Abstract The Humbly Grove Field has, for the UK, a unique development history. It was discovered as an oilfield in May 1980 and produced as an oilfield until 2000 along with small satellite fields Herriard (developed) and Hester's Copse (not developed). Peak production of 2219 bopd was achieved during July 1986 but, by October 1988, the rate had fallen to around 1000 bopd, a rate that was more or less maintained until October 1995 after which the production fell rapidly. At this point the decision was taken to reconfigure the field as a gas storage facility. Significant renewed pressure depletion occurred between 2000 and 2005, following which first cushion and then storage gas was injected into two reservoirs: the Middle Jurassic, Great Oolite Group and the uppermost Triassic, Rhaetian Westbury Formation. Gas storage operations commenced in 2005 and the reservoirs have undergone cyclical gas injection and gas withdrawal since that date. The cyclical injection of gas and re-pressuring of the Great Oolite reservoir causes mobile oil to be swept towards dedicated oil production wells. This operates effectively as an enhanced oil recovery scheme. The co-produced liquid hydrocarbons provide a valuable secondary income stream for the field.

Abstract: Folds associated with normal faults are potential hydrocarbon traps and may impact the connectivity of faulted reservoirs. Well-calibrated seismic reflection data that image a normal fault system from the Inner Moray Firth basin, offshore Scotland, show that folding was preferentially localized within the mechanically incompetent Lower–Middle Jurassic pre-rift interval, comprising interbedded shales and sandstones, and within Upper Jurassic syn-rift shales. Upward propagation of fault tips was initially inhibited by these weak lithologies, generating fault propagation folds with amplitudes of c. 50 m. Folds were also generated, or amplified, by translation of the hanging wall over curved, convex-upward fault planes. These fault bends resulted from vertical fault segmentation and linkage within mechanically incompetent layers. The relative contributions of fault propagation and fault-bend folding to the final fold amplitude may vary significantly along the strike of a single fault array. In areas where opposite-dipping, conjugate normal faults intersect, the displacement maxima are skewed upwards towards the base of the syn-rift sequence (i.e. the free surface at the time of fault initiation) and significant fault propagation folding did not occur. These observations can be explained by high compressive stresses generated in the vicinity of conjugate fault intersections, which result in asymmetric displacement distributions, skewed towards the upper tip, with high throw gradients enhancing upward fault propagation. Our observations suggest that mechanical interaction between faults, in addition to mechanical stratigraphy, is a key influence on the occurrence of normal fault-related folding, and controls kinematic parameters such as fault propagation/slip ratios and displacement rates.

Abstract The post-Caledonian development of the West Orkney Basin is regularly cited as a classic example of basement-influenced rifting. This paper presents the first detailed multidisciplinary analysis of the three-dimensional (3D) geometries and distribution of post-Caledonian faults in onshore northernmost Scotland, examining their relationships to basement fabrics and comparing them to rift-related structures developed offshore in the West Orkney Basin. Two phases of rift-related faulting are distinguished: 1) Devonian ENE–WSW extension localized in the east of the basin and related to regional sinistral transtension along the Great Glen Fault; and 2) Permo-Triassic NW–SE extension focused to the west of the basin and probably contemporaneous with movements along the Minch Fault. A complex North Coast Transfer Zone is developed along the northern Scottish coast linking Mesozoic rifts that reactivated Caledonian structures in the West Orkney Basin (Naver Thrust) to those bounding the North Minch Basin (Outer Hebrides Fault Zone). Polymodal faulting patterns are widespread in onshore exposures. Fault patterns vary due to changes in the obliquity between regional rifting vectors and variably orientated pre-existing structures in each basement terrane. The geometric complexity and spatial variations in fault patterns onshore can be correlated with changes in basement structures, despite limited direct reactivation of pre-existing fabrics.

Abstract Significant advances in geosciences data acquisition, visualization and analysis now allow highly detailed outcrop models to be constructed for a range of petroleum industry purposes. From a given field locality, a virtual outcrop is created from a centimetre-scale digital elevation model and colour photographs with geological information overlaid as appropriate. In a visualization environment, these datasets can be viewed sequentially to simulate undertaking a fieldtrip. These virtual fieldtrips allow geoscientists to improve and expand the traditional fieldwork experience in a number of ways, ranging from planning and health and safety considerations for management, to providing live supplemental technical content on a mobile device to the fieldtrip participant. The fieldtrips are easily archived and content can be reviewed in the office to provide analogue information during technical work. Examples of virtual fieldtrips are provided on the DVD that accompanies this volume.

