Fault Planes Materials Fill Characteristics, UAE

Abu Dhabi subsurface fault populations triggered basin system in diverse directions, because of their significant role as fluid pathways. Studying fault infill materials, fault geometries, zone architecture and sealing properties from outcrops as analogues to the subsurface of Abu Dhabi, and combining these with well data and cores are the main objectives of this paper. The fault core around the fault plane and in areas of overlap between fault segments and around the fault tip include slip surfaces and deformed rocks such as fault gouge, breccia, and lenses of host rock, shale smear, salt flux and diagenetic features. Structural geometry of the fault zone architecture and fault plane infill is mainly based on the competency contrast of the materials, that are behaving in ductile or in a brittle manner, which are distributed in the subsurface of Abu Dhabi sedimentary sequences with variable thicknesses. Brittleness is producing lenses, breccia and gouge, while, ductile intervals (principally shales and salt), evolved in smear and flux. The fault and fractures are behaving in a sealy or leaky ways is mainly dependent on the percentage of these materials in the fault deformation zone. The reservoir sections distancing from shale and salt layers are affected by diagenetic impact of the carbonates filling fault zones by recrystallized calcite and dolomite. Musandam area, Ras Al Khaima (RAK), and Jabal Hafit (JH) on the northeast- and eastern-side of the UAE represents good surface analogues for studying fault materials infill characteristics. To approach this, several samples, picked from fault planes, were analysed. NW-trending faults system show more dominant calcite, dolomite, anhydrites and those closer to salt and shale intervals are showing smearing of the ductile infill. The other linked segments and transfer faults of other directions are represented by a lesser percentage of infill. In areas of gravitational tectonics, the decollement ductile interval is intruded in differently oriented open fractures. The studied outcrops of the offshore salt islands and onshore Jabal Al Dhanna (JD) showing salt flux in the surrounding layers that intruded by the salt. The fractures and faults of the surrounding layers and the embedment insoluble layers are highly deformed and showing nearly total seal. As the salt behaving in an isotropic manner, the deformation can be measured clearly by its impact on the surrounding and embedment's insoluble rocks. The faults/fractures behaviour is vicious in migrating hydrocarbons, production enhancement and hydraulic fracturing propagation.

Fault Fictions: Systematic biases in the conceptualization of fault zone architecture

<p>Mental models are a human’s internal representation of the real world and have an important role in the way a human understands and reasons about uncertainties, explores potential options, and makes decisions. However, they are susceptible to biases. Issues associated with mental models have not yet received much attention in geosciences, yet systematic biases can affect the scientific process of any geological investigation; from the inception of how the problem is viewed, through selection of appropriate hypotheses and data collection/processing methods, to the conceptualisation and communication of results. This presentation draws on findings from cognitive science and system dynamics, with knowledge and experiences of field geology, to consider the limitations and biases presented by mental models in geoscience, and their effect on predictions of the physical properties of faults in particular. We highlight a number of biases specific to geological investigations and propose strategies for debiasing. Doing so will enhance how multiple data sources can be brought together, and minimise controllable geological uncertainty to develop more robust geological models. Critically, we argue that there is a need for standardised procedures that guard against biases, permitting data from multiple studies to be combined and communication of assumptions to be made. While we use faults to illustrate potential biases in mental models and the implications of these biases, our findings can be applied across the geoscience discipline.</p>

Permeability contrasts of fault zones - from conceptual model to numerical simulation

