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Porosity and Permeability affected by Juxtaposition

Juxtaposition of different units is a natural consequence of faulting. If the displaced units are of different hydraulic properties, which they almost always are, juxtaposition should be taken into account. Juxtaposition may have a significant impact on fault normal permeability. However, it won't have much influence on permeability of faults along the fault.

When no information about the actual fault zone is available, juxtaposition provides some estimate on the permeability changes that are imposed by a fault. In such estimation, a fault is treated as an ideal plane, ignoring the structure of the fault zone. For example, in a sequence of shale-sand layers, an ideal elliptical fault plane with linear slip variation would define a juxtaposition pattern shown in Figure 1. Layers that occur at the fault's center have large surface area, but only a small part is available for cross flow. Layers at the top and bottom of the fault, in contrast, have small surface area, though nearly completely available for cross flow (Willemse, 1996).

Plane view on an idealized ellipital fault plane in a sequence of shale-sand layers, showing positions of hanging wall, footwall, and areas available for flow across the fault. From Willemse (1996).Figure 1. Plane view on an idealized ellipital fault plane in a sequence of shale-sand layers, showing positions of hanging wall, footwall, and areas available for flow across the fault. From Willemse (1996).

When a permeable rock layer is juxtaposed upon a less permeable one across a fault, the fault effectively acts as a seal and reduces the fault normal permeability of the permeable layer. This is called juxtaposition seal.

Allan (1989) provides a method for constructing juxtaposition diagrams. For various examples of this method and its use in fault seal analyses, see Knipe et al. (1998), Knai and Knipe (1998), Koledoye et al. (2003), and Childs et al. (2000).

Reference:

Allan, U.S., 1989. Model for hydrocarbon migration and entrapment within faulted structures. American Association of Petroleum Geologists Bulletin 73: 803-811..

Childs, C., Sylta, O., Moriya, S., Walsh, J.J., Manzocchi, T., 2002. A method for including the capillary properties of faults in hydrocarbon migration models. In A.G. Koestler and R. Hunsdale (eds) Hydrocarbon Seal Quantification. Elsevier Science B.V., Amsterdam..

Knai, Knipe, R.J., 1998. The impact of faults on fluid flow in the Heidrun Field. In: Jones, G., Fisher, Q.J., and Knipe, R.J. (eds) Faulting, Fault Sealing and Fluid Flow in Hydrocarbon Reservoirs. Geological Society, London, Special Publications, 147, 269-282.

Koledoye, B., Aydin, A., May, E., 2003. A new process-based methodology for analysis of shale smear along normal faults in the Niger Delta. American Association of Petroleum Geologists Bulletin 87 (3): 445 - 463.

Willemse, E.J.M, 1996. Permeability anisotropy due to juxtaposition fault sealing. Stanford Digital Repository. Available at: http://purl.stanford.edu/tn055rn7737.



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