Abstract Faults are important controls on hydrocarbon migration and ore mineralization and, in areas of active deformation, are the most important source of seismic hazard. However, faults are rarely discrete surfaces and the internal structure of fault zones (e.g., the thickness, nature and continuity of the fault rocks, the distribution and segmentation of slip surfaces, and the orientation, distribution and connectivity of subsidiary faults and fractures) is a key control on their bulk fluid flow and mechanical properties. This Special Publication was inspired by two sessions held at the European Geosciences Union General Assembly in Vienna during 2005 and 2006 and contains 19 original papers divided into three sections. Part I addresses the controls on fault zone evolution, whilst Parts II and III focus, respectively, on the mechanical behaviour and fluid flow properties of fault zones. The introductory paper ( Wibberley et al. ) addresses each theme of the Special Publication: fault zone evolution, the permeability structure of ancient and active fault zones, the impact of faults on hydrocarbon sealing and migration, and the implications of fault zone geometry and material heterogeneity for seismogenic processes. In each section, Wibberley et al. identify important recent findings and suggest areas in which new conceptual advances in our understanding of fault zones are likely to occur. A key theme highlighted by many of the papers in Part I is the importance of pre-existing mechanical heterogeneities (e.g., bedding, joints) in controlling the internal structure of faults in sedimentary sequences. Johanssen & Fossen consider the contro

Abstract It is increasingly apparent that faults are typically not discrete planes but zones of deformed rock with a complex internal structure and three-dimensional geometry. In the last decade this has led to renewed interest in the consequences of this complexity for modelling the impact of fault zones on fluid flow and mechanical behaviour of the Earth's crust. A number of processes operate during the development of fault zones, both internally and in the surrounding host rock, which may encourage or inhibit continuing fault zone growth. The complexity of the evolution of a faulted system requires changes in the rheological properties of both the fault zone and the surrounding host rock volume, both of which impact on how the fault zone evolves with increasing displacement. Models of the permeability structure of fault zones emphasize the presence of two types of fault rock components: fractured conduits parallel to the fault and granular core zone barriers to flow. New data presented in this paper on porosity–permeability relationships of fault rocks during laboratory deformation tests support recently advancing concepts which have extended these models to show that poro-mechanical approaches (e.g., critical state soil mechanics, fracture dilatancy) may be applied to predict the fluid flow behaviour of complex fault zones during the active life of the fault. Predicting the three-dimensional heterogeneity of fault zone internal structure is important in the hydrocarbon industry for evaluating the retention capacity of faults in exploration contexts and the hydraulic behaviour in production contexts. Across-fault reservoir juxtaposition or non-juxtaposition, a key property in predicting retention or across-fault leakage, is strongly controlled by the three-dimensional complexity of the fault zone. Although algorithms such as shale gouge ratio greatly help predict capillary threshold pressures, quantification of the statistical variation in fault zone composition will allow estimations of uncertainty in fault retention capacity and hence prospect reserve estimations. Permeability structure in the fault zone is an important issue because bulk fluid flow rates through or along a fault zone are dependent on permeability variations, anisotropy and tortuosity of flow paths. A possible way forward is to compare numerical flow models using statistical variations of permeability in a complex fault zone in a given sandstone/shale context with field-scale estimates of fault zone permeability. Fault zone internal structure is equally important in understanding the seismogenic behaviour of faults. Both geometric and compositional complexities can control the nucleation, propagation and arrest of earthquakes. The presence and complex distribution of different fault zone materials of contrasting velocity-weakening and velocity-strengthening properties is an important factor in controlling earthquake nucleation and whether a fault slips seismogenically or creeps steadily, as illustrated by recent studies of the San Andreas Fault. A synthesis of laboratory experiments presented in this paper shows that fault zone materials which become stronger with increasing slip rate, typically then get weaker as slip rate continues to increase to seismogenic slip rates. Thus the probability that a nucleating rupture can propagate sufficiently to generate a large earthquake depends upon its success in propagating fast enough through these materials in order to give them the required velocity kick. This propagation success is hence controlled by the relative and absolute size distributions of velocity-weakening and velocity-strengthening rocks within the fault zone. Statistical characterisation of the distribution of such contrasting properties within complex fault zones may allow for better predictive models of rupture propagation in the future and provide an additional approach to earthquake size forecasting and early warnings.

