Fault surface

Hanging wall—[he block located above and bearing down on the fault. surface. 

The occurrence of earthquakes is a highly studied phenomena by geologists. The role of solid surfaces in such phenomena is obvious. Especially, faults are known to contribute to many earthquakes. Faults are treated as shear cracks, the propagation of which may be understood through the application of fracture mechanics. The stability of any fault movement, which determines whether the faulting is seismic or aseismic, is determined by the frictional constitutive law of the fault surface. It is well established that, once a fault has been formed, its further motion is controlled by friction (between the solid surfaces), which arises from contact forces across the two solid surfaces. 

Fig. 2 The intensity of the (0,1,0.3) X-ray reflection from Ag(ll 1) during Sb deposition at a substrate temperature of 250°C. The sudden rise in intensity at a coverage of 1/3 ML is associated with the formation of the ordered (V3xV3)R30° faulted surface alloy phase as described in the text. From de Vries et al [8]...

Lindsay, N.G., Murphy, F.C., Walsh, J.J. and Watterson, J. 1993. Outcrop studies of shale smears on fault surfaces. Int. Assoc. Sedimen-tol. Special Publication, 15 113-123. 

Data for characterisation of faults in the subsurface are limited to two sources, seismics and wells. Seismic reflection data allow the displacement distribution over a fault surface to be mapped while well and core data may allow determination of fault rock types and deformation mechanisms at specific points, in addition to characterising the lithologies of the host sequence. It is evident from outcrop studies that the internal geometries of fault zones are usually complex, in terms of the numbers of individual slip surfaces, the partitioning of slip between them and in the distribution of different fault rocks, all of which vary over a fault surface. This 3-D complexity of fault zone structure may not be apparent from either seismic or core data but is nevertheless crucial to the bulk hydraulic properties of a fault. 

Fig. I. Cartoon illustrating the asperity bifurcation model of fault zone widening. An irregularity on a fault surface (grey fill) in (a) is sheared off by the formation of a new slip surface in (b). Subsequent fault movement may result in deformation of the newly formed slip surface bounded lens.

Asperity bifurcation is due to the shearing off of fault surface irregularities by the formation of new slip surfaces. These irregularities may occur anywhere on a fault surface and on any scale. Irregular... 

Fig. 2. Successive stages of the tip-line bifurcation process of fault zone widening and generation of paired bounding slip surfaces (see text). The tip-line of a fault surface (e), part of which is shown shaded in (a)-(d), propagates upwards through a rock volume. The area shown in (a)-(d) is indicated by the rectangle in (e). With fault growth the elliptical tip-line bounding the fault surface propagates radially to the successive positions, a-d, shown in (e). The lines labelled I-III in (a) indicate successive positions of the fault surface tipline.

Fault surface bifurcation processes result in areas of a fault zone with paired bounding slip surfaces alternating with areas with only a single slip surface. Either laterally or up-/down-dip, the two slip surfaces of fault zone B may give way to a single slip surface. Similarly, it is unlikely that fault zone A is characterised everywhere by only a single slip surface. As... 

Lindsay et al. (1993) describe outcrop studies of shale smears in a carboniferous fluvio-deltaic sequence in northern England. As in the study described above, Lindsay et al. concentrated on the effects of individual shale beds in the sequence rather than the bulk properties of the sequence. Smear is observed to be thickest when derived from thicker source layers and with small fault throw values smear thicknesses commonly decrease with distance from the shale source bed. From a study of 80 faults they conclude that shale smears may become incomplete when the ratio of fault throw to shale layer thickness exceeds 7. Smaller ratios are more likely to correspond to continuous smears and therefore to a sealing layer on the fault surface. 

