Abstract -
Faults are often simplified as planar structures but are, in reality, complex zones comprised of multiple slip surfaces that contain variably deformed rock volumes, ranging from intact fault bound lenses to fault rock (breccia, gouge). This sub-resolution structure has a direct impact on the juxtaposition geometries across faults and ultimately their impact on fluid flow. Fault zone complexity and architecture will vary depending on the nature of the faulted sequence and the prevailing deformation conditions. Empirical constraints on the 2D and, in particular, 3D geometry and content of fault zones are, however, relatively sparse, in the sense that insufficient data are available on fault zones developed within multi-layered sequences, with different stacking patterns and rheological properties, and under different deformation conditions. Nevertheless, existing conceptual models for the formation of fault bound lenses, for example, suggest that their generation can often be explained by one of two processes (Childs et al. 1996): (i) Tip-line bifurcation in which a fault surface propagates through the rock volume and splits into two surfaces that enclose intact volume. (ii) Asperity bifurcation in which slip along a non-planar fault surface leads to the formation of a new fault strand that removes the fault plane irregularity. The newly formed fault bound lenses become progressively fractured with increasing fault displacement to become fault rock. However, as stated earlier, this and similar models for the formation of new fault strands, lenses and fault rock are conceptual and no data are available that can be used for assesing, for example, the likelihood that intact volumes of rock are contained within a fault zone that developed within a certain sequence at a given depth. Since this shortcoming will only partly be alleviated by future detailed outcrop investigations of faults, the aim of this study is to conduct numerical modelling using the Discrete Element Method (DEM), that could improve definition and prediction of the 3D geometry and growth of fault zones for a range of sequences and deformation conditions.
The fault zone evolution observed in the models demonstrates the main processes thought to be the cause of internal complexity in fault zone structure and the model faults replicate a range of features observed in normal faults at outcrop; these include multi-stranded fault zones, relay zones, normal drag, asperities and corrugated fault surfaces. These models, to our knowledge, represent the first time that the three dimensional internal structure of faults has been reproduced in numerical models. Systematic variation in the internal structure of model faults with both changes in the lithological sequence and confining pressure suggest that this type of modelling can provide a basis for evaluating the likely complexity of fault zone structure and associated sequence juxtapositions, which may be expected in different settings and its implications for fault-related flow in the subsurface.
Abstract of talk given to:
Fault and Top Seals - from Pore to Basin Scale, EAGE Conference, Montpellier, September 2009.