<p>Fault and fracture networks can govern fluid flow patterns in the subsurface and predicting fluid flow on a regional scale is of interest in a variety of fields like groundwater management, mining engineering, energy, and mineral resources. Especially the pore fluid pressure can have a strong impact on the strength of fault zones and might be one of the drivers for fault reactivation. Reliable simulations of the transient changes in fluid pressure need to account for the generic architecture of fault zones that comprises strong permeability contrast between the fault core and damage zone.</p><p>Particularly, the distribution and connectivity of large-scale fault zones can have a strong impact on the flow field. Yet, modelling numerically such features in their full complexity remains challenging. Often faults zones are conceptualized as forming exclusively either barriers or conduits to fluid flow. However, a generic architecture of fault zones often comprises a discrete fault core surrounded by a diffuse damage zone and conceptualizing large scale discontinuities simply as a barrier or conduit is unlikely to capture the regional scale fluid flow dynamics. It is known that if the fault zone is hosted in low-permeability strata, such as clays or crystalline rocks, a transversal flow barrier can form along the fault core whereas the fracture-rich fault damage zone represents a longitudinal conduit. In more permeable host-rocks (i.e. sandstones or carbonates) the reverse situation can occur, and the permeability distributions in the damage zones can be governed by the abundance of low-permeability deformation features. A reliable numerical model needs to account for the difference and strong contrasts in fluid flow properties of the core and the damage zone, both transversally and longitudinally, in order to make prediction about the regional fluid flow pattern.</p><p>Here, we present a numerical method that accounts for the generic fault zone architecture as lower dimensional interfaces in conforming meshes during fluid flow simulations in fault networks. With this method we aim to decipher the impact of fault zone architecture on subsurface flow pattern and fluid pressure evolution in fractures and faulted porous media. The method is implemented in a finite element framework for Multiphysics simulations. We demonstrate the impact of considering the more generic geological structure of individual faults on the flow field by conceptualizing discontinuities either as barriers, conduits or as a conduit-barrier system and show were these conceptualizations are applicable in natural systems. We further show that a reliable regional scale fluid flow simulation in faulted porous media needs to account for the generic fault zone architecture. The approach is finally used to evaluate the fluid flow response of statistically parameterised faulted media, in order to investigate the impact and sensitivity of each variable parameter.</p>

In-situ petrophysical and geomechanical characterization and 3D modelling of a mature normal fault zone (Goddo fault, Bømlo – Norway)

<p>The detailed characterization of internal fault zone architecture and  petrophysical and geomechanical properties of fault rocks is fundamental to understanding the flow and mechanical behaviour of mature fault zones. The Goddo normal fault (Bømlo – Norway) accommodated c. E-W extension related to North Sea Rifting from Permian to Early Cretaceous times [1]. It represents a good example of a mature, iteratively reactivated and thus long-lived (seismogenic?) fault zone, developed in a pervasively fractured granitoid basement at upper crustal conditions in a regional extensional setting.</p><p>Field characterization of the fault zone’s structural facies and analysis of background fracture patterns in the protolith have been integrated with in-situ petrophysical and geomechanical surveys of the recognized fault zone architectural components. In-situ air-permeability and mechanical directional tests (performed with NER TinyPerm III air-minipermeameter and DRC GeoHammer, L-type Schmidt hammer, respectively) have allowed for the quantification of the permeability tensor and mechanical properties (UCS and elastic modulus) within each brittle structural facies. Mechanical properties measured parallel to fault rock fabric of cataclasite- and gouge-bearing structural facies differ by up to one order of magnitude from those measured perpendicularly to it (~10 MPa vs. 100-200 MPa in UCS, respectively). Accordingly, permeability of cataclasite- and gouge-bearing facies is several orders of magnitude larger when measured parallel to fault-rock fabric than that perpendicular to it (10<sup>-0</sup>-10<sup>-1</sup> D vs. 10<sup>-2</sup>-10<sup>-3</sup> D, respectively). Virtual outcrop models (VOMs) of the fault zone were obtained from high-resolution UAV-photogrammetry. Field measurements of fracture orientations were used for calibration of the VOMs to construct a statistically robust fracture dataset. The results of VOMs structural analysis allowed for the quantification of fracture intensity and geometrical characteristics of mesoscopic fracture patterns within the different domains of the fault zone architecture.</p><p>Results from field, VOMs structural analysis, and in-situ petrophysical investigations have been integrated into a realistic 3D fault zone model with the software 3DMove (Petex). This model can be used to investigate the influence of mesoscopic fracture patterns, related to either the fault zone or the background fracturing, on the hydro-mechanical behaviour of a mature fault zone. In addition, the evolution of the hydro-mechanical properties through time can be assessed by integrating the progressive development of brittle structural facies and fracture sets developed during the incremental strain and stress history into the model. This contribution proposes a geologically-constrained method to quantify the geometry of 3D fault zones, as a possible tool for models to be adopted in stress-strain analysis, hydraulic characterization and in the mechanical analysis of fault zones.</p><p>[1] Viola, G., Scheiber, T., Fredin, O., Zwingmann, H., Margreth, A., & Knies, J. (2016). Deconvoluting complex structural histories archived in brittle fault zones. Nature communications, 7, 13448.</p>

An analysis of the factors that control fault zone architecture and the importance of fault orientation relative to regional stress

