characteristics and main structure that control the U mineralization in the tunnel. The method is by collecting data of fracture such as joints and fault planes, schistosity planes (S1), and bedding plane (S0) together with its cross-cutting relationship at depth 50 - 200 meter from the tunnel mouth which at this interval representing the presence of mineralization in the Remaja Sector. The data then plotted and analyzed on upper hemisphere stereographic projection. Fracture analysis conducted to understand the families of fractures developed and the force direction resulting fractures the tunnel. From the analysis, the mineralization of U in the tunnel controlled by N 280o E fractures that relatively parallel to the schistocity plane as vein and breccia mineralization and analytical fold axis on Eko-Remaja Tunnel is N 73 E° / 30 tilted to E-NE. Joints in the tunnel mainly affected by faults that formed by couple force.

Effect of nanoalumina in epoxy adhesive on lap shear strength and fracture toughness of aluminium joints

ABSTRACT Lap shear strength and opening mode fracture toughness of epoxy/alumina nanocomposite adhesives in bonded aluminium alloy joints were determined using a single lap joint, and double cantilever beam and contoured double cantilever beam joints, respectively. Epoxy/alumina nanocomposite adhesives were prepared with 0.5, 1.0, 1.5 and 2.0 wt.% of alumina nanospheres (diameter 23–47 nm) and alumina nanorods (diameter in the range of 10 nm and length less than 50 nm). A significant improvement in the lap shear strength and fracture toughness of nanocomposite adhesives was observed over that of neat epoxy adhesive. Maximum lap shear strength of joints bonded with nanocomposite adhesives was observed at 1.5 wt.% of nanospheres and at 1.0 wt.% of nanorods. Maximum fracture toughness of both types of joint was observed for adhesive having 1.5 wt.% of nanospheres and for 1.0 wt.% of nanorods. However, the fracture toughness of joints having 1.5 wt.% of nanospheres was higher in comparison to the fracture toughness of joints having 1.0 wt.% of nanorods. Lap shear strength and fracture toughness decreased on further increment in the wt.% of nanoalumina i.e. at 2.0 wt.% of nanospheres and, at 1.5 and 2.0 wt.% of nanorods.

Interface fracture of micro-architectured glass: Inverse identification of interface properties and a novel analytical model

Recently, it is demonstrated that bio-inspired interlocking micro-architectures within a brittle material can increase its ductility and fracture toughness remarkably. Despite showing tremendous promise, there is a lack of computational and analytical models for these interlocked systems to simulate opening-mode fracture behavior. Besides, due to curved geometry of the interface its properties are difficult to obtain from experiments alone. In the present paper the effective thickness, cohesive and contact properties of the interface, are inversely identified from the experimental data by developing and using a finite element (FE) model as a forward solver. The identification is challenging due to the fact that the influences of these parameters on the mechanics are interdependent. Present FE model has revealed important insights about the stress fields and the interface fracture process. Further, a novel approximate analytical model is derived to simulate the complete pullout response of the interlocking teeth, which involves the mixed mode cohesive fracture and the contact mechanics at the interface in addition to elastic deformation of the teeth. The analytical model is derived independently without any phenomenological input from the experiments or the finite element simulations. However, the predictions by the analytical model match accurately with both experiments and the finite element simulations. It is expected that the proposed experimentally validated, predictive computational and analytical models of interlocking micro-architectured materials would generate insights into their complex failure process and enable their performance optimization for large-scale applications.

Asymptotic Analysis of the Crack Tip Stress Field (Consideration of Higher Order Terms)

This paper presents multi-parameter asymptotic description of the stress field near the tip of a central crack in a linear-elastic plate under: (1) normal tensile stress; (2) transverse shear; (3) mixed mode deformation in the full range of mixed modes of loading, from the opening mode fracture to antiplane shear. A multi-parameter expansion of the stress tensor components including higher order terms has been constructed. All the scale (amplitude) factors—coefficients of the complete Williams asymptotic expansion—have been determined as functions of the crack length and parameters of loading. The expansion constructed and formulas obtained for the expansion coefficients can be used for keeping any preassigned number of terms in asymptotic representations of mechanical fields at a crack tip in a plate. The number of components to keep at different distances from the tip of defect was subjected to analysis. The angles of crack propagation under conditions of mixed-mode loading were calculated using a multi-parameter expansion of stress field by means of (1) the maximum tangential stress criterion and (2) the criterion of minimum elastic strain energy density.

