Abstract -
New models for fault zones recognize that faults are not simple planar features but are highly complex zones within which displacement and strain are concentrated onto one or several discrete slip surfaces or zones of intense shearing, enclosing variably strained rock volumes. Fault segments and intervening relay zones are a common form of propagation-related complexity which can be observed both in map view and in cross-section on a wide range of scales and for each mode of faulting. Relay zones, the rock volumes between adjacent kinematically related fault segments, are subjected to increasing strains and bed rotations as displacement increases on a segmented fault, eventually causing failure of the relay zone by the formation of a linking fault to form a ‘breached relay zone’. The former relay zone may become incorporated into the fault, initially as a fault-bounded lens and perhaps ultimately as a zone of thickened fault rock, which may be entrained along the fault surface in the displacement direction. Whatever the ultimate outcome of a relay zone, recent work indicates that their presence could have a major impact on fluid flow linked to a number of application areas. In this talk, we briefly review current thinking on the evolution of segmented faults and associated faults, followed by descriptions of a selection of case studies highlighting the different ways in which relays can impact flow in hydrocarbon and minerals flow systems, and how these effects are incorporated in industry workflows, either as conceptual models or as flow modeling solutions.
The impact of relays on fluid flow depends on their scale and the nature of flow system concerned. For example, in fault seal studies of clastic sequences in which faults are detrimental to hydrocarbon flow, the presence of relays on an otherwise continuous sealing fault may provide cross-fault pathways and the absence of a fault trap, but this need not be the case depending on the displacement on the fault and the integrity of the relay. Newly developed techniques provide a basis for quantifying the impact of either individual relays or a population of relays on reservoir production (Fig. 1), and have already shown how case specific the flow implications are: depending on many factors, such as the nature of the faulted sequence or the fault pattern, the flow impact can be insignificant or major. Similarly, relays can have a profound impact on hydrocarbon migration and the development of traps within a region, an issue which is, however, more often considered in qualitative terms, though there are also recently developed flow modeling approaches which could be applied to such problems. By contrast, in flow systems characterized by low-permeability host rocks and conductive fault-related fluid flow, relays can provide the locus for up-fault fluid flow. The inclusion of relay-related flow in so-called fractured reservoirs, such as limestones, is much less routine, partly because of the relatively poor parameterization of these conductive fracture flow systems, but also because of shortcomings in associated flow modeling schemes. Using 3D constraints from Irish Carboniferous Zn-Pb deposits we show that relays can have a profound impact on fluid flow and can be responsible for the formation of world-class mineral deposits (Fig. 2). Our work highlights the benefits of interactions between structural geologists in different application areas on flow-related issues.
Abstract of talk given to:
Industrial Structural Geology: Principles, Techniques and Integration, Geological Society of London, November 2012.