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Abstract Since the early descriptions published by Callaway in 1884, the gently dipping mylonites exposed along the Moine Thrust at the Stack of Glencoul have drawn generations of geologists to the northern part of the Assynt district. These mylonites, derived from Cambrian quartzites (footwall) and Moine pelites and psammites (hanging wall), have figured prominently in: a) early research into the influence of crystal plastic deformation and recrystallization on microstructural and crystal fabric evolution; b) debates on the kinematic interpretation of macro- and micro-structures and crystal fabrics; and c) debates on the tectonic significance of such kinematic data. In this paper first we briefly review the historical aspects of this research and then, using both previously published and unpublished data, document the finite strain and quartz fabric development at this classic mylonite locality. A tectonic interpretation of these data, together with quantitative estimates of flow vorticities associated with mylonite formation at the Stack of Glencoul, are presented in a companion paper by Law (2010) .
Abstract The effective computation and visualization of cross-fault sealing or flow, and parameters that infer or control that distribution, is a key step in the production of more reliable exploration and production simulation models. A better understanding of the impact of fault-related flow or baffling through visualization can lead to the development of more robust and useful geological models that better define the likely range in flow behaviour. A range of visualization tools are available, from the traditional fault plane juxtaposition map to the vector visualization of cross-fault fluid flux. Each tool has its applications and limitations. In this contribution we discuss the application of these different techniques and highlight situations where these are particularly successful. A number of existing visualization approaches will be reviewed and improvements to those techniques are shown. A series of existing property visualization techniques are critiqued, such as the imaging of shale gouge ratio (SGR) and fault transmissibility multipliers (TMs) on the fault faces, both of which are limited in their ability to act as a proxy for cross-fault fluid flux in many circumstances. Fault rock property visualizations, such as hydraulic resistance and fault transmissibility, are presented. More direct and hence more powerful indications of probable cross-fault fluid flux are also described, such as the effective cross-fault transmissibility (ECFT) and the effective cross-fault permeability (ECFP). These static proxies for cross-fault fluid flux are compared against back-calculated and visualized cross-fault fluid flux values derived from either streamline or full flow simulation data. The ECFT is shown to provide a useful and rapid indication of likely fluid flux from the static model; however, the direct imaging of cross-fault fluid flux derived from simulation results allows for a far better understanding of how the faults have contributed to the reservoir flow simulation result. Visualizations of the fault- and flow-related properties: (a) on the fault face; (b) in the grid cells adjacent to the fault face; (c) as vectors; or (d) as fault-wide summations, all provide useful insights for different parts of the reservoir evaluation workflow. This contribution highlights a series of new and efficient techniques to image and hence improve the understanding and modelling of fault sealing in both exploration and production settings.
Abstract In this paper, we present workflows, key relationships and results of multiple stochastic fault seal analyses conducted on geocellular geological or (static) reservoir grids. Ranges of uncertainties are computed from new and published datasets for the different input relationships (e.g. throw, VShale to VClay, fault clay prediction, fault rock clay content to permeability); these are used as input into stochastic modelling processes and the impact of each is assessed. The power of stochastic modelling to focus interpretation and risking effort is reviewed. Reducing the uncertainty distributions from the published data ranges has a massive impact on the range of predicted fault seal properties. Halving the uncertainties associated with the computation of the transmissibility multiplier, for instance, reduces this range from 7 to 1–1.5 orders of magnitude of the base-case value (no uncertainty). Importantly, when combined together, the median predictions from each individual parameter do not lead to the median value for the final prediction; average relationships combined together will not therefore produce the average final prediction. This is a powerful result for two reasons: first, current geological modelling packages use global trends to define fault properties and so are likely to predict spurious results; and secondly, reducing the uncertainty on specific relationships by around 50% is an achievable goal. Locally calibrated datasets and relationships (field-specific) based on carefully characterized samples should allow for this improvement in prediction accuracy. This paper presents a review of fault seal techniques, published data and the potential pitfalls associated with the analyses.
