Orthogonal Fracture Sets Research Articles

Estimating two groups of fracture weaknesses using azimuthal differences in partially incidence-angle-stacked seismic amplitudes

Geophysical and geologic data, e.g., results obtained using rock cores, outcrops, and borehole images, reveal the presence of multiple sets of fractures in rock. To analyze how fracture interactions affect seismic anisotropy and dispersion, we consider an effective model that contains two sets of orthogonal fractures. In seismic amplitude analysis, we focus on the case of subsurface target zones containing a fracture network composed of primary and secondary fracture sets. Focusing on gas-bearing rocks with small fracture densities, we formulate simplified stiffness parameters in terms of two groups of fracture weaknesses, which are related to the primary and secondary fractures, respectively. Using the simplified stiffness parameters, we derive the PP-wave reflection coefficient and the azimuthal elastic impedance (AEI) as functions of the two groups of normal and tangential fracture weaknesses. Based on the derived PP-wave reflection coefficient and AEI, we propose a method and workflow in which the azimuthal variations of the partial incidence-angle-stacked seismic data are used to estimate the AEI and differences in AEI (which we refer to as DEI) is input to a nonlinear inversion for two groups of fracture weaknesses. In the nonlinear inversion, the initial values of fracture weaknesses are obtained based on a two-term approximation of the PP-wave reflection coefficient. First- and second-order derivatives of DEI with respect to fracture weakness parameters are calculated to generate the update in the unknown parameter vector. Synthetic seismic gathers with varying signal-to-noise ratios are used to validate the robustness of the estimation. Applying the inversion method to a real data set, we obtain what we interpret to be reliable fracture weakness estimates that produce azimuthal amplitude differences matching those extracted from real seismic data. The results provide a potential tool for determining if two sets of fractures have developed in a reservoir.

Structure and fracture characterization of the Jizan group: Implications for subsurface CO2 basalt mineralization

The coastal region of southwest Saudi Arabia contains a thick sequence of Late Oligocene basalts in the Jizan Group, which accumulated along the continental rift that preceded the opening of the Red Sea. These basalts are targeted for the disposal of CO2 emitted from industrial sources by subsurface carbon mineralization processes. The disposal potential of the Jizan Group basalts depends on having adequate permeability along fracture networks capable of conducting injected fluids away from the wellbores. The basalts in the Jizan Group generally lack primary permeability due to hydrothermal alteration, but are cross-cut by a dense network of fractures. In this paper, we describe and interpret the structural geology of the area based on field and geophysical data, and characterize the fracture development in the Jizan Group. The Jizan Group in the area comprises a bimodal suite of 30–21 Ma volcanic and volcaniclastic rocks and lacustrine sediments that accumulated in a continental rift valley similar to the East African rift. It consists predominantly of basaltic lavas that were fed by dense swarms of sheeted basalt dikes intruded parallel to the rift axis. Structurally the area is composed of half grabens bounded from the west by antithetic normal faults, and from the east by a megaflexure. Fractures in the Jizan Group were characterized by ground and aerial digital photogrammetry of outcrops. Mean P21 fracture intensities from 12 scattered meter scale outcrops are in the range 5–54 m−1, which demonstrates that the Jizan Group is highly fractured. Fracture directions are multimodal. The dominant fracture trend is 140–160 N, which is parallel to the sheeted dike swarms and normal faults, and therefore parallel to the paleo-rift axis. Additional conjugate and orthogonal fracture sets are also recognized. The presence of pervasive fracture-based permeability in the Jizan Group will facilitate the injection and mineral carbonation of carbon dioxide in the mafic volcanic rocks in this region.