Fig. 6. Diagram illustrating the calculation of SGR at a point on a fault surface. The throw (t) at the point is defined from the offset horizons. The throw window in the hangingwall represents the thickness of the rock that has slipped past the point. The SGR at the point is equal to the percentage of shale in the throw window. For units composed of pure shale and non-shale, SGR is the sum of the shale thicknesses divided by the throw. For units of given shale fraction, these fractions are used as weighting factors in the summation such that the result is the net shale percentage within all units in the window.

Direct observations of sub-surface pressure allow a calibration to be made between the SGR and seal capacity. Ideally, an in situ measurement of the pore-pressure in the reservoir and that inside the fault zone would allow the capillary entry pressure of the fault to be calculated. However, fault-zone pressures are rarely available. Instead, the pressure difference between the two walls of the fault is a more general parameter that can be derived from pressure measurements in pairs of wells across the fault. Fig. 7a shows one such calibration, based on the Nun River dataset of Bouvier et al. (1989). From their strike projections of Fault K , values of SGR have been calculated on a dense grid across the fault surface. On the same grid, minimum across-fault pressure differences have also been derived, using the proven distribution of hydrocarbons in the footwall sands to calculate buoyancy pressures. Fig. 7a shows a cross-plot of these two parameters for the areas of sand-sand contact at the fault surface. The dashed line indicates the inferred relationship between SGR and seal capacity. At SGR < 20%, no fault-sealed hydrocarbons are observed the shale content of the slipped interval... 

Fig. 7. Examples of calibration of SGR against across-fault pressure difference, (a) Data from the Nun River field (Fault K of Bouvier et al., 1989). Each point represents one point (grid-node) on the fault surface. SGR was calculated using the sand-shale sequence shown by Bouvier et al. Across-fault pressure difference was calculated using densities of 1.0, 0.83 and 0.3 for water, oil and gas, respectively. The dashed line labelled seal capacity represents the maximum pressure difference that can be supported by a given value of SGR. (b) Data from the Columbus Basin, offshore Trinidad, based on Fig. 8 of Gibson (1994). Each point represents one reservoir top, with observations from many different faults (all reservoirs are oil-bearing). Data points falling well to the right of the seal capacity line are faults bounding relatively small dip closures (i.e., the seal capacity is not realised).

Our approach in this paper has been to examine juxtaposition relationships and compute fault seal attributes on strike projections of fault surfaces. The analysis was carried out using FAPS software (Freeman et al., 1989 Needham et al., 1996). 

Having constructed the fault grid, with detailed juxtapositions and compositional data for all layers, we calculate a SGR. It was stated earlier that the fault surfaces were gridded at 100 x 100m or 50 x 50 m. Whilst this is adequate for analysis of displacement... 

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. 

To calibrate the SGR calculation, we need to compare the observed pressure drops at every part of the fault surface with the SGR at the same point. The pressure profiles shown in Fig. 11 have been input to the fault model and are stored at every grid-point for footwall and hangingwall. At each grid node, the difference between the footwall and hangingwall pressures is the in situ pressure drop at the fault. Fig. 12a shows a cross-plot of SGR against across-fault pressure difference for every fault grid node in the Brent-... 

As with Fig. 7, Fig. 12a shows many data points that represent smaller pressure differences than the maximum, for a given SGR. These points correspond to structurally lower parts of the fault surface, for example near the hangingwall fluid contacts. Here seal is probably well-developed, but the in situ pressure difference is small. 

Fig. 15. Summary diagram indicating the observed relationship between SGR and across-fault pressure difference for all analysed faults in the study area. Each point represents the critical part of the fault surface, i.e., maximum pressure difference for small SGR values.

The study has provided essential information with respect to the juxtaposition of units across faults (information that otherwise is difficult to obtain from depth grids), telling us where the critical areas for leak in hangingwall blocks are present. In addition the SGR estimates indicate where clay smearing is more likely to occur on each fault surface. This information can guide transmissibility reduction factors to be implemented in reservoir simulators (e.g., ECLIPSE), in order to predict flow across faults. This has been tested for a small area in the Omega South structure. 

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