Front Matter
Structurally complex reservoirs: an introduction
Abstract Structurally complex reservoirs form a distinct class of reservoir, in which fault arrays and fracture networks, in particular, exert an over-riding control on petroleum trapping and production behaviour. With modern exploration and production portfolios commonly held in geologically complex settings, there is an increasing technical challenge to find new prospects and to extract remaining hydrocarbons from these more structurally complex reservoirs. Improved analytical and modelling techniques will enhance our ability to locate connected hydrocarbon volumes and unswept sections of reservoir, and thus help optimize field development, production rates and ultimate recovery. This volume reviews our current understanding and ability to model the complex distribution and behaviour of fault and fracture networks, highlighting their fluid compartmentalizing effects and storage-transmissivity characteristics, and outlining approaches for predicting the dynamic fluid flow and geomechanical behaviour of structurally complex reservoirs. This introductory paper provides an overview of the research status on structurally complex reservoirs and aims to create a context for the collection of papers presented in this volume and, in doing so, an entry point for the reader into the subject. We have focused on the recent progress and outstanding issues in the areas of: (i) structural complexity and fault geometry; (ii) the detection and prediction of faults and fractures; (iii) the compartmentalizing effects of fault systems and complex siliciclastic reservoirs; and (iv) the critical controls that affect fractured reservoirs.
Structural evolution of the Penguins Cluster, UK northern North Sea
Abstract The Penguins Cluster is a group of four oil and gas fields in the northern end of the East Shetland Basin. Its structural complexity is caused by the interaction between two or more fault trend populations, fault reactivation and the impact of faulting on the Brent reservoir architecture. This structural picture is further complicated by a NW–SE trending basement lineament interpreted as a Caledonian shear zone. The present day structural configuration is the result of two Mesozoic rifting episodes and their associated thermal subsidence phases. The Permo-Triassic rifting created a number of north–south-trending tilted fault blocks, and was followed by a period of tectonic quiescence until the Middle Jurassic, when a faulting episode coeval with the Brent Group deposition caused footwall rotation, uplift and erosion of the upper Rannoch Formation prior to the deposition of the Etive Formation across the area. The rifting climaxed in the late Jurassic, when the reactivation of pre-existing faults under oblique-slip conditions in the Penguin C Field created small-scale lozenge-shaped transpressional and transtensional fault blocks. The presence of reverse faults in the area is explained with a continuous kinematic model of structural evolution and oblique-slip fault reactivation rather than positive basin inversion.
Characterizing and producing from reservoirs in landslides: challenges and opportunities
Abstract Landslides can consist of rotational slips, translational glide blocks, topples, talus slopes, debris flows, mudslides and compressional toes which can combine in different proportions to form complex landslides. The mass movement can be subaerial or submarine, occur over wide ranges of scale and can vary in rate from creep to catastrophic failure. Complexity of the landslide reflects the controlling factors including the strength of the deforming material and triggering mechanisms such as earthquakes, imposed load, increasing topographic relief and removal of toe material. Processes of landslide deformation include slip on discrete surfaces, distributed shear within the landslide, vertical thinning and lateral spreading through shear, fluidization, porosity collapse and loss of material from the top or toe of the complex. These processes control the quality of the resultant reservoirs. This leads to a greater range of reservoir types than conventional faulted reservoirs, with a proportionate upside and downside potential and difficulty in quantifying uncertainty. This paper uses examples from the literature, outcrops and subsurface datasets (including the Statfjord Field and the Halten Terrace in Norway) to outline the complexity of reservoirs in landslides and the challenges and opportunities in finding and producing them. We present workflows for seismic and subseismic characterization for exploration and reservoir scale based on geomorphological principles. Seismic mapping is achieved by classifying the form of the reflectors (both slip surfaces and the bounding envelope of the landslide) from an atlas of geometric and structural styles and is applied to both the Halten Terrace example and the Statfjord Field. We present a new workflow for reservoir characterization in which integration of structural, biostratigraphic, sedimentological and dynamic data gives key information on process, timing and heterogeneity of the reservoir. For the Statfjord field, important maps of the landslide block stratigraphy derived from a subcrop map and communication maps based on a c . 130 well dataset can be correlated to outcrop analogues and used to develop a predictive tool for landslide reservoir extent and quality, both in this field and others.
