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  • 1
    Publication Date: 2008-01-02
    Description: 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.
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  • 2
    Publication Date: 2008-01-02
    Description: A range of unfaulted and faulted bed-scale models with sheet-like or lobate bed geometries and faults of comparable sizes to beds have been built and analysed in terms of bed connectivity and fractional permeability assuming permeable sands and impermeable shales and shale smears. A new method has been devised allowing amalgamation ratio to be included explicitly as model input and this property, rather than net:gross ratio, is found to be the dominant control on the connectivity of unfaulted sequences. At the geometrically representative scales considered (horizontal distances of 〉1 km for beds up to c. 1 m thick and faults up to c. 5 m throw), faulted sequences rarely have lower connectivities than their unfaulted sedimentological equivalents irrespective of whether fault rock properties are included. Models containing stochastically placed shale smears associated with each faulted shale horizon are generally better connected than if deterministic Shale Gouge Ratio cut-offs are applied. Despite the complex interactions between geological input and connectivity of the faulted sequences, the flow properties at representative scales are controlled by three geometrical variables describing connectivity, anisotropy and resolution. If two different faulted or unfaulted systems have identical values of these three variables they will have the same equivalent flow properties.
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  • 3
    Publication Date: 2001-01-01
    Description: The growth of normal fault arrays is examined in basins where sedimentation rates were higher than fault displacement rates and where fault growth histories are recorded by thickness and displacement variations within syn-faulting sequences. Progressive strain localization is the principal feature of the growth history of normal faults for study areas from the Inner Moray Firth, a sub-basin of the North Sea, and from the Timor Sea, offshore Australia. The kinematics of faulting are similar in both study areas. Fault displacement rates correlate with fault size, where size is measured in terms of either displacement or length. Small faults have higher mortality rates than larger faults throughout the growth of the fault system. Displacement and strain are progressively localized onto the larger faults at the expense of smaller faults at progressively larger scales. Strain localization and the preferential growth of larger faults are attributed to geometric factors, such as size and location, rather than to the mechanical properties of fault rock in individual faults. This conclusion is supported by numerical models that reproduce the main characteristics of fault system growth established from both study areas.
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  • 4
    Publication Date: 2016-09-17
    Description: The growth of normal faults in mechanically layered sequences is numerically modelled using three-dimensional Distinct Element Method (DEM) models, in which rock comprises an assemblage of bonded spherical particles. Faulting is induced by movement on a pre-defined normal fault at the model base whilst a constant confining pressure is maintained by applying forces to particles lying at the model top. The structure of the modelled fault zones and its dependency on confining pressure, sequence (net:gross) and fault obliquity are assessed using various new techniques that allow (a) visualization of faulted horizons, (b) quantification of throw partitioning and (c) determination of the fault zone throw beyond which theoretical juxtaposition sealing occurs along the entire zone length. The results indicate that fault zones become better localized with increasing throw and confinement. The mechanical stratigraphy has a profound impact on fault zone structure and localization: both low and high net:gross sequences lead to wide and relatively poorly localized faults. Fault strands developing above oblique-slip normal faults form, on average, normal to the greatest infinitesimal stretching direction in transtensional zones. The model results are consistent with field observations and results from physical experiments.
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
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  • 5
    Publication Date: 2016-03-12
    Description: Fault growth could be achieved by (1) synchronous increases in displacement and length or (2) rapid fault propagation succeeded by displacement-dominated growth. The second of these growth models (here referred to as the constant length model) is rarely applied to small outcrop-scale faults, yet it can account for many of the geometric and kinematic attributes of these faults. The constant length growth model is supported here using displacement profiles, displacement–length relationships and tip geometries for a system of small strike-slip faults (lengths of 1–200 m and maximum displacements of 0.001–3 m) exposed in a coastal platform in New Zealand. Displacement profiles have variable shapes that mainly reflect varying degrees of fault interaction. Increasing average displacement gradients with increasing fault size (maximum displacement and length) may indicate that the degree of interaction increases with fault size. Horsetail and synthetic splays confined to fault-tip regions are compatible with little fault propagation during much of the growth history. Fault displacements and tip geometries are consistent with a two-stage growth process initially dominated by propagation followed by displacement accumulation on faults with near-constant lengths. Retardation of propagation may arise due to fault interactions and associated reduction of tip stresses, with the early transition from propagation- to displacement-dominated growth stages produced by fault-system saturation (i.e. the onset of interactions between all faults). The constant length growth model accounts for different fault types over a range of scales and may have wide application.
