Abstract Naturally fractured reservoirs, within which porosity, permeability pathways and/or impermeable barriers formed by the fracture network interact with those of the host rock matrix to influence fluid flow and storage, can occur in sedimentary, igneous and metamorphic rocks. These reservoirs constitute a substantial percentage of remaining hydrocarbon resources; they create exploration targets in otherwise impermeable rocks, including under-explored crystalline basement, and they can be used as geological stores for anthropogenic carbon dioxide. Their complex fluid flow behaviour during production has traditionally proved difficult to predict, causing a large degree of uncertainty in reservoir development. The applied study of naturally fractured reservoirs seeks to constrain this uncertainty and maximize production by developing new understanding, and is necessarily a broad, integrated, interdisciplinary topic. Some of the methods, challenges and advances in characterizing the interplay of rock matrix and fracture networks relevant to fluid flow and hydrocarbon recovery are reviewed and discussed via the contributions in this volume.

Abstract Lateral structural variability and partitioning of fold–thrust belts often reflects lateral variations in the stratigraphy of the deforming foreland and interaction with inherited structures. The Keping Shan Thrust Belt, NW China, was initiated during the late Cenozoic and is a spectacular example of contractional deformation in a foreland setting. The belt is characterized by a series of imbricate thrusts which form a broadly arcuate salient and deform the thick (3–6 km) Phanerozoic sedimentary succession of the NW Tarim Basin (SW Tien Shan foreland). Abrupt lateral changes in the thickness of the sedimentary succession are associated with a series of major pre-existing basement faults which cross-cut the belt and which were probably initiated during early Permian times. These lateral variations in the basin template have impacted strongly on the structural architecture of the superimposed thrust belt. Variations in the thickness of the sediment pile affect the spatial distribution of thrusts, which increase in abundance where the sediment is thinnest. The inherited cross-cutting basement faults and the associated abrupt changes in sediment thickness combine to generate partitioning of the thrust belt.

Abstract The Upper Cretaceous (Senonian) Chalk in Kent, SE England, is considered with the aim of establishing the tectonic history of the basin in which it was deposited, based on the chronology of fractures and an understanding of the role of these fractures in controlling fluid movement in high-porosity-low-permeability sediments. The earliest brittle structures in the study area are NE-SW-striking, flint-filled shear fractures, with dips of c. 60°, which were formed when the maximum compression (σ 1 ) was vertical and were utilized as channels for fluid movement during flint filling. Flint also occurs along bedding planes, suggesting a diagenetic source. This phase was followed by the development of NW-SE-striking fracture swarms containing fractures ranging between vertical joints and steeply dipping hybrid fractures with acute dihedral angles of c. 40°. The absence of flint along these fractures indicates that they formed after diagenesis of the Chalk. NW-SE-striking, subvertical, regularly spaced, through-going joints then formed as a result of a NW-SE regional compression linked to the Alpine collision. The final stage in the basin history relates to the formation of bed-parallel and vertical (i.e. bed-normal), bed-restricted, systematic and unsystematic fractures associated with uplift and unloading. To model fluid flow through the fracture network present in the Chalk, a finite-element-finite-volume modelling was carried out. The fracture geometries mapped in the field were discretized using unstructured hybrid element meshes with discrete fracture representations. The permeability of fractures was calculated from the cubic law and the petrophysical properties of the rock matrix were taken from Chalk reservoirs in the North Sea. In the models, a constant pressure was applied at the top of the oil-saturated, fractured Chalk while water was injected at the base. In spite of greater density, the water preferentially displaced the oil from the fractures and migrated faster through the fracture swarms and joints than through bed-restricted fractures and the rock matrix. Almost 830f the total flow within the model occurred through the fractures. The results of the field study, combined with those of the numerical modelling, suggest that fracture swarms have a strong impact on the movement of fluids in fractured and faulted reservoirs.

Abstract This chapter focuses on the evolution of fractures during the inversion of the Bristol Channel Basin, and examines the lateral and vertical consistency of the resulting fracture network within the alternating Liassic limestones and shales. The study has two principal aims. The first is to determine the reliability of fracture systems deduced using more limited data from less well-exposed regions or unexposed regions sampled only by drilling, and the second is to assess whether the fractures are linked to a regional stress field or are the result of a local stress field controlled by the geometry and mechanism of formation of a fold. The joint patterns were studied using a combination of scanline and window sampling, and the results indicate that there are considerable variations in the fracture systems between adjacent limestone beds and also lateral variation within the same bed. Although there is little doubt that the independent development of fracture patterns in adjacent limestone beds is facilitated by the intervening shale horizons, which allow them to become mechanically decoupled, the reasons for these variations are still unclear.