Abstract Fault network modelling of complex faulted structures, those containing hundreds or even thousands of faults, can be an extremely difficult and time-consuming process. Although techniques for mapping and modelling faulted structures have been in existence for nearly forty years, asset teams still struggle to create correct portrayals of such complex faulted reservoirs due to the limitations of the commonly used techniques. We have developed a new approach to fault network modelling, using a new concept of ‘fused’ fault blocks. The identification of fault–fault intersections is based not on a manually drawn fault network or table of relationships, but rather is derived from the fault surfaces themselves. The calculated intersection lines are then used to truncate faults against each other. Because the truncation information can be stored with the fault model, this process yields a repeatable and easily updatable fault model. The name of our technique ‘fused fault blocks’ refers to the fact that when a section of a fault is removed, the two fault blocks that had been created by the fault are then fused together, forming a single fault block. The resultant fault model can then be used to create a 3D reservoir grid, one in which the fault geometry has not been compromised, and therefore better reflects the actual structure. The speed of the fault-building process ‘seconds or minutes, even for models with hundreds of faults’ also allows multiple interpretations, placing the emphasis of the fault network building on the evaluation of the interpretation and the effects of compartmentalization, and not on the manipulation of software.
Abstract Accurately positioning faults in a geological model is a major concern because they are responsible for offsets of geological sequences. In the tetrahedral models studied in this paper, faults are discontinuities: faces of tetrahedra on either side of a fault are disconnected. Building tetrahedral models can require a large amount of time, especially when there are many faults. We present a tool for making small, real-time, modifications of faults in tetrahedral models arising from geometrical changes required either by new subsurface data or by new interpretations of existing subsurface data. Fault editing is achieved by moving control points on the fault in the tetrahedral grid and by computing a distortion property over an editable volume relative to the control point and spreading this distortion to neighbouring points using the Discrete Smooth Interpolation technique. The editable volume in which tetrahedron vertices are allowed to move is defined by a given distance to the fault. This approach provides a means of editing faults and fault-related features, such as branch-lines.
Abstract Scaled analogue experiments with layered brittle and ductile materials have been used to simulate the development of listric growth-fault and expulsion rollover systems during gravitational spreading of a passive margin sedimentary wedge detached on salt. The experiments were performed with varying sedimentation patterns and rates to simulate different depositional scenarios. Deformation monitoring with 3D optical image correlation techniques was used to quantify the 3D surface evolution and strain history of model structures. Our results indicate that rollover structure kinematics is strongly coupled to sedimentation patterns and rates. Whereas differential loading governs the margin-scale state of stress and extensional spreading in the experiments, more localized feedback between the dynamic depositional systems, fault-controlled subsidence, and salt mobilization control the strain history of local fault structures. This is reflected in the characteristic succession of extensional structures that evolve from symmetrical grabens through early, mature and late (collapsed) basinward listric growth-fault and rollover systems into landward listric growth-fault and rollover systems. A lack of sedimentation enhances reactive diapir rise and passive diapirism, whereas low sedimentation rates favour development of long-lived basinward listric growth-fault or expulsion rollover systems. Conversely, high sedimentation rates lead to the development of landward listric growth-fault and rollover systems.
Abstract Improving the accuracy of subsurface imaging is commonly the main incentive for including the effects of anisotropy in seismic processing. However, the anisotropy itself holds valuable information about rock properties and, as such, can be viewed as a seismic attribute. Here we summarize results from an integrated project that explored the potential to use observations of seismic anisotropy to interpret lithological and fluid properties (the SAIL project). Our approach links detailed petrofabric analyses of reservoir rocks, laboratory based measurements of ultrasonic velocities in core samples, and reservoir-scale seismic observations. We present results for the Clair field, a Carboniferous–Devonian reservoir offshore Scotland, west of the Shetland Islands. The reservoir rocks are sandstones that are variable in composition and exhibit anisotropy on three length-scales: the crystal, grain and fracture scale. We have developed a methodology for assessing crystal-preferred-orientation (CPO) using a combination of electron back-scattered diffraction (EBSD), X-ray texture goniometry (XRTG) and image analysis. Modal proportions of individual minerals are measured using quantitative X-ray diffraction (QXRD). These measurements are used to calculate the intrinsic anisotropy due to CPO via Voigt-Reuss-Hill averaging of individual crystal elasticities and their orientations. The intrinsic anisotropy of the rock is controlled by the phyllosilicate content and to a lesser degree the orientation of quartz and feldspar; the latter can serve as a palaeoflow indicator. Our results show remarkable consistency in CPO throughout the reservoir and allow us to construct a mathematical model of reservoir anisotropy. A comparison of CPO-predicted velocities and those derived from laboratory measurements of ultrasonic signals allows the estimation of additional elastic compliance terms due to grain-boundary interactions. The results show that the CPO estimates are good proxies for the intrinsic anisotropy of the clean sandstones. The more micaceous rocks exhibit enhanced anisotropy due to interactions between the phyllosilicate grains. We then compare the lab-scale predictions with reservoir-scale measurements of seismic anisotropy, based on amplitude variation with offset and azimuth (AVOA) analysis and non-hyperbolic moveout. Our mathematical model provides a foundation for interpreting the reservoir-scale seismic data and improving the geological modelling of complex reservoirs. The observed increases in AVOA signal with depth can only be explained with an increase in fracturing beneath the major unit boundaries, rather than a change in intrinsic CPO properties. In general, the style and magnitude of anisotropy in the Clair field appears to be indicative of reservoir quality.