The moment magnitude 7.2 El Mayor–Cucapah (EMC) earthquake of 2010 in northern Baja California, Mexico produced a cascading rupture that propagated through a geometrically diverse network of intersecting faults. These faults have been exhumed from depths of 6–10 km since the late Miocene based on low-temperature thermochronology, synkinematic alteration, and deformational fabrics. Coseismic slip of 1–6 m of the EMC event was accommodated by fault zones that displayed the full spectrum of architectural styles, from simple narrow fault zones (< 100 m in width) that have a single high-strain core, to complex wide fault zones (> 100 m in width) that have multiple anastomosing high-strain cores. As fault zone complexity and width increase the full spectrum of observed widths (20–200 m), coseismic slip becomes more broadly distributed on a greater number of scarps that form wider arrays. Thus, the infinitesimal slip of the surface rupture of a single earthquake strongly replicates many of the fabric elements that were developed during the long-term history of slip on the faults at deeper levels of the seismogenic crust. We find that factors such as protolith, normal stress, and displacement, which control gouge production in laboratory experiments, also affect the architectural complexity of natural faults. Fault zones developed in phyllosilicate-rich metasedimentary gneiss are generally wider and more complex than those developed in quartzo-feldspathic granitoid rocks. We hypothesize that the overall weakness and low strength contrast of faults developed in phyllosilicate rich host rocks leads to strain hardening and formation of broad, multi-stranded fault zones. Fault orientation also strongly affects fault zone complexity, which we find to increase with decreasing fault dip. We attribute this to the higher resolved normal stresses on gently dipping faults assuming a uniform stress field compatible with this extensional tectonic setting. The conditions that permit slip on misoriented surfaces with high normal stress should also produce failure of more optimally oriented slip systems in the fault zone, promoting complex branching and development of multiple high-strain cores. Overall, we find that fault zone architecture need not be strongly affected by differences in the amount of cumulative slip and instead is more strongly controlled by protolith and relative normal stress.

Fault zone architecture of a large plate-bounding strike-slip fault: a case study from the Alpine Fault, New Zealand

New Zealand's Alpine Fault is a large, plate-bounding strike-slip fault, which ruptures in large (Mw>8) earthquakes. We conducted field and laboratory analyses of fault rocks to assess its fault zone architecture. Results reveal that the Alpine Fault Zone has a complex geometry, comprising an anastomosing network of multiple slip planes that have accommodated different amounts of displacement. This contrasts with the previous perception of the Alpine Fault Zone, which assumes a single principal slip zone accommodated all displacement. This interpretation is supported by results of drilling projects and geophysical investigations. Furthermore, observations presented here show that the young, largely unconsolidated sediments that constitute the footwall at shallow depths have a significant influence on fault gouge rheological properties and structure.

Fault fictions: systematic biases in the conceptualization of fault-zone architecture

Mental models are a human's internal representation of the real world and have an important role in the way we understand and reason about uncertainties, explore potential options and make decisions. Mental models have not yet received much attention in geosciences, yet systematic biases can affect any geological investigation: from how the problem is conceived, through selection of appropriate hypotheses and data collection/processing methods, to the conceptualization and communication of results. We draw on findings from cognitive science and system dynamics, with knowledge and experiences of field geology, to consider the limitations and biases presented by mental models in geoscience, and their effect on predictions of the physical properties of faults in particular. We highlight biases specific to geological investigations and propose strategies for debiasing. Doing so will enhance how multiple data sources can be brought together, and minimize controllable geological uncertainty to develop more robust geological models. Critically, there is a need for standardized procedures that guard against biases, permitting data from multiple studies to be combined and communication of assumptions to be made. While we use faults to illustrate potential biases in mental models and the implications of these biases, our findings can be applied across the geosciences.

Fault zone architecture and its scaling laws: where does the damage zone start and stop?

Damage zones of different fault types are investigated in siliciclastics (Utah, USA), carbonates (Majella Mountain, Italy) and metamorphic rocks (western Norway). The study was conducted taking measurements of deformation features such as fractures and deformation bands on multiple 1D scanlines along fault walls. The resulting datasets are used to plot the frequency distribution of deformation features and to constrain the geometrical width of the damage zone for the studied faults. The damage-zone width of a single fault is constrained by identifying the changes in the slope of cumulative plots made on the frequency data. The cumulative plot further shows high deformation frequency by a steep slope (inner damage zone) and less deformation as a gentle slope (outer damage zone). Statistical distributions of displacement and damage-zone width and their relationship are improved, and show two-slope power-law distributions with a break point at c. 100 m displacement. Bleached sandstones in the studied siliciclastic rocks of Utah are associated with a higher frequency of deformation bands and a wider damage zone compared to the unbleached zone of similar lithology. Fault damage zones in the carbonate rocks of Majella are often host to open fractures (karst), demonstrating that they can also be conductive to fluid flow.