Many natural rock fractures contain minerals or cements which reduce their permeability and the permeability of the fracture network. The example in Figure 1 shows barite cements filling a fracture network in aeolian dune sandstones - in the host rock the barite has a nodular, desert rose appearance. It has been suggested that these cements have a hydrothermal origin. Alternatively, Glennie (2002) assigns a Jurassic age to this mineralization and relates it to activity associated with the early Mid-Jurassic thermal dome over the central North Sea.
Figure 1. Fractures (in this case reactivated) deformation bands filled with barite. The barite also has a desert rose like appearance in the host sandstones (permian Hopeman Sst, Inner Moray Firth, UK).
Cemented fractures have been described from cored sandstones in the Rotliegende reservoirs of the Southern North Sea - anhydrite, for example, which could be sourced from pore waters coming from the overlying Zechtein evaporites during (fault dilation events) - saline fluids being dense and will move downwards. Alternatively the precipitation of these cements could be related to fault reactivation - as Zechstein-derived brines were expelled along a decreasing pressure/temperature gradient (Leveille et al. 1997). In this case, they are evidence of non-breached fault seals during a structural inversion phase (see blog - some benefits of fracture cements). Its also possible that the pore fluids were derived from the underlying Carboniferous Coal Measures.
Anhydrite has a lattice-like appearance in the fracture swarm in Figure 2 - this photo is from the foot of the cliff scale swarm on Figure 3. It is likely that these anhydrites have formed during burial - the gypsum to anhydrite transformation has released sulphide-rich brines which have travelled through fractures and along bedding planes and precipitated as cements. The host rocks are Zechstein dolomites (found close to Marsden Bay, NE England), which are analogues for Southern N Sea fractured dolomite reservoirs.
Figure 2. Close up of anhydrite cements (very intricate lattice) which occupy the fractures in the fracture swarm in Figure 3.
Figure 3. Cliff-scale fracture swarm in Zechstein dolomites, NE England. Anhydrite cements occupy portions of the swarms - see Figure 2.
Reference
Glennie, K.W. 2002. Permian and Triassic. In: Trewin N.H (ed). 2002 The Geology of Scotland. The Geological Society, London, p 301 – 321.
Leveille, G.P. Knipe, R. More, C, Ellis, D, Dudley, G, Jones, G, Fisher, Q.J. & Allinson, G.1997. Compartmentalisation of Rotliegendes gas reservoirs by sealing faults, Jupiter Fields area, southern North Sea. From Ziegler, K., Turner, P. & Daines, S.R. (eds), 1997, Petroleum Geology of the Southern North Sea: Future Potential, Geological Society Special Publication, 123, 87-104.