Abstract The geometry, orientation and distribution of deformation bands and fractures in eolian sandstones, siltstones and shales of the San Rafael Desert and Moab Fault area have been investigated. The results show that deformation bands, which are cataclastic in eolian sandstones and disaggregation structures in siltstones, are unevenly distributed throughout the damage zone in the form of individual bands, deformation band zones and deformation band clusters. The density of bands increases with increasing grain size. In thin (<3 m) eolian sandstones deformation band frequency is significantly lower than in thicker eolian sandstones, whereas above this thickness the frequency seems not to be related to layer thickness. Furthermore, large faults do not develop higher concentrations of deformation bands. Somewhat simplified, this suggests that damage zone growth occurs by expansion into its hanging wall and footwall. Still, the highest concentrations of deformation bands occur close to the main fault, which is of importance when considering their effect on fluid flow. Their general fault-parallel conjugate arrangements favour intra-damage zone flow parallel to rather than perpendicular to the fault.

Abstract This paper examines the role of mechanical stratigraphy on the evolution of normal fault geometry and fault zone internal structure, using a well-exposed normal fault system from the Permian Lodève Basin, southern France. Faults formed early during the syn-deformation tilting history of the basin tend to have steeper segments in the competent sandstone layers due to refraction, assisted by pre-existing early bedding-perpendicular joints, where displacement remained on the order of bed thickness. Faults which continued to slip during tilting have a more complex structure of splays due both to the space incompatability problem of slip at fault bends of this irregular geometry, and because tilting favours the generation of new splays at a different angle to the earlier faults experiencing rotation. Continued deformation between faults and their splays often causes both distributed deformation in between the two, and reconnection of splays to the main fault forming isolated lenses. Thus, fault zone complexity increases greatly as slip exceeds competent bed thickness, owing both to the presence of the mechanical layering, and the fact that this layering is being tilted.

Abstract In this study, we focus on the transition from the host rock to the damage zone within brittle shear zones in order to document the structures forming during the initial phases of deformation, i.e., the fractures that formed prior to the formation of fault breccias and cataclasites. Structural analyses of rock samples from sites of the Talhof– and Palten–Liesing faults in the eastern Alps show that in these cases well-known R- and P-fracture patterns do not play a dominant role in the early stages of the generation of brittle fracture zones. In the studied layered marble and foliated impure quartzite samples, the boundary between the host rock and the damage zone is characterized by the formation of closely spaced fractures at high angles (70–90°) to the shear zone boundaries, being parallel to pre-existing layering/foliation planes. These fractures bound and define slender slab-like or columnar rock elements, here being termed lamellae. It is assumed that subsequent rotation of these lamellae in the bookshelf and domino modes associated with impeded dilation across the actual shear zone boundaries leads to kinking, splitting and final granulation of the lamellae to generate breccias of later fault core zones parallel to the shear zone boundary. In some cases, the observed bending and buckling of lamellae indicate additional ductile deformation subsequent to the development of the dominant lamellar structures formed by brittle fracturing.

Abstract The spatial distribution of extensional strain in interbedded mudstones and carbonates from around Kimmeridge Bay in southern England is examined using a variety of line samples. Normal faults and tensile fractures (veins) from the same deformation event show displacements ranging over 6 orders of magnitude. The relative contribution of these structures to the overall extension varies, with large faults (>10 m heave) accommodating about 65%, smaller faults (1–10 m heave) about 25% and veins less than 10% of the overall extension. The heterogeneity of fracture density and strain can be quantified from cumulative plots by applying a non-parametric method based on Kuiper's test. Both the degree and statistical significance of strain heterogeneity can be determined and are shown to be scale-dependent. Thin veins accommodate a fairly constant background strain across the region, whilst thick veins and small faults take up localized higher strains in damage zones around larger faults. Fault-strain is relatively homogenously distributed across the region. The faults and veins do not share the same scaling relationship. Thus, this study shows that it is not possible to simply extrapolate fracture frequencies and strain from fault scale to vein scale, and that the heterogeneity of extensional strain is scale dependent.