Fracture intensity from geomechanical models: application to the Blue Forest 3D survey, Green River Basin, Wyoming, USA
Abstract A geomechanical method is presented for the prediction of subseismic fracture intensity associated with faulting using a boundary element model conditioned from 3D seismic interpretation within the Green River Basin, Wyoming, USA. Seismic data indicate two major phases of deformation: (1) an early phase of approximately ESE-verging contraction accommodated along a NNE–SSW striking system of basement-involved reverse faults and associated drape folds; and (2) a later phase of transtension accommodated by ENE–WSW-trending normal faults that are preferentially located in the hanging wall, above the crest of the drape fold. The models predict different spatial patterns of fracture intensity for each phase of deformation. For D 1 , enhanced probability for shear failure develops in the upper quadrant of the footwalls along the reverse faults (i.e. forelimb) and the greatest magnitudes occur adjacent to regions with the largest component of observable dip-slip displacement along the faults. During D 2 , enhanced probability for joint and fault formation occurs along-strike of the normal faults and in a general NE trend. Seismic velocity anisotropy data support the geomechanical predictions for both phases of deformation in that the location and azimuth of large anisotropies correlate with regions of predicted enhancement in joint and fault intensity.
Abstract Geomechanical simulations are used to demonstrate the importance of the way that models are loaded. In this paper the development of permanent damage during faulting using frictional-slip models of a reverse fault is investigated. Although the use of different loads and constraints can produce the same faulted geometry (for the same rock type, and at the same burial depth), the models develop very different stress and strain states. Permanent strain magnitudes and distributions between models are quite dissimilar, including the distributions of permanent dilation and compaction. This work demonstrates that boundary loads and boundary constraints are significant factors in determining what stress and deformation states evolve in the simulation model. The examples also illustrate that final (deformed) geometry alone is a very poor basis from which to predict either stress state or open fracture distribution. Bulk finite strain does not allow a prediction of local principal stress directions, magnitudes, or signs, at least in the vicinity of fault damage zones.
Abstract The concepts of damage mechanics are proposed as the logical basis for integration of seismic anisotropy based identification and geomechanics based prediction of subsurface fracturing. In such a context a damage tensor can be used to describe the evolution of (anisotropic) degradation of the elastic properties of a rock undergoing deformation. The concepts of damage mechanics are common to both geomechanical and seismic descriptions of rock elasticity but have not previously been connected. In geomechanical simulations of the evolution of geological structures, damage development can be predicted, described and recorded through a single damage tensor representation for each point in the model (with the inclusion of damage by a number of different mechanisms). With appropriate analysis, equivalent information may be determined from anisotropy analysis of seismic reflection data as the seismic wave propagation is sensitive to the total damage. Thus the proposed approaches for parameterization and modelling/analysis, using the damage domain, provide a common quantification of the damage (as detected using seismic data and predicted using geomechanical modelling) that permits integration of the two disciplines. This paper presents the damage-domain theory and linkage between the domains plus practicalities, with respect to both the geomechanical and seismic analyses, with illustration using a simple geomechanical model example.
