Abstract - Faults are frequently segmented both in map view and in cross-section. Displacement is transferred between adjacent fault segments with high displacement gradients via a zone of high strain, often referred to as a relay zone. For normal faults within sub-horizontally bedded sequences, segmentation in map view gives rise to relay ramps between overlapping segments. As displacement increases on the fault segments the relay ramp steepens and eventually fails with the formation of a through-going breaching fault that by-passes an associated splay. From the point of view of fault sealing, identification of relay zones is crucial because, if present, they can provide complete geometrical and hydraulic continuity of reservoir units across what may be an otherwise sealing fault. Intact relay zones occur on faults of all sizes so that for any seismically imaged fault system we can expect that there are relay zones, both intact and breached, which are below the limits of lateral seismic resolution i.e. the distance between faults below which they appear as a single fault on a seismic section. Here we address two questions: (i) If there is a sub-resolution relay zone on a fault which bounds a prospect, what is the likelihood that it is intact? (ii) If the relay zone is breached, what is the minimum likely displacement on the through-going fault?
We have collated a dataset of geometrical parameters for intact (131) and breached (74) map-view relay zones on normal faults defined from high quality outcrop and seismic datasets. The parameter set includes the fault throw, the relay zone length and separation (i.e. the fault normal distance between the relay bounding faults) and, for breached relays, the throws on both the breaching fault and the splay fault. Of the various relationships observed, the most relevant to fault sealing is a significant difference in the ratio of fault throw to relay separation for breached and intact relay zones. For a fault of a given throw and for a given lateral seismic resolution, these throw to separation ratios provide a basis for estimating the preservation potential of an intact sub-seismic relay. Furthermore, we can also estimate the minimum throw on the through-going fault of breached relays assuming that the throw on by-passed splays is equivalent to the fault throw at breaching. Though our database provides a firmer basis for predicting the geometry and preservation potential of sub-seismic relays, few constraints are available on the mechanical stratigraphy of the faulted sequences. Our dataset is therefore supplemented by distinct element numerical models of relays in which the mechanical stratigraphy is user defined. Although these models show similar relationships between the throw to separation ratio and breaching, they also show that relay zones in massive homogeneous rocks fail at lower throw values than those in layered sequences. Future work is, however, required before numerical modelling provides a more refined basis for predicting the geometry and preservation potential of relays.
Our findings provide a methodology for risking the integrity of relays, which when combined with a method for estimating fault seal capacity, provides risking estimates for trapped hydrocarbons. Here we illustrate such a methodology by assuming that fault seal capacity is related to the percentage shale which has slipped past a point on a fault i.e. Shale Gouge Ratio. Based on the throw distribution along the length of a fault and the lateral resolution of the seismic data, probability curves for reduction in estimated seal capacity due to the presence of relay zones can be constructed for a potential prospect bounding fault.
Abstract of poster presented at:
Fault and Top Seals, EAGE Conference, Montpellier, September 2003