Fault displacement

Whereas faults displace formerly connected lithologic units, fractures do not show appreciable displacement. They also represent planes of brittle failure and affect hard... 

Stacking faults are characterised by a fault plane and a fault displacement vector. On one side of the fault plane, the atoms that are located fer from the fault are displaced by a vector R in relation to the positions they would occupy in the absence of the fault. Strain fields emanating from any reconstructive bonding that is present near the fault plane will lead to additional displacements for atoms near the fault plane. Thus, the specification of R determines the positions of the atoms that are sufficiently distant so that the strain field generated by the fault is below some specified tolerance. For a planar fault, R may be determined experimentally by analysis of the diffraction contrast obtained with different diffraction vectors g. The positions of atoms near the fault may be determined theoretically by total energy minimisation calculations. Knowledge of these positions is essential to determine the electronic structure of the fault. 

FIGURE 2 Stacking sequence and associated stacking fault displacement vectors for three types of basal plane stacking fault in GaN. The double lines indicate the fault plane. The stacking fault vectors are shown for each fault. 

Results and conclusions The fault zones at Frechen contain clay fillings of up to 1 m in thickness, derived from extremely plastic shale source beds and smeared out over distances as much as 70 m in dip direction. The generation of substantial smears requires slow fault displacement rates and sufficient shale ductility. When a thick shale source bed is traversed by a normal fault, it is first flexed and eventually disrupted by a pull-apart mechanism that creates room for the emplacement of thick clay smears. Simple theoretical considerations suggest that the source bed thickness to some power n + 1 > 2 may be a key parameter in the ranking of seal quality. The length of continuous smears increases with source bed thickness, but will ultimately be controlled by the smearing process. The latter remains to be investigated. 

The last observation could indeed have major practical implications as a guiding principle in assessing fault trap prospects. It is therefore of some interest that Lindsay et al. (1993) have observed clay smears in tectonic faults that affected a Westphalian sand/shale sequences after lithification. These smears were apparently formed by abrasion of indurated shales. In this process, the surface of a sandstone becomes coated by a thin veneer of abraded material in much the same way as the surface of sandpaper. This veneer may run continuously along polished slip surfaces, but - as Lindsay et al. have documented in their study - with increasing fault displacement and de-... 

On either side of the shear zone proper (the term is used here to designate the zone that accommodates most of the fault displacement and is typically bounded by D-shears), a much wider zone of continuous and/or discontinuous (slip) deformation is usually discernible. Faulted monoclinal flexures in shale layers (Plate 3) furnish the most prominent examples of continuous, fault-related deformation. At places, the coal seam is flexed together with an over-lying or underlying shale. Almost everywhere along a fault, a fringe zone of (discolored) shears is observed in the sands (Plate 4). On the downthrown side, the flexed shales are often intersected by these shears... 

The development described from loosely anastomosing deformation bands to zones of highly interconnected deformation bands and slip planes has major implications for retention capacity (Loosveld and Franssen, 1992 Antonellini and Aydin, 1994, 1995). At low fault displacements the retention ca-... 

Faults in sandstones deformed at depths greater than 1 km tend to deform by cataclasis. Permeability and entry pressure of such faults can be predicted from estimates of matrix properties. Static seal capacities of cataclastic faults depend on the minimum sealing properties, which are related to the fault displacements. 

Brittle fault zones comprise discrete slip surface(s) and fault rocks. There is a general positive correlation between fault displacement and the thickness and complexity of the fault zones (Robertson, 1983 Hull, 1988). Complex fault zones generally comprise multiple slip surfaces or zones of intense shear (Childs et al., 1996). The simplest and most common multi-slip fault zones observed in outcrop are structures with two discrete bounding slip surfaces, enclosing fault rock which may vary from intensely deformed to virtually undeformed (Koestler and Ehrmann, 1991 Childs et al., 1996). Where sufficient data are available, areas of a fault zone with the paired slip surface geometry can be seen to alternate with areas with a... 

Robertson, E.C. 1983. Relationship of fault displacement to gouge and breccia thickness. Soc. Mining Eng., Am. Inst. Mining Eng. Trans., 35 1426-1432. 

For each fault, we calculate the shale gouge ratio (SGR) at all points of reservoir overlap. SGR is defined as the proportion (%) of shale in the rock interval that has moved past any point on the fault. This requires mapping of the fault displacement and combination with the shale percentage in the reservoir zones. RFT data provide a calibration of the value of SGR required to seal a fault plane. SGR values of 15-18% are consistent with adjacent fault blocks having small pressure differentials (18% correspond to significant seal (ca. 8 bar). 

The display of SGR on the fault surface (Fig. 10b) uses the shale fractions observed in the adjacent wells. Since the fault displacements are generally greater than the zone thicknesses, the calculated SGR values are relatively homogeneous. However, a significant point is the area of lower values (in yellow, <20%) near the upper part of the reservoir overlap zone. This represents the critical area for fault seal calibration. 

Fault seal probability analysis is a quantitative method that allows an assessment of the risk of a fault acting either as a barrier to hydrocarbon migration, or as a trapping element within a structure. Fault seal probability is a value ranging from one to zero where a value of one is the highest probability for sealing, and zero is the lowest. This value is derived from the equation that combines the main parameters involved in the formation of fault seal. These parameters, fault displacement, connectivity, and net to gross ratio, are related to the processes of cataclasis and cementation, juxtaposition, and shale smear. The parameters, their measurement and impact on fault seal, are discussed below. 

Fault displacement is usually taken as fault throw from depth structure maps. A relation has been determined, from empirical oil and gas field data (Knott, 1993) and outcrop studies (Knott, 1994), which shows that there is a positive correlation between the probability of a fault sealing and fault throw divided by reservoir thickness (Dn). 

Stepp, J. C., Wong, I., Whitney, J., Quittemeyer, R., Abrahamson, N., Toro, G. (2001). Yucca Mountain PSHA Project Members, Probabilistic seismic hazard analyses for ground motions and fault displacements at Yucca Mountain, Nevada. Earthquake Spectra, 17, 113-151. doi 10.1193/l.1586169... 

Geophysical method Depth to bedrock Stratigraphy Lithology Fractured zones Fault displacements Dynamic elastic moduli Density Rippability Cavity detection Buried artefacts... 

The hazards due to seismicity include the possibility of a structure being severed by fault displacement but a much more likely event is damage due to shaking (Fig. 8.4). The destruction wrought by an earthquake depends on many factors. Of prime importance are the magnitude of the event, its duration and the response of buildings and other elements of the infrastructure. In addition, other hazards such as landslides, floods, subsidence, tsunamis and secondary earthquakes may be triggered by a seismic event (Khazai and Sitar, 2004),... 

All major faults located in regions where strong earthquakes have occurred should be regarded as potentially active unless convincing evidence exists to the contrary (Sherard et al., 1974). In stable areas of the world, little evidence exists of notable fault displacements in the recent past. Nevertheless, an investigation should be carried out to confirm the absence of active faults at or near any proposed major dam in any part of the world. 

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