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Impact of Relay Ramps

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Seismic Resolution

Commonly, faults are mapped on seismic data as single continuous surfaces. One of the reasons is that seismic resolution does not in general allow for definition of faults located less than a few 100 m apart [1]. However, faults usually form shorter segments separated by relay ramps [1,2] (Fig. 1). This type of fault geometry is often called soft linking. With time and increasing displacement the relay ramp will get breached - this should occur earlier in stronger, brittle lithologies.

Figure 1. View along a relay ramp between 2 fault segments on the foreshore at Kilve Pill, Somerset, UK. Note the decrease of throw towards fault tips,

Figure 1. View along a relay ramp between 2 fault segments on the foreshore at Kilve Pill, Somerset, UK. Note the decrease of throw towards fault tips,

Impact in Oil Fields

These types of geometries need to be considered in a fault seal study [1] or during well planning as they will impact the flow of fluids in the subsurface. This is because a fault framework model with long, continuous faults will have a very different impact upon dynamic simulation than a framework dominated by short faults (Fig. 2).

Dynamic data if available should be used for weighting these cases and to provide calibration. For example, scenario (b), in Fig. 2, is consistent with a better than expected well to well communication in this case. It is therefore a more likely than (a) where the faults are long and continuous.

I’ve also come across examples where an appraisal well on one side of a seismically identifiable fault shows log evidence of pressure depletion from the other side of the fault, where a production well is located - relay ramps are one explanation for this.

Another thing to consider is that relay ramps can be an area of dense fracturing and may not be the optimal place to drill.

Figure 2. Different fault (dashed lines) interpretations on an anticline - the right hand is more likely here, as is consistent with better than expected communication between wells (pink dots).

Figure 2. Different fault (dashed lines) interpretations on an anticline - the right hand is more likely here, as is consistent with better than expected communication between wells (pink dots).

Weaker Rocks

In contrast to relay ramp development (Fig 1) in relatively strong rocks, weaker rocks can experience more bed rotations and drag. Monoclines can occur in between fault segments in weak, ductlie sequences (Fig. 3).

Figure 3. Monoclinal folding of Limestones at Kilve Pill, Somerset, UK. Indicative of weak, ductile deformation.

Figure 3. Monoclinal folding of Limestones at Kilve Pill, Somerset, UK. Indicative of weak, ductile deformation.

Some sketches on fault evolution are given in Fig. 4 after then fault analysis group in UCD, Dublin [2, 3]. Fig 4a (upper) is illustrated in Fig 1 and Fig. 4b lower in Fig. 3.

Figure 4. Fault geometry evolution (modified from Childs et al. 2016). (a) relay to breached relay (b) monoclines in ductile, weaker rocks.

Figure 4. Fault geometry evolution (modified from Childs et al. 2016). (a) relay to breached relay (b) monoclines in ductile, weaker rocks.

References

[1] Hesthammer, J, Fossen, H. 2000. Uncertainties associated with fault sealing analysis. Petroleum Geoscience, 6, 2000, 37 – 45.

[2] Outcrop images of relays and breached relays -

https://www.fault-analysis-group.ucd.ie/gallery/relay.htm

[3] Childs, C, Manzocchi, T., Nicol, A, Walsh, J.J, Soden, A.M., Conneally, J.C., Delogkos, E. 2016. The relationship between normal drag, relay ramp aspect ratio and fault zone structure. Geological Society Special Publications, 439, 355-372.