When fluids travel along fractures, cements can precipitate out, filling the fracture and destroying it's permeability and any storage capacity that it may have. Calcite is a common fracture cement but many minerals form cements e.g., fluorite in the fractured sandstone in Figure 1.
Figure 1. Fluorite cemented fractures (deformation bands) in Permian Hopeman Sandstone, Inner Moray Firth, UK.
Although cemented fractures (Figure 1) will form flow barriers in the subsurface (impacting for example producer-injector communication), they are not always bad news as,
1. Fractures are often partially filled with cements (Figure 2), preserving some aperture - imparting a stiffness to the fractures, keeping them open under high effective stresses. This together with fracture surface roughness (where asperities prop open the fracture even as the normal stress increases) may explain why pressure drawdown does not always result in closed fractures and reduced hydrocarbon production. If the fracture is reactivated, these asperities may collapse (depending upon the rock) and new points of contact taken up, creating new apertures.
Figure 2. Partially cemented fracture in dolomite. The cement will help preserve aperture during hydrocarbon production (Middle East outcrop).
2. Fracture cements can add to the sealing potential of a prospect. The deformation band in the thin section in Figure 3, (backscattered SEM) has crushed grains (cataclasis) and is cemented by anhydrite (white patches). It is likely that the broken grain surfaces have proven attractive for the cements. They were introduced during the Southern North Sea basin inversion phase; showing that inversion (fault reactivation) did not result in seal breaching.
However, fracture cements often occur in dilatational jogs, separated by non-cemented areas - and the barrier/sealing potential of fault zones are more likely to be controlled by the petrophysical properties of the fault rock in between the cements, rather than the cements themselves (Fisher & Knipe, 1998).
Figure 3. Deformation band that has been reactivated during the Southern North Sea inversion phase - when the anhydrite cements (white patches) were introduced. Backscattered scanning electron microscope image, from Ogilvie & Glover, (2001).
References
Fisher, Q.J. & Knipe, R.J. 1998. Fault sealing processes in siliciclastic sediments. In Jones, G., Fisher, Q.J. and Knipe, R.J. (eds) Faulting, Fault Sealing and Fluid Flow in hydrocarbon Reservoirs. Geological Society of London Special Publications, 147, 117-135.