Quantified fracture (joint) clustering in Archean basement, Wyoming: application of the normalized correlation count method

We demonstrate statistically significant self-organized clustering over a length scale range from 10 −2 to 10 1 m for north-striking opening-mode fractures (joints) in Late Archean Mount Owen Quartz Monzonite. Spatial arrangement is a critical fracture network attribute that until recently has only been assessed qualitatively. We use normalized fracture intensity plots and the normalized correlation count (NCC) method of Marrett et al. to discriminate clustered from randomly placed or evenly spaced patterns quantitatively over a wide range of length scales and to test the statistical significance of the resulting patterns. We propose a procedure for interpreting cluster patterns on NCC diagrams generated by the freely available spatial analysis software CorrCount. Results illustrate the efficacy of NCC to measure fracture clustering patterns in texturally homogeneous Archean granitic rock in a setting distant (>2 km) from folds or faults. In their current geological setting, these regional fractures are conduits for water flow and their patterns – and the NCC approach to defining clusters – may be useful guides to the spatial arrangement style and clustering magnitude of conductive fractures in other, less accessible fractured basement rocks. Thematic collection: This article is part of the Naturally Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/naturally-fractured-reservoirs

Degradation of fracture porosity in sandstone by carbonate cement, Piceance Basin, Colorado, USA

Cretaceous Mesaverde Group sandstones contain opening-mode fractures lined or filled by quartz and, locally, calcite cement. Fracture occlusion by quartz is controlled primarily by fracture size, age and thermal history. Fracture occlusion by calcite is highly heterogeneous, with open and calcite-sealed fractures found at adjacent depths. In the Piceance and in other basins, processes that control the distribution of these calcite cements have been uncertain. Using pore and fracture cement petrography, fluid inclusions, and isotopic and elemental analysis, we show that host-rock calcite distribution and remobilization govern porosity degradation and occlusion of fractures >1 mm wide by calcite. Fluid-inclusion analyses indicate calcite cement precipitation at 135–165°C. 87 Sr/ 86 Sr ratios of calcite and the presence of porous albite suggest that detrital feldspar albitization released Ca 2+ , driving carbonate cement precipitation. In host rock, both albite and calcite content decreases with depth along with greater fracture porosity preservation. Although the cement sequence Fe-dolomite → ankerite → calcite is widespread, Fe-dolomite and ankerite occur as host-rock cements only, with detrital dolomite as preferred precipitation substrate. We find that the rock-mass calcite cement content correlates with fracture degradation and occlusion, and can be used to accurately predict where wide fractures are sealed or open. Thematic collection: This article is part of the Naturally Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/naturally-fractured-reservoirs

Propagation of toughness-dominated fluid-driven fractures in reactive porous media

Fluid-driven open-mode fractures in porous media are the result of multiple physical processes governed by fluid properties, porous media properties, geometrical fracture properties, and fluid injection parameters. For example, a propagating open-mode fracture in which most work is spent on creating new fracture surface rather than on moving fluid through the fracture is said to be in a “toughness-dominated” regime. However, other multiphysical phenomena such as thermal dilation and mineral-dissolution can also affect fracture propagation. This study focuses on the facilitation of fracture propagation in toughness-dominated regime by weakening of the rock at the fracture tip by injection of a reactive fluid that causes mineral dissolution. We explore this coupled problem through numerical simulation using a finite element method (FEM) formulation based on the phase-field approach in order to solve equations for fluid flow, poroelasticity, linear elastic fracture mechanics and reactive transport. We couple mechanics and reactive fluid flow by assuming that fracture toughness decreases due to increases in rock porosity caused by mineral dissolution. The numerical simulation results show that acid injection and mineral dissolution (1) lower fracture breakdown pressure and (2) can bridge a transition from toughness-dominated to viscosity-dominated fracture propagation at constant injection pressure. Our results demonstrate that acid injection can change fracture propagation regimes and enable new phenomena that are not possible in unreactive porous media. The understanding of fracture propagation patterns and geometry manipulation is critical for safe and effective use of reactive fluids in the subsurface, such as in hydraulic fracturing, harnessing of deep geothermal energy, and carbon geological sequestration applications.

