Deformation Bands are fault-like structures that reduce the quality of reservoir rocks. They are often white coloured although the compound zone of deformation bands on a wave-cut platform (coastal) in Figure 1 is heavily covered in barnacles. They can seal considerable columns of hydrocarbons. But deformation bands are generally bad news for production as they can trap hydrocarbons in isolated fault blocks and for example hamper the ability of an injection well to support a production well. The trouble is that they are hard to detect as are usually below seismic and image log resolution, often providing barriers at sand-on-sand contacts (Figure 1).
Figure 1. Compound zone of deformation bands in permian dune ssts, (Hopeman Sst, Inner Moray Firth, UK). The structures are near vertical, but the dune bedding dips 20-30 deg to left to right.
Deformation bands have a large impact upon effective permeability - a small increase in the volume fraction of rock occupied by deformation bands causes a big decline in effective permeability. If we consider 3 blocks of equal thickness (for simplicity) in parallel – the central one is the fault zone and has a permeability of 1 mD. The host rock blocks on either side have permeabilities of 1000 mD and 200 mD respectively. The harmonic mean of these permeabilities is 2.98 mD. Clearly, the permeability is controlled by the smallest permeability because all the fluids that pass easily through the higher permeability layers are held up by the low permeability layer or deformation band zone.
Figure 2. Impact of Deformation Bands Upon Effective Permeability using a harmonic mean.
The challenge here is that deformation bands have little effect upon porosity logs but have a large effect upon effective permeability. That’s because the actual volume fraction of deformation bands in the most degraded rock is quite low (Figure 3) – perhaps no more than 10-15%. In the Entrada/Navajo sandstones (Utah), the porosity loss is from about 25% to 6% - but this loss is unlikely to affect storage properties of a porous sandstone at reservoir scale – due to localised nature of these structures.
Figure 3. Volumetric Fraction of Deformation Bands vs. Effective Permeability calculated using a harmonic average.