Now showing 1 - 10 of 11
  • Publication
    Definition of a fault permeability predictor from outcrop studies of a faulted turbidite sequence, Taranaki, New Zealand
    Post-depositional normal faults within the turbidite sequence of the Late Miocene Mount Messenger Formation of the Taranaki basin, New Zealand are characterised 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 cutoffs at higher ratios. Although cataclastic fault rock permeabilities may be significantly lower (c. 2 orders of magnitude) than host rock sandstone permeabilities, the occurrence of continuous clay smears, combined with low clay permeabilities (10's to 100's 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.
  • Publication
    Geometrical analysis of the refraction and segmentation of normal faults in periodically layered sequences
    Normal faults contained in multilayers are often characterised by dip refraction which is generally attributed to differences in the mechanical properties of the layers, sometimes leading to different modes of fracture. Because existing theoretical and numerical schemes are not yet capable of predicting the 3D geometries of normal faults through inclined multilayer sequences, a simple geometric model is developed which predicts that such faults should show either strike refraction or fault segmentation or both. From a purely geometrical point of view a continuous refracting normal fault will exhibit strike (i.e. map view) refraction in different lithologies if the intersection lineation of fault and bedding is inclined. An alternative outcome of dip refraction in inclined multilayers is the formation of segmented faults exhibiting en échelon geometry. The degree of fault segmentation should increase with increasing dip of bedding, and a higher degree of segmentation is expected in less abundant lithologies. Strike changes and associated fault segmentation predicted by our geometrical model are tested using experimental analogue modelling. The modelling reveals that normal faults refracting from pure dip-slip predefined faults into an overlying (sand) cover will, as predicted, exhibit systematically stepping segments if the base of the cover is inclined.
      479Scopus© Citations 40
  • Publication
    Structural evolution and the partitioning of deformation during basin growth and inversion: A case study from the Mizen Basin Celtic Sea, offshore Ireland
    The Celtic Sea basins lie on the continental shelf between Ireland and northwest France and consist of a series of ENE–WSW trending elongate basins that extend from St George’s Channel Basin in the east to the Fastnet Basin in the west. The basins, which contain Triassic to Neogene stratigraphic sequences, evolved through a complex geological history that includes multiple Mesozoic rift stages and later Cenozoic inversion. The Mizen Basin represents the NW termination of the Celtic Sea basins and consists of two NE–SW-trending half-grabens developed as a result of the reactivation of Palaeozoic (Caledonian, Lower Carboniferous and Variscan) faults. The faults bounding the Mizen Basin were active as normal faults from Early Triassic to Late Cretaceous times. Most of the fault displacement took place during Berriasian to Hauterivian (Early Cretaceous) times, with a NW–SE direction of extension. A later phase of Aptian to Cenomanian (Early to Late Cretaceous) N–S-oriented extension gave rise to E–W-striking minor normal faults and reactivation of the pre-existing basin bounding faults that propagated upwards as left-stepping arrays of segmented normal faults. In common with most of the Celtic Sea basins, the Mizen Basin experienced a period of major erosion, attributed to tectonic uplift, during the Paleocene. Approximately N–S Alpine regional compression-causing basin inversion is dated as Middle Eocene to Miocene by a well-preserved syn-inversion stratigraphy. Reverse reactivation of the basin bounding faults was broadly synchronous with the formation of a set of near-orthogonal NW–SE dextral strike-slip faults so that compression was partitioned onto two fault sets, the geometrical configuration of which is partly inherited from Palaeozoic basement structure. The segmented character of the fault forming the southern boundary of the Mizen Basin was preserved during Alpine inversion so that Cenozoic reverse displacement distribution on syn-inversion horizons mirrors the earlier extensional displacements. Segmentation of normal faults therefore controls the geometry and location of inversion structures, including inversion anticlines and the back rotation of earlier relay ramps.