Intraplate Strike-Slip Corridor within South America (NE Border of the Paraná Basin) Unveiled by Structural Analysis of Faults and Fracture Swarms

We present the effect of neotectonics in intracratonic settings as revealed by the surface, brittle deformation associated to a regionally-sized shear corridor, which affects Southeastern Brazil. The deformation zone is characterized by the presence of nearly orthogonal fracture sets, interpreted as systematic and non-systematic joints often cutting Quaternary deposits. An original methodology of fault and joint inversion by the Monte Carlo converging approach is used to infer multiple paleostress fields. The method provides the best orientation of the principal paleo-stresses responsible for the observed fracturing. At each step of the inversion process, structures are uniquely associated to the stress tensor that provides the lowest error. The results showed the poly-phased tectonic history of the shear corridor studied and paleostresses compatible with a regional strike-slip motion. Specifically, an E-W, left-lateral shear was followed by an E-W, right-lateral kinematics related to the post-Paleogene drifting of South American Plate and its clockwise rotation. The latter tectonic event is presently responsible for brittle deformation observed in Quaternary deposits. The proposed deformation corridor may represent the Cenozoic reactivation of an ancient weakness zone. We speculate that the described intraplate strike-slip deformation belt represents the continental prosecution of the Rio de Janeiro fracture zone.

Fracture stratigraphy and oil first migration in Triassic shales, Favignana Island, western Sicily, Italy

This study aims at evaluating the control exerted by fracture stratigraphy and diagenetic processes on oil first migration through an outcropping, mixed carbonate-siliciclastic succession. The present work included results of sedimentological, paleontological, mineralogical, petrographic, structural, and microstructural analyses carried out on organic-rich shales exposed at the Favignana Island of Sicily, Italy. The analyses focus on Upper Triassic yellowish siltstones and greyish laminated dolomitic limestones, which form a 10's of m-thick succession exposed along the westernmost portion of the Sicilian fold-and-thrust belt. The studied succession deposited in a coastal lagoon associated to a wide carbonate platform, in which anoxic bottom conditions allowed the preservation of the organic matter forming catagenetic patches and veins/fractures infill. In fact, two orthogonal fracture sets perpendicular to bedding are pervaded by organic matter. They are hence interpreted as structural elements that affected the paleofluid circulation and oil migration within the Triassic source rock. These two orthogonal sets form the background fracture network predating the formation of three other sets infilled mainly with calcite cements. The latest fracture set also includes barite cements implying a hydrothermal origin (<200 °C), as supported by the R1 and R3 mixed-layer Illite-Smectite (I–S) stacking. Data are consistent with a burial depth of the sedimentary succession of about 3 ± 0.5 km. In light of these considerations, the study area might represent an outcropping stratigraphic and structural analogue of the source rocks buried in the offshore of western Sicily and northern Tunisia.

Evaluation of 3D shear-wave anisotropy based on elastic-wave velocity variations around the borehole

Aligned fractures/cracks in rocks are a primary source of elastic anisotropy. In an azimuthally anisotropic formation surrounding a borehole, shear-waves (S-waves) split into fast and slow waves that propagate along the borehole and are recorded by a borehole logging tool. However, when the formation has conjugate fractures with orthogonal strike directions, the azimuthal anisotropy vanishes. Hence, azimuthal anisotropy measurements may not be adequate to detect orthogonal fracture sets. We have developed a method for obtaining azimuthal and radial S-wave anisotropy parameters simultaneously from 4C array waveforms. The method uses a velocity tomogram around the borehole. The azimuthal and radial anisotropy were determined by integrating a shear velocity radiation profile along the radial direction at different azimuthal angles. Results indicate that this approach is reliable for estimating anisotropy properties in aligned crack systems. The advantage of this interpretation method is shown in multiple conjugate crack systems. Field data processing examples are used to verify the application of the proposed technique. Comparison of results against those obtained with a conventional procedure shows that the new method can not only provide estimates of azimuthal anisotropy, but also of the radial anisotropy parameter, which is important in fracture network evaluation.