Abstract The curvature of geological surfaces has been used to predict areas of elevated strain, deformation and fracture density. The research presented here tests if and to what extent a geometric measure such as normal surface curvature can be used as a proxy for deformation by comparing field observations with geometric modelling results. This is achieved by first quantifying fracturing in outcrop and then by performing a curvature analysis of the deformed bedding surface. This research suggests that curvature analysis by itself does not allow for the prediction of deformation: fracture density in the Emigrant Gap anticline is unrelated to horizon curvature, and synfolding fractures are aligned with prefolding fractures instead of the directions of principal curvature. Normal surface curvature by itself has only limited value in predicting strain or fracture density; however, surface curvature is a unique descriptor of shape. Describing the geometry of a horizon quantitatively is an essential first step when attempting to compare physical and numerical models with natural surfaces. The tools presented here allow for unique descriptions of three dimensional folded surfaces that are based on the normal surface curvature, and they provide the necessary mathematical rigour and flexibility to allow for descriptions of non-cylindrical folded surfaces.
Stratigraphic control on extensional fault propagation folding: Big Brushy Canyon monocline, Sierra Del Carmen, Texas
Abstract Mechanical stratigraphy exerts a first-order control on deformation at a range of scales from oilfield-scale structural style to deformation (e.g. fracturing) within an individual reservoir stratum. This paper explores an outcrop example where mechanical stratigraphy in a limestone and shale sequence directly influenced the structural style and distribution of deformation related to the propagation of a ‘seismic-scale’ normal fault that has maximum displacement on the order of 100–500 m and extends for more than 10 km. A monocline developed in Cretaceous Buda Limestone above tectonically thinned Del Rio Clay and faulted Santa Elena Limestone is here interpreted as an extensional fault propagation fold. Monocline limb dips reach 59°. The Del Rio Clay is thinned from approximately 36 m to 1.5 m, whereas the underlying Santa Elena Limestone is offset vertically by approximately 74 m along a steep (approximately 80°) normal fault. This large fault displacement of the Santa Elena Limestone is not transferred upward to the Buda Limestone because of ductile flow within the intervening Del Rio Clay. Although upward fault propagation has been inhibited, thinning of the Del Rio Clay and the resultant extreme displacement gradient at the tip of the fault have forced the Buda Limestone into a monoclinal fold. Two competent packstone and grainstone beds, 6 m and 2.7 m thick and separated by 10.5 m of less competent calcareous shale, comprise the Buda Limestone at this location. Deformation features within the competent Buda beds include bed-perpendicular veins that accommodate bed-parallel extension, and bedding plane slip surfaces with an up-dip sense of shear that offset the veins. Deformation is concentrated in the monoclinal limb and not in the monoclinal hinge regions. Consequently, bed-parallel extension and shear strain are associated with monoclinal dip, not with curvature. These results show that for this structure, bed dip is a better proxy for bed-parallel extension and related fracture dilation than is curvature.
Treatment of faults in production simulation models
Abstract This paper describes basic ‘rules-of-thumb’ that offer an indication of common uncertainties and pitfalls, as well as the analytical methods, data requirements and work elements required to replicate the impact of faults on fluid flow in production simulation models successfully. The first, and most important, stage in this modelling process is to ensure that an accurate structural interpretation is incorporated into the simulation model. In particular, that all fault linkages and cross-fault juxtapositions are taken from the seismic interpretation into the simulation grid. Fault rocks sometimes reduce the rate of cross-fault flow in which case it is important to account for this reduction in flow within simulation models. This is best achieved if databases of fault rock properties, measured from the field of interest or nearby similar reservoirs, are up-scaled to calculate fault transmissibility multipliers. It is sometimes necessary to consider not just the single-phase permeability but also the capillary pressure and relative permeability characteristics of the fault rocks present. Finally, all the relevant static and dynamic data must be appraised critically. However, the interpretation of such data is usually non-unique and misinterpretations can create errors in the production-related fault seal analysis. Where these basic guidelines are followed, it has been our experience that the project time required to achieve a history match of production data is dramatically reduced. In addition, as the history match is more geologically reasonable, the model is often more reliable for predicting the long-term behaviour of the reservoir. This gives confidence in the model's forecast to guide development planning and day-to-day field management decisions.