Fracture Propagation in Heterogeneous Porous Media: Pore-Scale Implications of Mineral Dissolution

Open-mode fractures in sediments and rock occur both in natural processes and in engineering practice. In contrast to long planar fractures predicted by solutions in homogeneous media, laboratory and field observations often evidence complex and non-planar open-mode fracture networks in geological media. One cause of fracture complexity is the heterogeneity of material mechanical properties. This study focuses on the effects of mineral dissolution on pore structure modification in heterogeneous porous media and ensuing changes on fracture propagation characteristics. We performed semicircular bending experiments on limestone samples pre- and post-acidizing, mapped 3-D pore structure with X-ray microtomography, and used a finite element formulation based on the phase-field approach and linear elastic fracture mechanics to model initiation and propagation of open-mode fractures. Semicircular bending experiments on acidized carbonate rock samples show that non-planar fractures follow high porosity regions and large pores, and that fracture toughness correlates well with local porosity. Numerical modeling shows that a direct relationship between fracture toughness and porosity permits replicating fracture stress intensity at initiation and non-planar fracture propagation patterns observed in experiments. The results of this study indicate that mineral dissolution can affect fracture propagation patterns and that a coupling of mechanical and reactive processes in porous media formulations can be achieved through porosity and toughness. The understanding of fracture propagation patterns and geometry manipulation is critical for safe and effective use of reactive fluids in the subsurface, such as in hydraulic fracturing and carbon geological sequestration applications.

An Inside Perspective on Magma Intrusion: Quantifying 3D Displacement and Strain in Laboratory Experiments by Dynamic X-Ray Computed Tomography

Magma intrusions grow to their final geometries by deforming the Earth’s crust internally and by displacing the Earth’s surface. Interpreting the related displacements in terms of intrusion geometry is key to forecasting a volcanic eruption. While scaled laboratory models enable us to study the relationships between surface displacement and intrusion geometry, past approaches entailed limitations regarding imaging of the laboratory model interior or simplicity of the simulated crustal rheology. Here we apply cutting-edge medical wide beam X-ray Computed Tomography (CT) to quantify in 4D the deformation induced in laboratory models by an intrusion of a magma analogue (golden syrup) into a rheologically-complex granular host rock analogue (sand and plaster). We extract the surface deformation and we quantify the strain field of the entire experimental volume in 3D over time by using Digital Volume Correlation (DVC). By varying the strength and height of the host material, and intrusion velocity, we observe how intrusions of contrasting geometries– cryptodomes, cup shapes, cone sheets and dikes – grow, and induce contrasting strain field characteristics and surface deformation in 4D. We observe dominantly mixed-mode (opening and shear) fracture localisation in low-cohesion material overburden versus opening-mode fracture localisation in high-cohesion material overburden. The results demonstrate how the combination of CT and DVC can greatly enhance the utility of optically non-transparent crustal rock analogues in obtaining insights into shallow crustal deformation processes. This unprecedented perspective on the spatio-temporal interaction of intrusion growth coupled with host rock deformation provides a conceptual framework that can be tested by geological field observations at eroded volcanic systems and by the ever increasing spatial and temporal resolution of geodetic data at active volcanoes.