A three‐dimensional enhanced dual‐porosity and dual‐permeability approach for hydromechanical modeling of naturally fractured rocks

AbstractThe natural fracture system plays an important role in the development of naturally fractured reservoirs. Traditionally, those reservoirs are simulated using dual‐porosity and dual‐permeability models. Conventional dual‐porosity models adopt over‐simplifications in terms of characterization of the fractured system. Generally, they focus on the hydraulic problem and do not consider the rock and fracture deformability. Besides, those models assume equally sized block matrix and orthogonal fracture sets with uniform properties. This work proposes a new hydromechanical formulation to represent a fractured porous formation more realistically using the finite element method. The enhanced dual‐porosity and dual‐permeability (EDPDP) model allows incorporating multiblock domains formed by several multiscale fracture sets with arbitrary orientations, permeabilities, and sizes. The fully coupled hydromechanical formulation includes fracture orientations and stress‐induced aperture changes to update stiffness and permeability tensors. The EDPDP model is implemented in an in‐house framework to study the effects of fractures with multiple scales on a hydrocarbon reservoir's hydromechanical behavior. We compare the proposed model against the discrete fracture method to assess its accuracy. The comparative results show excellent agreement and validated the fully coupled hydromechanical formulations. Finally, we present a case study of a reservoir that contains multiple sets of fractures with different scales and arbitrary orientation to demonstrate the applicability and robustness of the proposed model.

Enhancing subsurface imaging and reservoir characterization in the Marcellus Shale play, northeast Pennsylvania, through advanced reprocessing of wide-azimuth 3D seismic data

The Marcellus Shale sits in a geologically complicated region, characterized by faults and salt-cored compressional folds. Large lateral velocity variations associated with this complex geology make seismic imaging difficult. Recently acquired 3D seismic with wide azimuthal coverage and long offsets have poorly imaged folds and faults within the Marcellus Shale. Wells drilled using the existing 3D seismic volumes often encounter incorrect bed dips and folds mistakenly interpreted as faults (or vice versa) because of the poor imaging quality. We carefully examined each processing step in the seismic processing and imaging workflow and identified a few key drivers that could potentially lead to improvements in subsurface imaging. Starting from field seismic records, we identified and corrected geometry errors. To improve signal-to-noise ratio, we applied advanced noise attenuation practices including land surface-related multiple elimination. We carefully tested and validated an extension patch in the 5D seismic data interpolation that improved the offset coverage in the crossline direction. Furthermore, we performed orthorhombic prestack time migration (PSTM). An orthorhombic velocity model fits the geology better than the traditional vertical transverse isotropic (VTI) model for layered subsurface because a dominant set of orthogonal fracture sets is present in the basin. We achieved a significant improvement of subsurface imaging by implementation of these processing steps as key drivers. The new orthorhombic PSTM correctly images steeply dipping (75° and above) faults that were elusive or absent on the previous VTI migration. Additionally, previously interpreted faults are now clearly imaged as folds, small and large in scale. These imaging improvements have enabled the accurate drilling of lateral wells in the target that otherwise would have been drilled out of zone. Some large faults, which pass through the Marcellus, Mahantango, and Tully Formations, are now well-imaged but were previously barely visible. Successful imaging of such large faults helps identify and avoid geohazards. The dramatic improvement in the subsurface imaging and amplitude-preserving processing enhance the reservoir description of the Marcellus Shale.

Incipient karst formation in carbonate rocks: Influence of fracture network topology

Karst features in limestone aquifers often exhibit highly heterogeneous patterns, which are strongly governed by structural complexities such as the spatial distribution and organization of fractures in natural carbonate rocks. However, until now, little is known about the impact of structural complexity on karstification processes. In this paper, we use numerical models to investigate how structural heterogeneity affects the generation of incipient karst in natural fractured systems. We develop a numerical model which couples the processes of fluid flow, mass transport and dissolution kinetics that govern the growth of fracture aperture, based on discrete fracture networks. We apply the model to simulate conduit evolution in a natural joint network that displays a distinctive topological pattern. A wide range of hydraulic gradient and initial fracture aperture is considered. Our results show that the network topology may generate a significant impact on dissolution pattern depending on how flow is organized within the discrete network. If a channelized structure occurs in the flow network, the dissolution pattern depends highly on the initial hydrogeological settings, while if the flow network consists of branched segments, a similar dissolution pattern emerges for a broad range of initial hydrogeological settings. When the flow direction is normal to the persisting fracture orientation, intermittent interruptions to the positive feedback process between flow and dissolution occur. Therefore, the breakthrough is delayed. We discuss the link between our results of dissolution modeling in fracture networks to previous modelling efforts in the context of single rough fractures and fracture network formed by orthogonal fracture sets.