      738Scopus© Citations 9
  • Publication
    Calibrating fault seal using a hydrocarbon migration model of the Oseberg Syd area, Viking Graben
    It is widely acknowledged that fault rock capillary properties are important in controlling the distribution of hydrocarbons in sedimentary basins, and methods exist for predicting the capillary seal capacity of prospect bounding faults. However, fault seal capacity is rarely incorporated into models of hydrocarbon migration. This paper presents the results of migration modelling of the Oseberg Syd area of the Viking Graben incorporating fault rock capillary properties. Seal capacity is calculated in the model as a function of Shale Gouge Ratio (SGR), i.e. the percentage shale in the sequence moved past a point on a fault. Over 3 000 model realisations were run for different SGR to fault seal capacity relationships and the calculated hydrocarbon distributions were compared with known distributions. Realisations were ranked according to the closeness of fit between model and actual oil-water contacts for 7 traps. The best-fit to all 7 traps was provided by realisations with significant seal capacity at SGR values greater than ca. 0.2; a value which is in agreement with an independently derived fault-by-fault calibration between SGR and seal capacity. The level of fill calculated for an individual trap is extremely sensitive to minor changes in the seal capacity relationship because it is controlled not only by the seal capacities of the faults that bound the trap, but also by the pattern of fill-spill of upstream traps. This sensitivity to minor changes in seal capacity introduces large uncertainties when fault seal capacity relationships are used in a predictive mode and emphasises the requirement for migration modelling in fault seal prospect evaluation.
      1064Scopus© Citations 27
  • Publication
    Geometric and kinematic controls on the internal structure of a large normal fault in massive limestones : the Maghlaq Fault, Malta
    The Maghlaq Fault is a large, left-stepping normal fault (displacement >210 m) cutting the Oligo-Miocene pre- to syn-rift carbonates of SW Malta. Two principal slip zones separate the deformed rocks of the fault zone from the undeformed wall rocks. Fault rocks derived from fully lithified, pre- to early syn-rift sediments comprise relatively continuous fine-grained veneers of cataclasite and localised fault-bound lenses of wall rock, occurring over a range of scales, which are commonly brecciated. The lenses result from the linkage of slip surfaces, the inclusion of asperities and the formation of Riedel shears within the fault zone. In contrast, fault rock incorporated from unlithified syn-rift sediments comprise relatively continuous veils of rock that deformed in a ductile manner. Anomalously thick parts of the fault zone with highly complex structure and content are associated with breached relay zones, branch-lines and bends; these structures represent progressive stages of fault segment linkage. The progressive evolution and bypassing of fault zone complexities to form a smoother and more continuous active fault surface, results in complex fault rock distributions within the fault zone. Segment linkage structures have high fracture densities which combined with their significant vertical extents suggest they are potentially important up-fault fluid flow conduits.
      1408Scopus© Citations 79
  • Publication
    The impact of porosity and crack density on the elasticity, strength and friction of cohesive granular materials : insights from DEM modelling
    Empirical rock properties and continuum mechanics provide a basis for defining relationships between a variety of mechanical properties, such as strength, friction angle, Young’s modulus, Poisson’s ratio, on the one hand and both porosity and crack density, on the other. This study uses the Discrete Element Method (DEM), in which rock is represented by bonded, spherical particles, to investigate the dependence of elasticity, strength and friction angle on porosity and crack density. A series of confined triaxial extension and compression tests was performed on samples that were generated with different particle packing methods, characterised by differing particle size distributions and porosities, and with different proportions of pre-existing cracks, or uncemented grain contacts, modelled as non-bonded contacts. The 3D DEM model results demonstrate that the friction angle decreases (almost) linearly with increasing porosity and is independent of particle size distribution. Young’s modulus, strength and the ratio of unconfined compressive strength to tensile strength (UCS/T) also decrease with increasing porosity, whereas Poisson’s ratio is (almost) porosity independent. The pre-eminent control on UCS/T is however the proportion of bonded contacts, suggesting that UCS/T increases with increasing crack density. Young’s modulus and strength decrease, while Poisson’s ratio increases with increasing crack density. The modelling results replicate a wide range of empirical relationships observed in rocks and underpin improved methods for the calibration of DEM model materials.
