reservoir geomecanics

345 Critically stressed faults and fluid flow
feldspars to clays cause faults (and the secondary fractures and faults in the damage
zones adjacent to them) to seal over time. Mechanical processes associated with active
faulting such as brecciation counteract these tendencies and help maintain permeability
within the faults and in the damage zones adjacent to them. Townend and Zoback (2000)
argue that it is the presence of critically stressed faults deep within the brittle crust that
keeps the bulk permeability of the crust about four orders of magnitude greater than
intact rock samples subjected to appropriate confining pressures.
It is well known that laboratory studies show that the permeability of faults and
fractures is a strong function of effective normal stress (Kranz, Frankel et al. 1979;
Brace 1980; Brown and Scholz 1985). The data shown in Figure 11.3 would seem to
contradict this as the tendency for faults to be permeable appears to be independent
of normal stress. However, while the critically-stressed-fault hypothesis may help us
understand which faults are likely to be permeable, a number of other factors determine
what the actual permeability is likely to be. For example, for a given permeable fault,
a number of geologic factors control its permeability such as the degree of alteration
and cementation of the brecciated rock within the fracture and its diagenetic history
(Fisher and Knipe 1998; Fisher, Casey et al. 2003), as well as the current effective
normal stress.
It is probably useful to discuss briefly why the critically-stressed-fault hypothesis
works, and what might be its extent of usefulness (some of which will be discussed
below in the context of specific case studies). In the context of the critically-stressedfault
hypothesis, the increase in permeability associated with critically stressed faults
results from brecciation during shearing (Figure 5.2) and formation of a damage zone
adjacent to the faults (e.g. Chester and Logan 1986; Antonellini and Aydin 1994;
Davatzes and Aydin 2003, and many others). In formations like the diagenetically
immature shales of the Gulf of Mexico (the so-called gumbo shales) or diagenetically
immature siliceous rocks in which silica is in the form of Opal A (a form of SiO 2
that deforms ductily), shearing would not cause brecciation so slip on active faults
may not contribute significantly to formation permeability. The effects of fractures and
faults in both of these lithologies are discussed in case studies below. In carbonates,
both dissolution and precipitation influence permeability, but there still might be an
important role for faulting to contribute to permeability. For example, faulting and
brecciation may occur along planes originally formed through dissolution processes
and there is no reason to reject out-of-hand the critically-stressed-fault hypothesis for
carbonate rocks. The importance of brittle faulting in contributing to bulk permeability
in chalk reservoirs of the North Sea is discussed in Chapter 12.
Another geologic setting in which critically stressed faults may not directly contribute
significantly to formation permeability is in the case of porous, poorly cemented sandstones
and diatomites. In such lithologies, either compaction bands, planes of reduced
porosity but little shearing (see Mollema and Antonellini 1996 and Sternlof, Rudnicki
et al. 2005), or shear bands, planar bands of reduced porosity that form in association