Revisiting the Analytical Solutions of Heat Transport in Fractured Reservoirs Using a Generalized Multirate Memory Function

AbstractNumerous analytical solutions have been developed for modeling thermal perturbations to underground formations caused by deep‐well injection of fluids. Each solution has been derived for a specific boundary value problem and a simplified flow network with one set of parallel fractures. In this paper, new generalized solutions G*(x, s) are developed using (existing) global transfer functions and a new memory function g*(s), where x and s are the space and Laplace variable. The memory function represents the solutions of conductive heat exchange between fractures and matrix blocks and between fractured aquifers and unfractured aquitards. The memory function is developed to account for multirate exchange induced by different shapes, sizes, properties, and volumetric fractions of matrix blocks bounded by multiple sets of orthogonal fractures with different spacing. The global transfer functions represent the fundamental solutions to convective, convective‐conductive, and convective‐dispersive heat transport in fractures (or aquifers) without exchange and are available for various (1‐D linear, 1‐D radial, 2‐D dipole, and single‐well injection‐withdrawal) flow fields. The new solutions with exchange are developed using , thereby greatly simplifying solution development in a novel way, where ϑ and B*(s) are a fracture‐matrix scaling factor and the boundary condition function. The new solutions are applied to several example problems, showing that heat transport in fractured aquifers is significantly impacted by (1) thermal dispersion in fractures that is rarely considered, (2) multirate heat exchange with a wide range of size and anisotropy of rectangular matrix blocks, and (3) heat exchange between aquifers and aquitards.

Pseudo-static failure modes and yield accelerations in rock slopes

Earthquake-induced landslide databases indicate that failure modes, observed in the field following earthquakes, include sliding, toppling, and slumping. Methods based on simplified, discrete, single block models are limited to rectangular blocks on a plane or wedge blocks that restrict the potential failure modes to sliding or toppling. This paper expands the geometry assumptions so that additional failure modes will be kinematically admissible. A simple, yet broadly applicable, two-dimensional (2D) single block framework is introduced that does not restrict the geometry to orthogonal fracture sets. This framework allows for the slumping failure mode to naturally occur and can identify a new failure mode, confined toppling. Using simple failure mode charts, this framework can be easily applied to a wide range of analysis and design applications. In addition to the identification of new failure modes, seismic yield acceleration equations are presented for all four modes of failure: sliding, toppling, slumping, and confined toppling. The equations for slumping and confined toppling are derived for the first time. Although the model may be simple in its formulation and implementation, it is quite powerful, allowing for significant implications to be developed. Complex shaped blocks can be easily evaluated knowing just their centers of mass and contact points with supporting fractures. The failure mode of a discrete rock block is shown to be independent of the primary fracture inclination on which the block rests upon and the scale of the block itself. Seismic failure modes are demonstrated to be different from those induced by static forces alone and can even change modes depending on the amount of displacement during the ground motion. In addition, it is shown that the characteristics of an earthquake ground motion acting on these blocks in combination with geometric variability can influence the abundance of failure types observed in the field. Finally, an example mode and yield acceleration evaluation of Alaskan rock slope is presented.

Effects of fracture intersections on seismic dispersion: theoretical predictions versus numerical simulations

