Fault zone structure

Can sub-seismic fault zone structures and population characteristics be predicted ... 

Fig. 13. Conceptual model of a fault damage zone using a Sierpinski carpet. The fault zone structure can be considered to be characterised by a population series (i.e., the number of faults of different sizes, 1 8 64) and a dimension series (i.e., the size cascade of the faults present, 1, 1/3, 1/9).

How can a more refined evaluation of fault zone structure and flow behaviour be achieved ... 

Complexity in fault zone structure and implications for fault seal prediction... 

Due to seismically irresolvable complexities of fault zone structure, the juxtapositions of footwall and hangingwall rocks predicted from seismic data will in most cases be different from those actually present. The importance of such differences to the prediction of across-fault connectivity, of both hydraulically passive and hydraulically active fault zones, is strongly dependent on the reservoir sequence. Connectivities are calculated for hydraulically passive and active faults offsetting an Upper Brent Reservoir sequence. Shaley fault rocks within brittle fault zones often represent a spatially persistent, although variable thickness, component of the zones and provide a basis for the application of empirical methods of fault seal prediction to brittle faults. 

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. 

A model for the development of the complex internal structures of fault zones has recently been proposed (Childs et al., 1996). Although this model does not increase the predictability of sub-surface fault zone structure, it demonstrates how complexity can arise from the operation of simple processes and provides a framework for consideration of the uncertainties inherent in prediction. The purpose of this paper is to describe and develop this model in terms relevant to the problems of fault seal prediction. While... 

The bifurcation mechanisms for formation of multi-slip fault zones suggest that maximum fault zone thickness will often correspond to the strike-normal distance between the traces of two overlapping slip surfaces (Fig. 2c). Fault overlaps and their breached equivalents occur on faults of all sizes as do, by implication, paired and multi-slip surface fault zones. Complex and paired slip surface fault zone structures will occur on scales below that resolvable by even high quality seismic data (lateral resolution is no better than 50-100 m at North Sea reservoir depths). The possible impact of sub-seismic complexity and paired slip surfaces on connectivity and sealing across faults offsetting an Upper Brent type sequence are briefly considered below. 

Empirical risking methods implicitly take account of the unpredictable complexities in fault zone structure which inevitably are present. The way forward is by further refinement of current empirical methods, i.e., by more detailed characterisation of sub-surface faults to allow more objective comparison of target faults and faults of known sealing behaviour. 

Childs, C., Walsh, J.J. and Watterson, J. 1997. Complexity in fault zone structure and its implication for fault seal prediction. In P. Mpller-Pedersen and A.G. Koestler (Editors), Hydrocarbon Seals Importance for Exploration and Production, NPF Special Publication 7. Elsevier, Singapore, pp. 61-72. 

Molli G, Cortecci G, Vaselli L, Ottria G, Cortopassi A, Dinelli E, Mussi M, Barbieri M. 2010. Fault zone structure and fluid-rock interaction of a high angle normal fault in Carrara marble (NW Tuscany, Italy). 

---