Now showing 1 - 8 of 8
  • Publication
    The state of stress in the shallow crust of the Hikurangi Subduction Margin hangingwall, New Zealand
    Knowledge of in situ stress fields is critical for a better understanding of deformation, faulting regime, and earthquake processes in seismically active margins such as the Hikurangi Subduction Margin (HSM), North Island, New Zealand. In this study, we utilize Leak-off Test (LOTs) data, borehole breakout widths measured from borehole image logs, and rock unconfined compressive strengths (UCS) derived from empirical P-wave velocity log relationships to estimate vertical (Sv), minimum (Shmin), and maximum horizontal stress magnitudes (SHmax) and interpret the likely faulting regime experienced in four boreholes (Kauhauroa-2, Kauhauroa-5, Titihaoa-1, and Tawatawa-1). Using the standard Anderson’s stress regime classification, relative stress magnitudes in Kauhauroa-5 at 1200-1700 m depth and Kauhauroa-2 at 1800-2100 m and indicate that the stress state in the shallow crust of the central and northern part of HSM is predominantly strike-slip (SHmax≥Sv≥Shmin) and normal Sv≥SHmax> Shmin respectively. Moving to the offshore, southern HSM a dominant compressional stress regime (SHmax> Shmin >Sv), with some possible strike slip stress states are observed in Titihaoa-1 from 2240-2660 m and Tawatawa-1 from 750-1350 m. The observed normal/strike-slip stress state in Kauhauroa-2 and Kauhauroa-5 is consistent with the average SHmax orientation of 64° ± 18° (NE-SW) determined from borehole breakouts and dominantly NE–SW striking normal faults interpreted from seismic reflection data. The normal/ strike-slip regime in this area suggests that the stress regime here is probably influenced by the effect of the clockwise rotation of the HSM hangingwall associated with oblique Pacific-Australia plate convergence (ENE-WSW). Alternatively, these stress states could be the result of gravitational collapse due to rapid uplift of the subducting plate during the mid-Miocene. The compressional stress regime in the southern HSM in Titihaoa-1 and Tawatawa-1 is in agreement with the SHmax orientations of 148° ± 14° (NW-SE ) and 102° ± 16° (WNW-ESE) obtained from image logs and mapped NE–SW striking reverse faults in this region. This observation suggests that the tectonics here are strongly linked to the subduction of Hikurangi plateau under Australian Plate (NW-SE) or active frontal thrusts in the overriding plate.
  • Publication
    The role of antithetic faults in transferring displacement across contractional relay zones on normal faults
    Contractional relay zones between pairs of normal faults are sometimes associated with multiple antithetic faults in a geometry similar to that found in Riedel shear zones. Detailed fault displacement profiles of outcrop examples of this geometry demonstrate that the antithetic faults accommodate the transfer of displacement between the synthetic faults that bound the relay zones. The throw on individual antithetic faults, or R′ shears, is typically constant across relay zones while the throw profile on the synthetic faults, or R shears, is stepped; the steps occurring across branchpoints with abutting R’ shears. Transfer of fault displacement occurs by a combination of block rotation and irrotational block translation within the relay zone. As fault throw increases, contractional relay zones are by-passed by the linkage of the synthetic faults, in a manner analogous to the formation of P-shears by the linkage of R shears in classic Riedel shear experiments, but with the original relay zone structure still preserved within the fault zone. With yet further strain bedding may rotate into near-parallelism with the fault surface, with the original geometrical configuration of the relay zone difficult to unravel.
  • Publication
    Variability in the three-dimensional geometry of segmented normal fault surfaces
    Normal faults are often complex three-dimensional structures comprising multiple sub-parallel segments separated by intact or breached relay zones. Relay zones are classified according to whether they step in the strike or dip direction and whether the relay zone-bounding fault segments are unconnected in 3D or bifurcate from a single surface. Complex fault surface geometry is described in terms of the relative numbers of different types of relay zones to allow comparison of fault geometry between different faults and different geological settings. A large database of fault surfaces compiled primarily from mapping 3D seismic reflection surveys and classified according to this scheme, reveals the diversity of 3D fault geometry. Analysis demonstrates that mapped fault geometries depend on geological controls, primarily the heterogeneity of the faulted sequence and the presence of a pre-existing structure, as well as on resolution limits and biases in fault mapping from seismic data. Where a significant number of relay zones are mapped on a single fault, a wide variety of relay zone geometries occurs, demonstrating that individual faults can comprise segments that are both bifurcating and unconnected in three dimensions.
      5Scopus© Citations 14
  • 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.
      489Scopus© Citations 40
  • Publication
    Hierarchical parameterization and compression-based object modelling of high net: gross but poorly amalgamated deep-water lobe deposits
    (Geological Society of London, 2020-01-27) ; ; ;
    Deepwater lobe deposits are arranged hierarchically and can be characterized by high net:gross ratios but poor sand connectivity due to thin but laterally extensive shale layers. This heterogeneity makes them difficult to represent in standard full-field object-based models, since the sands in an object-based model are not stacked compensationally and become connected at a low net:gross ratio. The compression algorithm allows generation of low connectivity object-based models at high net:gross ratios, by including the net: gross and amalgamation ratios as independent input parameters. Object-based modelling constrained by the compression algorithm has been included in a recursive workflow, permitting generation of realistic models of hierarchical lobe deposits. Representative dimensional and stacking parameters collected at four different hierarchical levels have been used to constrain a 250 m thick, 14 km2 model that includes hierarchical elements ranging from 20 cm thick sand beds to 30+ m thick lobe complexes. Sand beds and the fine-grained units are represented explicitly in the model, and the characteristic facies associations often used to parameterize lobe deposits are emergent from the modelling process. The model is subsequently resampled without loss of accuracy for flow simulation, and results show clearly the influence of the hierarchical heterogeneity on drainage and sweep efficiency during a water-flood simulation.
      292Scopus© Citations 8
  • 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.
      680Scopus© Citations 42
  • 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.
      1095Scopus© Citations 27