Why are Deformation Bands important?
Deformation bands can contribute to fault seal in sandstones and are therefore key structures during exploration for hydrocarbons as well as geothermal energy. They can act as baffles and can compartmentalise reservoirs and for that reason they are detrimental to field production.
What are Deformation Bands?
Deformation Bands are fault-like structures that reduce the quality of reservoir sandstones. World-class examples of those occur in sandstones in Utah and as coastal exposures in the Hopeman Sandstone (Inner Moray Firth, UK), (Fig. 1).
The Hopeman Sandstone is a well-sorted, relatively clay-free sandstone that was deposited in the Permian period as a series of aeolian sand dunes. Therefore, the main fault rock processes are grain crushing (cataclasis), not those that involve clay (clay mixing, smear). Cataclasis and later cementation results in a considerably lower porosity and permeability than the host sandstones.
The compound zone of deformation bands in Figure 1a has evolved by amalgamation of individual bands (Fig. 2). Each band grows through a process of strain hardening. Further bands form and eventually a compound zone is created with a well-developed (fault-like) slip surface. This is illustrated on the right hand side of Fig. 2. The compound zones in the Hopeman Sandstone tend to form knife-edge ridges which stand out along the foreshore as the hard fault rock makes them resistant to erosion unlike the softer host sandstones.
Why are Deformation Bands important in reservoirs?
They can seal considerable columns of hydrocarbons, which can potentially be good news. However, 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 they are usually below seismic and image log resolution, often providing barriers at sand-on-sand contacts. Note that the displacement on the outcrop example in Fig 1a is clearly sub-seismic in scale (few metres).
How are they handled?
We clearly cannot use clay algorithms such as the Shale-Gouge-Ratio (SGR) to predict sealing potential of cataclastic deformation bands (in high porosity, clay-free sandstones).
Instead, we need to rely upon our knowledge of burial (and temperature) histories at the time of faulting. Burial in the order of 2 – 3 km can cause a significant mechanical reduction in grain size and in these situations cataclastic deformation bands can be better seals than clay rich ones.
Strain maps are also useful, showing regions that have experienced most strain e.g., steep forelimbs of anticlines where deformation bands may be concentrated. Also, deformation bands occur in fault damage zones, increasing in intensity towards major faults. The damage zones may be wider when a fault changes dip with depth or if there are linking fault segments.
In conclusion, deformation bands can seal large columns of hydrocarbons and can therefore be beneficial during the exploration stage. However, they can reduce hydrocarbon recovery and significantly impact well productivity. For that reason, the risks of compartmentalisation and rock degradation need to be captured in field development risk registers.
Steven Ogilvie – Ogilvie Geoscience Ltd
info@ogilviegeoscience.com
