What is Juxtaposition Fault Seal ?
This is when a sand reservoir unit is completely offset against non-reservoir (commonly shale), (Fig 1a) and fault seal can usually be expected. If fault throw is less than reservoir thickness then the sands in Fig 1a will be self-juxtaposed and leakage of fluids can occur (Fig 1b).
Could the communication pathway be different ?
Yes, for the juxtaposition seal case in Fig. 1. In fact, seismic may not be able to resolve the level of detail that is illustrated in Fig 1b. Note how the presence of small fault blocks (sub - seismic faults) allow for communication between the sands. Outcrop data and/or conceptual understanding of the fault network would help reduce this uncertainty (Fig. 2). These types of observations could explain well to well communication across a seismically mapped fault.
Fig. 1. (a) Clear case of juxtaposition seal where the sands are not in communication. (b) Small fault blocks below seismic resolution could allow for communication between the 2 sands.
Analysis of seismic data may indicate that a sand reservoir is juxtaposed against non-reservoir. The non reservoir in the outcrop photograph in Figure 2b is conglomerate. The splaying nature of the fault zone structure here shows that this is not simply a sand juxtaposed against non-reservoir (conglomerate in this case but shale in many cases) - there are opportunities for communication across this fault.
Fig. 2. Outcrop example of a normal fault in Devonian Sandstones/Conglomerates, Arbroath, Scotland: Showing there are opportunities for communication across this fault.
An example taken from a seismic interpretation of relatively poor quality seismic (without the seismic backdrop) of a reverse fault is given in Fig. 3. A simple interpretation in (a) means that there is no opportunity for communication across this fault as the sands are not in contact with each other. However, some sub-seismic structure in (b) could allow for communication across this fault. Usually we assume leakage/communication across sand on sand contacts unless there is some fault rock sealing potential of the smaller faults.
Fig.3. 2 Interpretations of a reverse fault zone where the seismic data is relatively poor (a) simple interpretation where there is no opportunity for communication across either reservoir (b) Sub-seismic splay allows for potential communication through Reservoir B across the fault.
Takeaway Points
Even although a trap has the appearance of a juxtaposition fault seal (reservoir against non-reservoir), sub-seismic structure may be such that communication is possible across the fault.
This is because sands are in contact with each other in much the same way as a self-juxtaposed reservoir.
In these cases, we should consider fault rock seal i.e., the presence of clay smears and/or cataclastic fault rocks.