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The Upper Cretaceous and Palaeogene rocks of S England and N France are cut by a NW-trending system of fractures, characterized by swarms of vertical extension joints accompanied by conjugate sets of hybrid and shear joints, and normal mesofaults. Many major photolineaments exhibit the same NW trend as the fractures, and are particularly conspicuous in N France where they are coincident with rectilinear drainage channels, suggesting a relationship between fracture orientation and physiography. In S England, this relationship is absent; possibly because of the influence of additional fracture systems related to major flexures trending obliquely to the NW-SE direction. These major E-W trending flexures and associated fracture systems were formed during the Oligocene to Early Miocene 'Helvetic' phase of deformation, and are cross-cut obliquely by fractures in the NW-trending system. The NW-trending fractures and lineaments are parallel to neotectonic normal faults in the Lower Rhine embayment, and become less well developed to the west with increasing distance from these major structures. The Lower Rhine faults were reactivated and propagated into the Quaternary cover as a consequence of NE-SW regional tension generated during the late Neogene to Recent 'Jura' phase of NW-SE Alpine convergence. The NW-trending extensional structures in S England and N France are considered to be of about the same age and related to the same stress regime as the neotectonic normal faults of the Lower Rhine embayment. However, because fractures of the NW-system cutting Palaeogene sediments in SE England are truncated by the erosion surface beneath the Plio-Pleistocene Red Crag deposits, it is possible that in England fracture initiation did not continue into Quaternary times.

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Length frequency distributions of fractures are shown to have a power-law trend across many orders of magnitude. This allows an estimate to be made of the full number of fractures and faults existing in a reservoir over that which can be measured from core and seismic data. This relationship has been applied to reservoir studies involving the continuity of juxtaposed sands, permeability impairment, and estimating the number of fractures that a horizontal well may intersect. Many more fractures and faults exist in reservoirs than those that can he observed from seismic and core data, which have significant implications for reservoir characterisation, geomechanical properties and waterflood behaviour. Introduction There is a tendency for natural fractures in reservoirs to gain the attention of development teams only when the fractures provide permeability significantly greater than that of the matrix: usually in reservoirs which would not be commercially productive without the presence of those fractures. These reservoirs are deemed as 'naturally fractured' and are, of course, an important resource in the hydrocarbon industry. However, the somewhat loose application of this term tends to lead to the neglect of natural fractures as a component in the description of a much larger set of reservoirs. By the term 'fractures' we encompass both joints (extension fractures) and faults (all fractures whose walls have undergone shear displacements). The wider importance of natural fractures includes the following:Juxtaposition of different lithologies across faults can cause either an interruption in reservoir continuity, or enhancement due to the juxtaposition (across non-sealing faults) of depositionally isolated units.Fractures provide surfaces with an intrinsic conductivity that is almost certainly different from that of the surrounding matrix, occurring either through genesis (e.g. the cataclastic grain comminution in a granulation seam), diagenesis (i.e. mineralisation) or aperture governed by present-day effective stress. These altered surfaces can form a first-order perturbation to the permeability variations arising from depositional processes. In addition, the intrinsic conductivity of a fracture can change as stresses change in the reservoir rock during the development life of a field. There is growing evidence of the dominant influence of fractures in the lateral anisotropy of waterfloods, a process in which the reduction in stresses due to injection probably plays a significant role. The geomechanical behaviour of natural fractures during depletion may manifest itself as an influence upon compaction drive, subsidence and even casing failures. The presence of natural fractures carries particular implications when development of a reservoir with horizontal or high-angle wells is considered. The most obvious change is that high-angle wells have a greater probability than conventional wells of intersecting sub-vertical or vertical fractures. This fact has led to spectacular successes in the exploitation of reservoirs containing conductive natural fractures, for example in the Austin chalk of Texas. Additionally, the influence of natural fractures on the viability of horizontal or high-angle wells may be effective in at least two other ways:The productivity benefit of a high-angle well is very dependent upon the effective ratio of vertical to horizontal permeability. Fractures can increase or decrease this ratio.Conductive natural fractures and faults may provide pathways forextraneous water production from an underlying aquifer or gas production from a gas cap. In all these applications a knowledge of the true frequency distribution of fractures is a necessary component in a complete synthesis of the process. However, direct measurement of the distribution is hindered because reservoir fractures are observable only in the relatively limited ranges in scale of seismic and core data. We examine in this paper the concept that scaling relationships in fracture distributions enable the quantitative interpolation between the ranges of observable fracture sizes. We are concerned here primarily with spatial frequency distributions of fractures. A knowledge of the true form of these is a first step towards being able to predict the effect of fractures on fluid flow, but the subsequent steps of connectivity and conductivity are beyond the scope of this paper. REVIEW OF FRACTURE DISTRIBUTIONS The literature dealing with fractures in rock covers many disciplines and industries. Many assessments of frequency distributions of fracture lengths and spacings have been reported. The distributions have usually been given as one of log normal, negative exponential, gamma, Weibull, or occasionally power law or Pareto. P. 367

A common model for normal fault growth involves a single fault at depth splaying upwards into a series of en-echelon segments. This model is applied to faults as well as a range of extension fractures, including veins, joints and igneous dykes. Examples of splaying growth fault systems in the Columbus Basin, offshore Trinidad, are presented. They include the commonly described upwards splaying type, but also one fault zone with an upward change from disconnected overlapping synthetic faults to a continuous fault. One fault zone with high-displacement fault segments is separated by a relay ramp at depth, becomes breached higher up, developing into a continuous fault at its upper part, where displacements are least. This example suggests that whilst kinematic linkage typically precedes geometric linkage in the evolution of relay ramps, low-displacement parts of a fault system may be geometrically linked whereas higher displacement areas are only kinematically linked.

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.Global estimates of conventional hydrocarbon resources are typically subdivided based on lithological reservoir types for example, carbonate or siliciclastic (e.g. Roehl & Choquette 1985). However, many of these sedimentary rock reservoirs may contain fractures to a greater or lesser degree. The recent boom of unconventional reservoirs highlights once again the key role that natural fractures can play in helping production of fluids. It also requires an improved understanding of the geology and physics of natural fracture networks to meet public expectations regarding safety issues. Moreover, fracture networks can be present in otherwise impermeable crystalline basement rocks (e.g. Sanders et al. 2003;Murray & Montgomery 2012;Slightam 2012) and igneous intrusions (e.g. Gudmundsson & Løtveit 2012), also allowing these rocks to form potential fractured reservoirs. Historically, fractured crystalline basement rocks have been under-explored as potential hydrocarbon reservoirs. Naturally fractured reservoirs constitute a substantial percentage of remaining hydrocarbon resources. Naturally fractured reservoirsA reservoir fracture is a general term used to describe a 'naturally occurring macroscopic planar discontinuity in rock due to deformation, or physical diagenesis' (Nelson 2001). Reservoir fractures encompass both extensional ( joints) and shear (faults) structures. Fractures formed by brittle tectonic deformation are the most common focus for studies of naturally fractured reservoirs. However, reservoir fractures may also include structures that formed by desiccation (e.g. shrinkage cracks) and syneresis (e.g. chickenwire texture) in sediments, and structures that formed by thermal , as used here. Naturally fractured reservoirs are generally defined as such when the fracture network has a significant influence on fluid flow in the reservoir such that: (1) the fracture networ...

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