Mini thief zones: Subcentimeter sedimentary features enhance fracture connectivity in shales

This study examines the influences on fluid flow within a shale outcrop where the networks of two distinct paleoflow episodes have been recorded by calcite-filled veins and green alteration halos. Such direct visualization of flow networks is relatively rare and provides valuable information of fluid-flow behavior between core and seismic scale. Detailed field mapping, fracture data, and sedimentary logging were used over a 270 m2 (2910 ft2) area to characterize the paleo–fluid-flow networks in the shale. Distal remnants of turbidite flow deposits are present within the shale as very thin (1–10 mm [0.04–0.4 in.]) fine-grained sandstone bands. The shale is cut by a series of conjugate faults and an associated fracture network, all at a scale smaller than seismic detection thresholds. The flow episodes used fluid-flow networks consisting of subgroups of both the fractures and the thin turbidites. The first fluid-flow episode network was mainly comprised of thin turbidites and shear fractures, whereas the network of the second fluid-flow episode was primarily small joints (opening mode fractures) connecting the turbidites. The distribution of turbidite thicknesses follows a negative exponential trend. which reflects the distribution of thicker turbidites recorded in previous studies. Fracture density varies on either side of faults and is highest in an area between closely spaced faults. Better predictions of hydraulic properties of sedimentary-structural networks for resource evaluation can be informed from such outcrop subseismic scale characterization. These relationships between the subseismic features could be applied when populating discrete fracture networks models, for example, to investigate such sedimentary-structural flow networks in exploration settings.

Combined control of décollement layer thickness and cover rock cohesion on structural styles and evolution of fold belts: A discrete element modelling study

A series of numerical experiments based on the discrete element method were performed to simulate the formation of fold belts, with varied décollement layer thickness and cover rock cohesion, to investigate their controls on structural styles of fold belts. Each model consists of a weak, unbonded basal décollement layer with a relatively low to high thickness, and a bonded cover with a relatively low to high cohesive strength. Horizontal shortening of the particle assemblage was achieved by horizontal motion of a vertical boundary wall, resulting in deformations in the system. The results show that shortening was mainly accommodated by detachment folds (sinusoidal and box folds), fault-related folds and opening-mode fractures in the models. The combination of a lower cohesion and a thinner décollement resulted in more distributed strain and a larger number of folds, whilst the combination of a higher cohesion and a thicker décollement led to more pronounced strain localisations in fewer folds. Surface uplift and fold amplitude (diapir height) are mainly positively affected by the décollement thickness, i.e. the thicker the décollement is, the greater the surface uplift and fold amplitude are. The propagation rate of deformation is predominately controlled by the cover rock cohesion. A lower cohesion led to a higher propagation rate of deformation. The model with a relatively low cohesion and décollement thickness produced regularly-spaced box folds, which are comparable to those in the southern Zagros Fold-and-Thrust Belt. The models presented demonstrate that the combined effect of décollement layer thickness and cover rock cohesion can play a critical role in the structural styles and kinematic evolution of fold belts.

Universal and Nonuniversal Aperture‐to‐Length Scaling of Opening Mode Fractures Developing in a Particle‐Based Lattice Solid Model

AbstractOpening‐mode fractures, such as joints, veins, and dykes, frequently exhibit power‐law aperture‐to‐length scaling, with scaling exponents typically ranging from 0.5 to 2. However, published high quality outcrop data and continuum‐based numerical models indicate that fracture aperture‐to‐length scaling may be nonuniversal, with scaling being superlinear for short fractures and sublinear for long fractures. Here we revisit these published results by means of a particle‐based lattice solid model, which is validated using predictions from linear elasticity and linear elastic fracture mechanics. The triangular lattice model composed of breakable elastic beams, with strengths drawn from a Weibull distribution, is used to investigate the fracture aperture‐to‐length scaling that emerges in a plate subjected to extension. The modeled fracture system evolution is characterized by two stages which are separated by the strain at which peak‐stress occurs. During the pre‐peak‐stress stage, aperture‐to‐length scaling is universal with a power‐law exponent of about one. Shortly after the material has attained its maximum load bearing capacity, which coincides with the formation of a multiple‐segment fracture zone, aperture‐to‐length scaling becomes nonuniversal, with power‐law exponents being consistent with earlier studies. The results presented here confirm that deviation from universal scaling laws is a consequence of fracture interaction. More specifically, the onset of nonuniversal aperture‐to‐length scaling coincides with the formation of a multiple‐segment fracture zone.