ABSTRACTThe detection and characterisation of domains of intersecting fractures are important goals in several disciplines of current interest, including exploration and production of unconventional reservoirs, nuclear waste storage, CO2 sequestration, and groundwater hydrology, among others. The objective of this study is to propose a theoretical framework for quantifying the effects of fracture intersections on the frequency‐dependent elastic properties of fluid‐saturated porous and fractured rocks. Three characteristic frequency regimes for fluid pressure communication are identified. In the low‐frequency limit, fractures are in full pressure communication with the embedding porous matrix and with other fractures. Conversely, in the high‐frequency limit, fractures are hydraulically isolated from the matrix and from other fractures. At intermediate frequencies, fractures are hydraulically isolated from the matrix porosity but can be in hydraulic communication with each other, depending on whether fracture sets are intersecting. For each frequency regime, the effective stiffness coefficients are derived using the linear‐slip theory and anisotropic Gassmann equations. Explicit mathematical expressions for the two characteristic frequencies that separate the three frequency regimes are also determined. Theoretical predictions are then applied to two synthetic 2D samples, each containing two orthogonal fracture sets: one with and another without intersections. The resulting stiffness coefficients, Thomsen‐style anisotropy parameters, and the transition frequencies show good agreement with corresponding numerical simulations. The theoretical results are applicable not only to 2D but also to 3D fracture systems and are amenable to being employed in inversion schemes designed to characterise fracture systems.

Wave Propagation in Isotropic Media with Two Orthogonal Fracture Sets

Orthogonal intersecting fracture sets form fracture networks that affect the hydraulic and mechanical integrity of a rock mass. Interpretation of elastic waves propagated through orthogonal fracture networks is complicated by guided modes that propagate along and between fractures, by multiple internal reflections, as well as by scattering from fracture intersections. The existence of some or all of these potentially overlapping modes depends on local stress fields that can preferentially close or open either one or both sets of fractures. In this study, an acoustic wave front imaging system was used to examine the effect of bi-axial loading conditions on acoustic wave propagation in isotropic media containing two orthogonal fracture sets. From the experimental data, orthogonal intersecting fracture sets support guided waves that depend on fracture spacing and fracture-specific stiffnesses. In addition, fracture intersections have stronger effects on propagating wave fronts than merely the superposition of the effects of two independent fractures because of energy partitioning among transmitted/reflected waves, scattered waves and guided modes. Interpretation of the properties of fractures or fracture sets from seismic measurements must consider non-uniform fracture stiffnesses within and among fracture sets, as well as considering the striking effects of fracture intersections on wave propagation.

Seismic characterization of fractured reservoirs by focusing Gaussian beams

Naturally fractured reservoirs occur worldwide, and they account for the bulk of global oil production. The most important impact of fractures is their influence on fluid flow. To maximize oil production, the characterization of a fractured reservoir at the scale of an oil field is very important. For fluid transport, the critical parameters are connectivity and transmittivity plus orientation. These can be related to fracture spacing, compliance, and orientation, which are the critical seismic parameters of rock physics models. We discovered a new seismic technique that can invert for the spatially dependent fracture orientation, spacing, and compliance, using surface seismic data. Unlike most seismic methods that rely on using singly scattered/diffracted waves whose signal-to-noise ratios are usually very low, we found that waves multiply scattered by fractures can be energetic. The direction information of the fracture multiply scattered waves contains fracture orientation and spacing information, and the amplitude of these waves gives the compliance. Our algorithm made use of the interference of two true-amplitude Gaussian beams emitted from surface source and receiver arrays that are extrapolated downward and focused on fractured reservoir targets. The double beam interference pattern provides information about the three fracture parameters. We performed a blind test on our methodology. A 3D model with two sets of orthogonal fractures was built, and a 3D staggered finite-difference method using the Schoenberg linear-slip boundary condition for fractures was used to generate the synthetic surface seismic data set. The test results showed that we were able to not only invert for the fracture orientation and spacing, but also the compliance field.

Physical Modelling of Stress-dependent Permeability in Fractured Rocks

This paper presents the results of laboratory experiments conducted to study the impact of stress on fracture deformation and permeability of fractured rocks. The physical models (laboratory specimens) consisted of steel cubes simulating a rock mass containing three sets of orthogonal fractures. The laboratory specimens were subjected to two or three cycles of hydrostatic loading/unloading followed by the measurement of displacement and permeability. The results show a considerable difference in both deformation and permeability trends between the first loading and the subsequent loading/unloading cycles. However, the micrographs of the contact surfaces taken before and after the tests show that the standard deviation of asperity heights of measured surfaces are affected very little by the loadings. This implies that both deformation and permeability are rather controlled by the highest surface asperities which cannot be picked up by the conventional roughness characterization technique. We found that the dependence of flow rate on mechanical aperture follows a power law with the exponent n smaller or larger than three depending upon the loading stage. Initially, when the maximum height of the asperities is high, the exponent is slightly smaller than 3. The first loading, however, flattens these asperities. After that, the third loading and unloading yielded the exponent of around 4. Due to the roughness of contact surfaces, the flow route is no longer straight but tortuous resulting in flow length increase.