Abstract Fluid flow through fractured media is dependent upon a variety of parameters including fracture length, orientation and density, and also on the relative magnitude of the lithostatic stress and the fluid pressure. Because of this it is often difficult to determine the fluid pathway through a particular fracture system even when the geometry of the network is known. Although many fluids (e.g. gas, water etc.) leave little or no evidence of their passage through the rock others such as magmas and fluidized sediments preserve the pathways they follow by forming dykes and sills. It is found that the pathways preserved by the two types of fluids are different i.e. the spacing of the clastic dykes follows a power-law distribution and that of the igneous dykes a log-normal distribution. It is suggested that this in part might reflect the different properties of the dyke material (specifically its permeability) which determines whether or not the fracture containing the dyke can continue to act as a channel of easy fluid migration once the dyke has been emplaced.

Abstract The Zagros Mountains are situated along the NE margin of the Arabian plate and are the product of complex deformation which began in Late Cretaceous time as a result of the collision between the Arabian and Central Iranian plates. During Pliocene time, deformation increased when plate convergence was accelerated by the opening of the Red Sea. This stimulated the migration of a deformation front from the collision zone towards the SW into the undisturbed Zagros basin and led to the creation of the Zagros Mountain Belt. The type and distribution of the deformation in the Zagros are controlled mainly by plate velocity, which is linked to the anticlockwise rotation of the Arabian plate around a pole in Syria, and the regional stratigraphy. The sedimentary cover and the underlying metamorphic basement decouple along an important detachment horizon, the Hormuz Salt Formation, and the uneven thickness and distribution of this salt plays a key role in determining the geometry of the deformation belt. Analysis of the distribution and geometry of the folds provides evidence for the southwestwards migration of the deformation front into the Arabian plate. The analyses are consistent with field evidence for serial folding, which indicates that each fold takes c. 600 ka to develop fully, and with the model of a southwestward advancing deformation front driven by the processes of serial folding and footwall collapse.

Abstract A considerable body of work exists in the geological literature dealing with the formation of buckle folds, (i.e. folds formed by compression either parallel or at a low angle to the layering or fabric of the rock) and a summary of much of this is presented in Price & Cosgrove 1990. In addition, fractures associated with these folds have been reported and discussed extensively for many decades, (e.g. Stearns 1964). This in part reflects the fact that the formation of folds and their associated fracture patterns frequently plays an important role in controlling the migration and concentration of fluids within the crust and thus has important implications regarding the disposition of water, hydrocarbons and zones of mineralization. However, there are many mechanisms other than buckling operating in the crust which can give rise to folds. One of the most important is that of ‘forced folding’ defined by Stearns (1978) as ‘folding in which the final overall shape and trend [of the fold] are dominated by the shape of some forcing member below’ and these folds and their associated fracture patterns have received relatively little attention in the literature. The present volume is an attempt to redress this imbalance. Unlike buckle folds, which are only generated during layer parallel compression, forced folds can be formed in any tectonic environment and are equally common in extensional and compressional regimes. The dominant mechanism operation during forced folding is ‘bending’, defined as the flexuring of a layer or surface by a compression acting at

Abstract In this paper the three-dimensional geometry and spatial organization of folds (both buckle folds and various types of forced folds) are considered, together with their associated fracture patterns, in an attempt to determine if these features can be used in regions of poor exposure or in areas where the geologist must rely on seismic data to indicate the type of folding that has occurred. This study of the relationship between the various fold types and associated fracture patterns draws on theoretical considerations of ideal conceptual models of folds, analogue models and field studies.

Abstract The mechanics of deformation in multilayer flexures is analysed by comparing field observations of joint clusters from the East Kaibab Monocline, Utah with fracture patterns produced in analytical and numerical experiments. Dune boundaries (bedding planes) were mapped through the thickness of the aeolian Navajo Formation, and the occurrence of joints related to dune boundary slip and fold curvature was documented. Slip along dune boundaries, as evidenced by joint clusters oblique to bedding, occurs along the steep limb of the fold and in the middle of the Navajo. Joints perpendicular to bedding and parallel to the fold axis occur near the synclinal hinge. Numerical experiments examine a layer flexed to match the Navajo at Hackberry Canyon with both uniform and observed distribution of dune boundaries. Within the numerical experiments, horizontal frictional interfaces slip within the centre of the layers where dips are steepest, and opening-mode fractures related to curvature form within the anticlinal and synclinal hinges of the fold. Thus, the first-order numerical results match field observations. This study illustrates the important roles of mechanical stratigraphy and interlayer slip in multilayered folding and the contribution of bedding-plane faults and fold curvature in the production of joint clusters.