Definition of a fault permeability predictor from outcrop studies of a faulted turbidite sequence, Taranaki, New Zealand
Abstract Post-depositional normal faults within the turbidite sequence of the Late Miocene Mount Messenger Formation of the Taranaki Basin, New Zealand are characterized by granulation and cataclasis of sands and by the smearing of clay beds. Clay smears maintain continuity for high ratios of fault throw to clay source bed thickness ( c . 8), but are highly variable in thickness, and gaps occur at any point between the clay source bed cut-offs at higher ratios. Although cataclastic fault rock permeabilities may be appreciably lower ( c . two orders of magnitude) than host rock sandstone permeabilities, the occurrence of continuous clay smears, combined with low clay permeabilities (10s to 100s nD) means that the primary control on fault rock permeability is clay smear continuity. A new permeability predictor, the Probabilistic Shale Smear Factor (PSSF), is developed which incorporates the main characteristics of clay smearing from the Taranaki Basin. The PSSF method calculates fault permeabilities from a simple model of multiple clay smears within fault zones, predicting a more heterogeneous and realistic fault rock structure than other approaches (e.g. Shale Gouge Ratio, SGR). Nevertheless, its averaging effects at higher ratios of fault throw to bed thickness provide a rationale for the application of other fault rock mixing models, e.g. SGR, at appropriate scales.
Abstract Traditionally, the analysis of fault seal has been purely deterministic or a combination of deterministic and stochastic methods. In a deterministic model, prediction of the locations of reservoir overlaps is made from the static model of the reservoir horizon and fault geometry. The principal aim is to map faulted reservoir overlaps and determine their sealing character. This is usually performed using a predictive algorithm such as the shale gouge ratio (SGR) that relates the shale content of the formations that have moved past a point on the fault zone to the sealing capacity of the fault rock. Deterministic fault seal studies are sensitive to the uncertainties associated with mapping of horizons in proximity to faults and the inherent uncertainty in a static fault interpretation in both position and fault zone complexity. Uncertainty in the static structure model can be addressed by convolving uncertainty in throw magnitude with juxtapositions at the fault. However, this does not address the uncertainty in the distribution of reservoirs on either side of the fault. With stochastic models multiple realizations of the stratigraphy can be tested. Stochastic models capture the uncertainty in the position of the reservoir at the fault by allowing multiple realizations of stacking geometries, where the principal assumption is that these stacked reservoir zones are laterally continuous covering the entire likely fill area. Despite the conceptual differences between these two approaches to fault seal analysis, comparison of the predictions they make on the Ling Gu field shows a surprising degree of conformity. The cut-off used to determine the number of sand and shale beds in the stochastic workflow appears to account for seal by fault zone materials, since a conservative cut-off implies fewer sand beds with lower probability of leak and correlates with more shale in the section and higher SGR values.
Testing fault transmissibility predictions in a structurally dominated reservoir: Ringhorne field, Norway
Abstract At Ringhorne field, in the North Sea, judicious well placement and high quality 3D seismic data allow good control over stratigraphic and structural frameworks. In particular, two near-horizontal producing wells about 150 m apart on both sides of a critical normal fault are key for deciphering the fault effects on flow. These elements make this field ideal for using production data to constrain a range of process-based fault permeability predictions in a siliciclastic reservoir. A high resolution (50 m×50 m×1.8 m) faulted geological model, constructed in Petrelâ„¢, was used as input to process-based fault permeability predictions. Subsequent multiphase simulation and testing identified critical stratigraphic connections across shale layers and structural connections along a faulted relay around an isolated fault block. The simulations were used systematically as a probe to investigate both these and other controls on production and determine the likely range for permeability of fault zone materials, which are inferred to include deformed shales, sands, and minor cements. This study leverages the most pertinent observations and best constrained interpretations in the field to attempt to extract accurate, quantitative information on fault properties. A range of predicted fault permeability cases, linked to particular fault movement timing scenarios, were tested. The middle case, from a fault timing perspective, was determined to provide the best overall flow simulation match to all actual production information, providing valuable feedback for our process-based fault property prediction approach. Establishing the link between predicted and actual flow, and pressure history in response to critical reservoir plumbing elements, is paramount for evaluating and improving fault permeability predictions.