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
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  • 6
    Publication Date: 2013-05-18
    Description: Whether a reduction in overpressure across a hydrocarbon column and its seal will reduce the column height that the seal is capable of supporting has been much debated. Recent studies have focused on measuring water relative permeability at high capillary pressure, and have concluded that hydrocarbon columns may be sufficiently permeable to water for the seal capacity to be unaffected by hydrodynamic flow in most situations. A new method for estimating the capillary seal capacity is developed which properly takes into account viscous as well as gravity and capillary forces. A reassessment of existing analyses indicates that hydrodynamic flow has a greater influence on the seal capacity than previously thought. Sets of sensitivity models, including a range of reservoir and seal geometrical and petrophysical characteristics, indicate that – as a general rule – hydrodynamic flow should not be neglected when assessing capillary seal capacity. The sealing capacity of thick top seals above reservoirs with a basal aquifer are least likely to be affected by hydrodynamic flow. The sealing capacity of membrane fault seals, irrespective of whether they have a basal or edge aquifer, may be influenced by realistic overpressure differences. Simple end-member equations define the possible influence of hydrodynamic flow in a particular situation, and only if this is significant should a more comprehensive evaluation be considered.
    Print ISSN: 1354-0793
    Topics: Chemistry and Pharmacology , Geosciences
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  • 7
    Publication Date: 2017-09-30
    Description: Extract Normal faults are the dominant structures found in extensional sedimentary basins developed in continental rifts and passive margins. The geometry and growth of faults are intimately linked, and much of our understanding of how faults grow is derived directly from observations of fault geometry. The key geometric relationship that has underpinned the study of fault growth since the 1980s is the relationship between fault maximum displacement ( D ) and fault length ( L ) as defined by Elliott (1976) and Watterson (1986). This relationship is expressed as D α L n . The value of the exponent n in this relationship has been a topic for discussion for the last 30 years and values ranging between 0.5 and 2.0 have been advocated (e.g. Walsh & Watterson 1988; Cowie & Scholz 1992; Schultz et al. 2008). The range of values reflects the natural variation between different areas and uncertainties in data quality and sampling (Gillespie et al. 1992; Kim & Sanderson 2005). Irrespective of the value of the exponent, the recognition of a positive correlation between displacement and length suggests that faults grow progressively as their displacement increases (Watterson 1986; Walsh & Watterson 1988). ... This 250-word extract was created in the absence of an abstract.
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
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  • 8
    Publication Date: 2016-08-19
    Description: The boundaries between pairs of adjacent fault segments within normal fault arrays define a spectrum of structures, from relay ramps where the length of overlap between the fault segments is much larger than the separation, through low aspect ratio (overlap/separation) relay ramps and ultimately to underlapping fault segments. Where fault segments underlap, transfer of displacement between them is accommodated by a connecting monocline. When displacement increases and a through-going fault forms, relay ramps are preserved as fault-bounded zones of elevated bed dip and monoclines are preserved as areas of normal drag. Therefore, the orientation and magnitude of bed dips within and adjacent to a fault zone, and the numbers of segments seen on a cross-section through it, depend largely on the aspect ratios of relay ramps in the initial fault array. The aspect ratio of relay ramps varies between different fault systems. An analysis of the geometry of 512 relay ramps from 13 different fault systems suggests that the main controls on aspect ratio are the strength of the sequence at the time of faulting and the underlying structure.
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
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  • 9
    Publication Date: 2016-11-23
    Description: The total throw across a fault zone may not occur entirely on a single fault strand but may be distributed onto several strands or may be accommodated by distributed deformation within or adjacent to the fault zone. Here we conduct a quantitative analysis of the partitioning of throw into three components, the throw accommodated by: (a) the largest fault strand; (b) subsidiary faults; and (c) continuous deformation in the form of bed rotation in sympathy with the fault downthrow direction. This analysis is applied to seven seismic-scale fault zones at outcrop resolution (maximum throw 50 m) that were mapped over a four-year period during open-cast lignite mining within the late Miocene–Pliocene Ptolemais Basin, West Macedonia, Greece. The analysis shows that the fault zones offsetting the lignite–marl sequence are more localized at higher throws with progressively more of the total throw accommodated by the largest fault strand. Normal drag, which can account for up to 12 m of the total throw, accommodates a lower proportion of the total throw on larger faults. It appears that initial fault segmentation is the main control on the degree of, and spatial variation in, fault throw partitioning.
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
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  • 10
    Publication Date: 2017-02-02
    Description: The transmissibility expression generally used for connections across faults in industrial flow-simulation models built using corner-point geometry is inaccurate, because of cell misalignments across the faults. A comprehensive suite of high-resolution flow-simulation models has been designed to assess the magnitude of the error, which is greatest for connections with smaller juxtaposition areas; between cells with higher k V : k H ratios; between cells with greater length: height aspect ratios; in more heterogeneous sequences; for connections containing more permissive fault rocks; and for connections between cells with lower angular misalignments. Transmissibility can be underestimated by a factor of 10 or more in the absence of fault rock; however, the inclusion of realistic fault transmissibility multipliers virtually eliminates the error. The expression for transmissibility for corner-point geometry models contained in commercial simulators is therefore within acceptable limits for most realistic faulted full-field models.
    Print ISSN: 1354-0793
    Topics: Chemistry and Pharmacology , Geosciences
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