Abstract The Newberry Springs Fault Zone experienced slip associated with the 1992 Landers earthquake in the Mojave Desert of California, USA. Detailed analysis of scaling relationships from single-event ground ruptures in the Newberry Springs Fault Zone mapped in the field shows an average maximum displacement to length ( D max / L ) relationship for fault segments (rupture lengths in the range of 100–1000 m) of 8×10 −5 –consistent with previously published D max / L ratios for normal fault earthquake ground ruptures (rupture lengths in the range of 1–100 km) of 7×10 −5 . To explore the ability of remote sensing (interferometric synthetic aperture radar or InSAR) to map small-displacement single-event fault ruptures and add constraints on segment displacements, we applied established interferometry methods with phase unwrapping to produce maps of line-of-sight displacement and displacement gradient. These maps highlight fault traces that experienced displacement during the time between collection of the synthetic aperture radar images. Comparison of published 1992 single-event ground rupture maps with mapping based on photogeologic interpretation of 1950s vintage aerial photographs indicates that most of the 1992 ruptures occurred as reactivation of pre-existing slip surfaces. In general, D max / L for total fault displacement is approximately 100 times D max / L for single-event ruptures. Evidence from the Newberry Springs Fault Zone indicates that, since the Pleistocene, at least 10–20 Landers-like slip events have occurred, reactivating the Newberry Springs Fault Zone. Evidence of wide damage zones and reactivation of individual segments developed in alluvial floodplain deposits, at relatively small (order of metres) fault displacements, supports a conceptual model of fault damage zone width being established early, during fault propagation. With continued displacement by accumulation of additional slip events, fault zone damage intensifies. The fault zone width may remain relatively stable, although the active portion of the fault zone will likely narrow as faulting continues and a throughgoing slip surface develops and accumulates the bulk of displacement.

Abstract The geometry and evolution of normal fault terminations were studied in Tithonian limestones exposed on a vertical cliff in the French SE-Basin. The rocks consist of mainly limestone layers alternated with thin clayey interlayers. All studied fault zones die out vertically into bed-perpendicular veins striking approximately parallel to the fault. Displacement decreasing to zero towards the fault tip is accommodated horizontally by bed-parallel opening of calcite veins, and vertically by bed-perpendicular localized compaction. The latter mechanism leads the clayey interlayers to be thinned and in places completely pushed out, and enhances pressure solution in bed-parallel seams. The respective thicknesses of the limestone layers and clayey interlayers, and the ratio between local displacement amount and bed thickness influence the geometries of the fault termination and of the steps between slip surfaces. Relatively thick clayey interlayers localize low-angle slip surfaces and may impede the vertical propagation of the slip surface. Vertical fault restriction is also related to thick limestone layers, which are deflected and affected by outer arc extensional fractures, localized pressure solution and dilational jogs connecting adjacent propagating slip surfaces. However, beds keep their continuity if thicker than the local displacement amount. Where the local displacement is larger than the layer thickness, limestone beds are disconnected and clayey interlayers are completely cut but the slip surfaces. Tip-point veins, as well as outer arc veins, do not cross the clayey interlayers and fluid flow is local and confined within one limestone layer. In contrast, dilational jogs in places cut through several layers, and the breaking of clayey interlayers causes an increase in fluid flow. Conduits can be opened along the fault zone and fluids are driven into the jog, so slip surfaces may communicate separate reservoirs, until dilational jogs are sealed by mineral precipitation.