Modeling of strategies for performance monitoring of groundwater contamination at sites underlain by fractured bedrock

A three dimensional flow and transport modeling using FRAC3DVS was undertaken to examine factors which influence plume detection in a performance monitoring network for a site where an unconfined aquifer composed of uniform unconsolidated sediments overlies fractured bedrock. The bedrock is assumed to contain a fracture system with three orthogonal fracture sets embedded in a low permeable homogeneous rock matrix. A dissolved phase, non-reactive contaminant is released from a source zone located at the ground surface. The processes which influence plume geometry, and probabilities of plume detection for a performance monitoring network located between the contaminant source and a downstream compliance boundary, are evaluated. Factors considered include the hydraulic conductivity of the unconfined aquifer, the geometric properties of the fracture network and the matrix permeability of the bedrock, and the contaminant detection threshold concentration. The simulations demonstrate that the character of the fracture network not only controls contaminant transport and plume detection in the bedrock but also influences plume detection in the overlying unconfined aquifer. The ratio of the hydraulic conductivity of the unconfined aquifer to the effective hydraulic conductivity of the fractured bedrock, and the contaminant detection threshold concentration, are principal factors influencing detection probability in the performance monitoring network. Results suggest that in many instances encountered in field practice, the unconfined aquifer and fractured bedrock should be viewed as an integrated hydrogeologic system from a monitoring perspective.

Mechanical stratigraphy and (palaeo-) karstification of the Murge area (Apulia, southern Italy)

Abstract The Cretaceous Apulia Platform, exposed in the Murge area (southern Italy), suffered intense (palaeo)karstification. This study focuses on the controlling factors of karstification with emphasis on fracturing. Mechanical stratigraphy was used to calculate the fracture density within different sedimentary sequences. Several mechanical units were defined and a characteristic relationship was found between unit thicknesses and fracture density, that is, fracture density increases if layer thickness decreases. In some of the quarries studied, sedimentary cycles are clearly present that are also reflected in the fracture density logs. The degree of karstification within a mechanical unit is proportional to the mean fracture spacing. Based on fracture orientation data extracted from LIDAR scans, different orientation clusters were observed between fractures that are karstified and fractures that are not karstified, post-dating karstification. The clusters of karstified fractures are related to the compression of the southern Apennines. The fractures became dissolution enlarged during the Pleistocene uplift caused by bulging of the Apulia Platform. This main karstification phase occurred prior to Late-Pleistocene deposition and before the formation of orthogonal fracture sets.

Multiple cracks in VTI rocks: Effective properties and fracture characterization

The existing fracture-characterization techniques are based on assuming the unfractured host rock either to be isotropic or the magnitudes of both the background and crack-induced anisotropies to be small. I relax both assumptions and examine the effective media caused by fractures with realistic (not small) crack densities in a strongly anisotropic, primarily transversely isotropic (TI) host rock. The analysis of penny-shaped cracks in the noninteraction approximation (NIA) reveals the dependence of their excess fracture compliance tensors on the orientation of the background symmetry axis. As a result of this dependence, the excess fracture compliance tensor generally becomes rotationally noninvariant even when the cracks are circular. One of the consequences of this complication (compared to the background isotropy) is a reduction of symmetry from TI to monoclinic resulting from the presence of a single oriented fracture set. Verti–cal dry cracks in a vertically transversely isotropic (VTI) host constitute an important exception to this general rule.The effective symmetry for this arrangement is approximately orthorhombic (or orthotropic) even in the presence of multiple fracture sets that have arbitrary azimuths. I perform finite-element simulations on the so-called digital rocks to verify both the proximity of effective symmetry to orthotropy and the accuracy of the NIA up to the crack density of 0.15. Multiple sets of dry vertical cracks in a VTI host not only result in nearly orthorhombic effective symmetry but also their cumulative influence is equivalent to that of just two orthogonal (or principal) fracture sets. The possibility of replacing multiple fracture sets with two orthogonal ones paves the way for their characterization. The inverse problem of estimating the parameters of two orthogonal crack systems in a VTI background from the effective elasticity, however, is known to be nonunique. I suggest overcoming its ambiguity by combining 3D, wide-azimuth, multicomponent seismic data with sonic logs.

