Fractured basement is an emerging hydrocarbon play e.g., the Lancaster Field (West of Shetland) and well known fields in Vietnam. The matrix has virtually no storage or permeability potential (Figure 1) and the fractures provide the (hydrocarbon) storage and essential fracture permeability to bring the oil to the well.
Figure 1. (a) Opposing fracture surfaces in granite with matrix petrophysical properties from offcuts, KL = core permeability, He = helium porosity & Hg is mercury injection porosity (b) optically Profiled fracture surfaces - the distance between the surfaces is the aperture. Methodology in isakov et al. (2001).
The permeability of the fracture network can be calculated using well tests. As single fracture permeability (kf) is often calculated using the parallel plate model or Cubic Law (using cube of fracture aperture) - a small change in aperture has a big impact. Permeability is overestimated as fracture surface roughness (that we see in Figure 1), is not taken into account. The related flow (Reynold's) equation fails for rough surfaces (Figure 2) although the improved Ge's equation does manage to represent the bump in Figure 2.
Figure 2. Finite Element Model (FEM) showing that the Reynold’s Equation (upper) does not handle curved surfaces whereas the Ge’s Equation (lower) does a better job of this.
If there is shear movement (e.g., stress-induced reactivation of fractures due to oil production) the asperities that touch (from opposing fracture surfaces) may be sheared off and new contact points taken up. In strong rocks like the granite in Figure 1, the contact points (& aperture) may be preserved but as you can imagine the situation would be quite different in soft sands and chalks - and this stress dependency on fracture permeability becomes a reservoir management issue as the fractures can close. Ekofisk a giant chalk field in the Norwegian N Sea experienced fracture closure and compaction but good production was a result of new fractures forming, following quite an unusual (triaxial) stress path. In addition to roughness, a small amount of fracture cement can be beneficial as imparts a stiffness to the fracture, keeping it open and productive (see older blog "Some benefits of fracture cements")
References
Isakov, E, Ogilvie, S.R., Taylor, C.W., Glover, P.W.J. 2001. Fluid flow through rough fractures in rocks I: High resolution aperture determinations. Earth & Planetary Science Letters 191, 267 - 282.