Structure and flow properties of syn-rift border faults: The interplay between fault damage and fault-related chemical alteration (Dombjerg Fault, Wollaston Forland, NE Greenland)

Publisher's version, source: http://dx.doi.org/10.1016/j.jsg.2016.09.012.Structurally controlled, syn-rift, clastic depocentres are of economic interest as hydrocarbon reservoirs; understanding the structure of their bounding faults is of great relevance, e.g. in the assessment of fault-controlled hydrocarbon retention potential. Here we investigate the structure of the Dombjerg Fault Zone (Wollaston Forland, NE Greenland), a syn-rift border fault that juxtaposes syn-rift deep-water hanging-wall clastics against a footwall of crystalline basement. A series of discrete fault strands characterize the central fault zone, where discrete slip surfaces, fault rock assemblages and extreme fracturing are common. A chemical alteration zone (CAZ) of fault-related calcite cementation envelops the fault and places strong controls on the style of deformation, particularly in the hanging-wall. The hanging-wall damage zone includes faults, joints, veins and, outside the CAZ, disaggregation deformation bands. Footwall deformation includes faults, joints and veins. Our observations suggest that the CAZ formed during early-stage fault slip and imparted a mechanical control on later fault-related deformation. This study thus gives new insights to the structure of an exposed basin-bounding fault and highlights a spatiotemporal interplay between fault damage and chemical alteration, the latter of which is often underreported in fault studies. To better elucidate the structure, evolution and flow properties of faults (outcrop or subsurface), both fault damage and fault-related chemical alteration must be considered. Highlights • Faults juxtaposing syn-rift clastics against crystalline basement are investigated. • Early fault-zone diagenesis profoundly influences later fault-related deformation. • Spatiotemporal interplay between fault damage and chemical alteration. • Findings have implications for fault-bounded syn-rift reservoirs in the subsurface

Line sampling of fracture swarms and corridors

Scanlines across a range of fracture networks are analysed using cumulative frequency plots, spacing distribution and correlation integral plots. Synthetic samples from known distributions are used to distinguish uniform and random positioning of fractures from various types of clustering and the results used to interpret fracture corridors from the Bergen area, Norway. The Kuiper test provides a means of establishing the statistical significance of clustering, and the correlation integral allows assessment of clustering at different scales. A simple workflow is proposed that uses quantitative measures for the definition and characterisation of the spatial organisation of fractures. The form of fracture networks is shown to vary from uniform or random distributions, through the development of simple corridors, to clustering at various scales.</p

Making rose diagrams fit-for-purpose

Rose diagrams are powerful tools for visually representing two-dimensional orientation (and other cyclic) data. The conventional practice of scaling the wedge radius to frequency leads to exaggeration of modal orientations and should be replaced by using wedges whose area is proportional to frequency (the equal-area wedge diagram). We outline a workflow in which the type of data (vectorial or axial), sample size and the degree of preferred orientation are used to control the type of plot (discrete or binned) and the parameters used for binning the data. This allows visual comparison of data using rose diagrams that are fit-for-purpose

Brecciation driven by changes in fluid column heights

A mechanism is presented for the pulses of high fluid pressure (PF) necessary for fluid-assisted brecciation. Establishment of hydraulic- or pneumatic-connectivity between rock masses with different PF can cause overpressure in the higher rocks because the PF gradient is parallel to the hydrostatic gradient (the centroid effect). PF can become high enough to create a fracture network, with an influx of fluids and mineralisation occurring as fluids migrate to areas of lower PF. Changes in PF caused by the centroid effect can cause other structures and seismicity.</p

Modeling tip zones to predict the throw and length characteristics of faults

A map of faults in a 60 km 2 area of the southern North Sea has been produced from three-dimensional seismic data. The faults shown on the map obey power-law cumulative-frequency distributions for throw (power-law exponent, D, nearly equal 2.7) and length (D nearly equal 1.1). Simulations have been carried out to correct for sampling biases in the data and to make predictions of the throw and length scaling characteristics of the faults. The most important bias is caused by poor resolution of the small displacement tip zones of faults. The raw data show considerable scatter in their length:throw ratios, but they more closely fit a linear relationship if a length of 500 m is added to each fault, thereby making up for the zones near the fault tips with throws ( nearly equal 15 m) below seismic resolution. Further variability in the data may be caused by such geological factors as fault interaction. Tip lengths have been extended to simulate the actual fault pattern in the study area. Maps produced by this procedure can be used to estimate the true connectivity of the fault network. Extending the faults results in greater connectivity than shown by the raw data, which may cause greater compartmentalization of the rock mass. This greater compartmentalization has implications for hydrocarbon exploitation if the faults are sealing. A problem with the model, however, is that it does not deal effectively with the interaction of subparallel, noncoplanar faults. To test the reliability of the procedure, we analyzed exposure-scale faults in Somerset, United Kingdom, where the tips are well constrained. Both length-throw relationships and map-pattern connectivity for the simulated fault networks agree closely with the actual data. <br/

Graph theory and the analysis of fracture networks

Two-dimensional exposures of fracture networks can be represented as large planar graphs that comprise a series of branches (B) representing the fracture traces and nodes (N) representing their terminations and linkages. The nodes and branches may link to form connected components (K), which may contain fracture-bounded regions (R) or blocks. The proportions of node types provide a basis for characterizing the topology of the network. The average degree &lt;d&gt; relates the number of branches (|B|) and nodes (|N|) and Euler's formula establishes a link between all four elements of the graph with |N| - |B| + |R| - |K| = 0. Treating a set of fractures as a graph returns the focus of description to the underlying relationships between the fractures and, hence, to the network rather that its constitutive elements. Graph theory provides a wide range of applicable theorems and well-tested algorithms that can be used in the analysis of fault and fracture systems. We discuss a range of applications to two-dimensional fracture and fault networks, and briefly discuss application to three-dimensions