      1793Scopus© Citations 230
  • Publication
    2D distinct element modeling of the structure and growth of normal faults in multilayer sequences : 1. Model calibration, boundary conditions, and selected results
    (American Geophysical Union, 2007) ; ;
    The distinct element method is used for modeling the growth of normal faults in layered sequences. The models consist of circular particles that can be bonded together with breakable cement. Size effects of the model mechanical properties were studied for a constant average particle size and various sample widths. The study revealed that the bulk strength of the model material decreases with increasing sample size. Consequently, numerical lab tests and the associated construction of failure envelopes were performed for the specific layer width to particle diameter ratios used in the multilayer models. The normal faulting models are composed of strong layers (bonded particles) and weak layers (nonbonded particles) that are deformed in response to movement on a predefined fault at the base of the sequence. The modeling reproduces many of the geometries observed in natural faults, including (1) changes in fault dip due to different modes of failure in the strong and weak layers, (2) fault bifurcation (splaying), (3) the flexure of strong layers and the rotation of associated blocks to form normal drag, and (4) the progressive linkage of fault segments. The model fault zone geometries and their growth are compared to natural faults from Kilve foreshore (Somerset, United Kingdom). Both the model and natural faults provide support for the well-known general trend that fault zone width increases with increasing displacement.
      822Scopus© Citations 62
  • Publication
    Static and dynamic connectivity in bed-scale models of faulted and unfaulted turbidites
    A range of unfaulted and faulted bed-scale models with sheet-like bed geometries have been built and analysed in terms of static bed connectivity and fractional permeability assuming permeable sands and impermeable shales. The models are built using a new method which allows 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 inter-bed connectivity. The connectivity of faulted sequences is much more complex and is dominated by interactions of variables. A comprehensive modelling suite illustrates these results and highlights the extremely rare combinations of circumstances in which faulted sequences have lower connectivities than their unfaulted sedimentological equivalents, irrespective of whether fault rock properties are included or not. In general, models containing stochastically placed shale smears associated with each faulted shale horizon are better connected than if deterministic Shale Gouge Ratio cut-offs are applied. Despite the complex interactions between geological input and bed-scale connectivity, the flow properties of a system are controlled by only three geometrical, rather than geological, 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 flow properties.
      632Scopus© Citations 41
  • Publication
    Reconciliation of contrasting theories for fracture spacing in layered rocks
    Natural and man-made brittle layers embedded in a weaker matrix and subjected to layer-parallel extension typically develop an array of opening-mode fractures with a remarkably regular spacing. This spacing often scales with layer thickness, and it decreases as extension increases until fracture saturation is reached. Existing analytical one-dimensional (1-D) 'full-slip' models, which assume that interfacial slip occurs over the entire length of the fracture-bound blocks, predict that the ratio of fracture spacing to layer thickness at saturation is proportional to the ratio of layer tensile to interface shear strength (T/s). Using 2-D discontinuum mechanical models run for conditions appropriate to layered rocks, we show that fracture spacing at saturation decreases linearly with decreasing T/s ratio, as predicted by 1-D models. At low T/s ratios (ca. <3.0), however, interfacial slip is suppressed and the heterogeneous 2-D stress distribution within fracture-bound blocks controls further fracture nucleation, as predicted by an existing 2-D 'fracture infill criterion'. The applicability of the two theories is hence T/s ratio dependent. Our models illustrate that fracture spacing in systems permitting interfacial slip is not necessarily an indicator of fracture system maturity. Fracture spacing is expected to decrease with increasing overburden pressure and decreasing layer tensile strength.
      633Scopus© Citations 73
  • Publication
    Localisation of normal faults in multilayer sequences
    Existing conceptual growth models for faults in layered sequences suggest that faults first localise in strong, and brittle, layers and are later linked in weak, and ductile, layers. We use the Discrete Element Method (DEM) for modelling the growth of a normal fault in a brittle/ductile multilayer sequence. The modelling reveals that faults in brittle/ductile sequences at low confining pressure and high strength contrast localise first as Mode I fractures in the brittle layers. Low amplitude monoclinal folding prior to failure is accommodated by ductile flow in the weak layers. The initially vertically segmented fault arrays are later linked via shallow dipping faults in the weak layers. Faults localise, therefore, as geometrically and kinematically coherent arrays of fault segments in which abandoned fault tips or splays are a product of the strain localisation process and do not necessarily indicate linkage of initially isolated faults. The modelling suggests that fault tip lines in layered sequences are more advanced in the strong layers compared to weak layers, where the difference in propagation distance is most likely related to strength and/or ductility contrast. Layer dependent variations in fault propagation rates generate fringed rather than smooth fault tip lines in multilayers.
      746Scopus© Citations 126