Abstract Cretaceous and Tertiary coal beds in the western United States typically contain subvertical opening-mode fractures (cleat). However, closely spaced normal faults abruptly substitute for opening-mode fractures in coal beneath some sandstone lenses having blunt terminations. Differential forced-fold compaction of coal beds around and beneath lens-shaped sandstone bodies accounts for such shifts in fracture style. Finite element modelling of coal deformation shows that shear stress is augmented in coal layers below abruptly tapering edges of sandstones lenses, favouring fault development, whereas under gradually tapering lenses shear stresses are not sufficiently enhanced to cause shifts in fracture style. Upper Cretaceous Mesaverde Group coal beds in southwest Wyoming have significant variations in fracture style over distances of a few to tens of metres. Because these faults have little or no porosity, the coal that contains them is likely to have low permeability compared to coal having typical (generally porous) opening-mode fractures. Thus, shifting fracture style may affect regional and local gas and water flow in coal beds.

Abstract It is argued that the present trapping geometry within the Eocene Alba Field of the Outer Moray Firth, UKCS, has combined structural-stratigraphic elements. The structural component formed by significant topographic inversion of the deep-marine channel sandstones that constitute the field’s reservoir. A study of core and three-dimensional (3D) seismic demonstrates that topographic inversion took place in response to two processes. The first was differential compaction of mudstone over the channel resulting in a forced fold. This structure was further enhanced by post-depositional sandstone fluidization and remobilization, the latter being focused toward the structural crest of the sandstone body which formed the core of the forced fold. The process of post-depositional remobilization caused significant sand injection into the mudstone overburden and the formation of seismic-scale dykes at the margins of the channel fairway. The growth of the forced fold (by sand remobilization) and the position of the peripheral dykes is analogous to that of igneous intrusions, specifically laccoliths.

Abstract On the western margin of the Rhine graben, forced folding and décollement of competent Dogger strata occurred during the latest Eocene-Early Oligocene (Priabonian) as a result of normal faulting in the basement. The Dogger series are represented by NNE-striking thick layers of oolitic limestones strongly disrupted by extensional structures and incipient boudinage. To the east, the overlying Priabonian (syn-rift) sequences exhibit divergent onlaps, steeply dipping intraformational unconformities and a general drag syncline geometry. These syn-depositional structures attest to a close control by the progressive tilting and normal dip-slip sliding of the Dogger strata. Interpretation of a shallow seismic reflection profile reveals that this tilting is related to an underlying master normal fault which dips 60–70° to the east. This fault offsets the Variscan crystalline basement and the gently dipping Triassic cover by about 2 km. To be balanced, the structural arrangement requires the presence of a décollement layer located between the top of the Triassic series and the Dogger series, corresponding to the Late Triassic incompetent gypsiferous clays and marls. The response of the competent Dogger to the imposed forced folding is through the development of a large-scale extensional syncline in the hangingwall of the master detachment, compensated by the development of a piggy-back half-graben in the footwall. In the light of this new interpretation, a mechanical hypothesis is proposed including a two-stage evolution model of extension and subsidence for the Rhine graben.

Abstract Field and seismic data in northern Nova Scotia, eastern Canada, demonstrate that displacement transfer from steep basement faulting to bedding-parallel detachment is necessary in the development of forced folds. Lateral translation of the strata above horsted and down-dropped blocks generates a monoclinal structure which, as the faults are kinematically linked, evolves in a manner similar to fault-bend folds in thrust-and-fold belts. In the case of partial transfer of displacement a breached drape syncline is developed. The breached syncline is characterized by steep upturned beds against the fault that truncates them. In the Nova Scotia example the detachment horizon is located at the base of a Visean evaporitic sequence, and is exposed in the study area showing shearing structures within the evaporites. Brittle fault rock types (gouge and cataclasite) and meso- to microstructures were formed, including stretching lineation, principal schistosity plane and secondary shear planes, as well as intrafolial to upright asymmetrical folds. The regionally extensive weak evaporitic layer was remarkably effective in transferring displacement between the two faults, with mechanical decoupling of the strata above the evaporitic detachment being observed in the horsted block 70 km away from the steep basement fault. Moreover, displacement was also transferred as much as 40 km onto the down-dropped block at the frontal part of the system.