Abstract Elucidation of the internal structure of fault zones is paramount for understanding their mechanical, seismological and hydraulic properties. In order to observe representative brittle fault zone structures, it is preferable that the fault be passively exhumed from seismogenic depths and the exposure must be in arid or semi-arid environments where the fragile rocks are not subject to extensive weathering. Field observations of two such faults are used to constrain their likely mechanical properties. One fault is the Carboneras fault in southeastern Spain, where the predominant country rocks are phyllosilicate-rich lithologies, and the other is part of the Atacama fault system in northern Chile, where faults pass through crystalline rocks of acidic to intermediate composition. The Carboneras fault is a left lateral fault with several tens of kilometres offset exhumed from approximately 4 km depth, and displays multiple strands of clay-bearing fault gouge, each several metres wide, that contain variably fractured lenses of protolithic mica schists. The strain is evenly distributed across the gouge layers, in accordance with the measured laboratory mechanical behaviour which shows predominantly strain hardening characteristics. The overall width of the fault zone is several hundred metres. Additionally, there are blocks of dolomitic material that are contained within the fault zones that show extremely localized deformation in the form of faults several centimetres wide. These are typically arranged at an angle of c . 20° to the overall fault plane. These differing types of fault rock products allow for the possibility of ‘mixed mode’ seismicity, with fault creep occurring along the strands of velocity strengthening clay-rich gouge, punctuated by small seismic events that nucleate on the velocity weakening localized faults within the dolomite blocks. The Caleta Coloso fault in northern Chile has a left-lateral offset of at least 5 km and was exhumed from 5–10 km depth. The fault core is represented by a 200–300 m wide zone of hydrothermally altered protocataclasite and ultracataclasite. This is surrounded by a zone of micro and macro-fractures on the order of 150 m thick. The fault core shows a heterogeneous distribution of strain, with alternate layers of ultracataclasite and lower strain material. The strain-weakening behaviour of crystalline rocks might be expected to produce highly localized zones of deformation, and thus the wide core zone must be a result of additional process such as precipitation strengthening or geometric irregularities along the fault plane.

Abstract Previously hypothesized fault weakening mechanisms include faults lined by low-friction clay gouges, elevated pore pressures within fault cores and/or the operation of dynamic weakening during seismic slip. Geological studies to support dynamic weakening are still in their infancy and there is little geological evidence for the widespread occurrence of low-friction gouges. The cores of some ancient faults exhumed from <5 km depth contain sheared syntectonic mineral veins. This observation is consistent with elevated pore pressures, but the implications for long-term fault weakening are unclear. Experimental data and microphysical modelling suggest that frictional–viscous flow within phyllosilicate-rich fault rocks (phyllonites, some foliated cataclasites) can cause sufficient weakening of crustal faults to satisfy published heat flow constraints. These predictions are consistent with the common occurrence of phyllonite in the cores of large-displacement faults exhumed from >5 km depth. Comparison with seismological data suggests that some faults with phyllonitic cores are likely to generate large earthquakes. Future studies should establish the geological evidence for seismic slip within phyllonitic fault cores and quantify the partitioning between seismic slip and frictional–viscous flow. Further geological observations are also required to test the hypothesized mechanisms by which earthquakes can nucleate and propagate along phyllosilicate-rich faults.

Abstract The influx of fluids into fault zones can trigger two main types of weakening process that operate over different timescales and facilitate fault movement and earthquake nucleation. Short- and long-term weakening mechanisms along faults require a continuous fluid supply near the base of the brittle crust, a condition satisfied in the extended/extending area of the Northern Apennines of Italy. Here carbon mass balance calculations, coupling aquifer geochemistry to isotopic and hydrological data, define the presence of a large flux ( c . 12 160 t/day) of deep-seated CO 2 centred in the extended sector of the area. In the currently active extending area, CO 2 fluid overpressures at ∼85% of the lithostatic load have been documented in two deep (4–5 km) boreholes. In the long-term, field studies on an exhumed regional low-angle normal fault show that, during the entire fault history, fluids reacted with fine-grained cataclasites in the fault core to produce aggregates of weak, phyllosilicate-rich fault rocks that deform by fluid assisted frictional–viscous creep at sub-Byerlee friction values (μ<0.3). In the short term, fluids can be stored in structural traps, such as beneath mature faults, and stratigraphical traps such as Triassic evaporites. Both examples preserve evidence for multiple episodes of hydrofracturing induced by short-term cycles of fluid pressure build-up and release. Geochemical data on the regional-scale CO 2 degassing process can therefore be related to field observations on fluid rock interactions to provide new insights into the deformation processes responsible for active seismicity in the Northern Apennines.