Study of fracture-induced anisotropy from discrete fracture network simulation of well test responses

Abstract Different conceptualizations of fracture networks have been generated in order to simulate well tests in fractured reservoirs. These models have two purposes: firstly to investigate whether the behaviour of realistic fracture networks can be emulated with more simple models; and secondly to assess whether different sources of anisotropy have characteristic and recognizable influences on the pressure derivative. The anisotropy of the fracture networks was increased by decreasing the angle between two originally orthogonal fracture sets, and by increasing the permeability contrast between two orthogonal fracture sets. Results indicate that simple models can capture the first-order behaviour of more realistic examples. However because early time data are strongly influenced by the connectivity of the fracture network, the degree of anisotropy can only be assessed at later times in a test. Increasing anisotropy results in increasing heterogeneity and compartmentalization, and permeability anisotropy in an orthogonal system can only partially replicate the behaviour of a geometrically anisotropic system.

The Khaviz Anticline: an outcrop analogue to giant fractured Asmari Formation reservoirs in SW Iran

Abstract The carbonate reservoirs in the Late Oligocene—Early Miocene Asmari Formation in the Dezful Embayment of SW Iran are characterized by low matrix permeability, and effective drainage is dependent on the occurrence of open fractures. Limited information on fracture orientation and fracture density is available from core and borehole image data, and high-quality/high-resolution three-dimensional seismic is often lacking in this area. Well and core data do not contain information on important fracture parameters like length distribution, crosscutting relationships, fracture density v. lithology and bed thickness. The understanding of fracture distribution and formation in the region and their effects on fluid flow has been greatly improved by the use of outcrop analogue data. Exposures of the Asmari Formation in the Khaviz Anticline are in close vicinity to the giant hydrocarbon fields. The Khaviz Anticline has a similar geometry and structural history to the major hydrocarbon fields in the area, and represents an excellent analogue for these. Two types of fracture features were observed: diffuse fracturing and fracture swarms. The diffuse fractures form networks and comprise structures grouped into four fractures sets, which are the typical for this type of anticline. Two orthogonal fracture sets are oriented parallel and perpendicular to the fold axis, and two conjugate fracture sets are oblique to the fold axis with their obtuse angle intersecting the trend of the fold axis. The fractures are typically stratabound, sub-perpendicular to bedding and commonly about the bounding stratigraphic surfaces. To a large extent the density and height of fractures in the Asmari Formation are controlled by the mechanical stratigraphy, which is controlled by the depositional environment and cycles. These outcrop data have been essential in the generation of discrete fracture network (DFN) models and the population of the fracture properties in the reservoir models.

Switches in the minimum compressive stress direction induced by overpressure beneath a low‐permeability fault zone

AbstractThe Zuccale fault is a gently east‐dipping normal fault exposed on Elba. Its displacement of 7–8 km occurred from the mid‐Miocene to the Early Pliocene and the fault has been exhumed from a depth of 3–6 km. A complex hydrofracture system exposed in the footwall block consists of three orthogonal vein sets: two vertical sets trending N–S and E–W and one sub‐horizontal. The veins show a crack‐and‐seal texture and mutually cross‐cut each other. Throughout the period when the Zuccale fault was active, the regional stress field was extensional with the minimum principal stress oriented E–W, consistent only with the N–S trending set of vertical hydrofractures. We attribute the three sets of orthogonal fractures beneath the low‐permeability phyllosilicate‐rich fault core to switches in the minimum compressive stress direction induced by cyclic build‐up and release of overpressure.