Deformation Bands are fault like structures that can trap hydrocarbons and also be flow barriers within a producing field. Their effect is biggest in high porosity sandstones, where grain breakage (cataclasis) can significantly reduce the permeability and porosity of the rock. World-class examples of those occur in sandstones in Utah (Aydin & co-workers) and in the Hopeman Sandstone in the UK (Inner Moray Firth), (Figure 2).
Here we focus on cataclastic deformation bands, although there are other types including clay-rich deformation bands (see Gibson, 1998). Firstly, a solitary band is joined by new bands (process known as strain hardening) which eventually coalesce to form a fault with a well defined slip surface (Figures 1, 2).
Figure 1. Formation of deformation bands (a) Single deformation band with mm scale offset (b) compound zone of deformation bands with 0.2 to 0.3 m offset (c) Compound zone with well developed slip surface with c. 0.5 m offset. Modified from Edwards et al. (1993)
Figure 2. Outcrop examples of the stages in Figure 1, from Hopeman Sandstone, Inner Moray Firth, UK. Upper: single deformation band at label 1, 20 cm wide compound zone at label 2 which runs through the centre of the photograph. Lower: compound zone with well developed slip surface (steeply downthrowing to left). Dune bedding dips more gently to the right.
Impact & Prediction
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, where the fault throw may be less than the reservoir thickness. They have a large impact upon permeability - a small increase in volume fraction of rock occupied by deformation bands causes a big decline in effective permeability. Permeability maps (profile permeametry) of slabbed sandstone containing deformation bands are shown in Figure 3. The 5 mm grid resolution is effective at delineating the permeability of individual deformation bands. There is a mean reduction of 4 orders of magnitude in permeability in the deformation band compared with the permeability of the immediately adjacent host rock pod (Ogilvie & Glover, 2001).
On the positive side, they can seal significant columns of hydrocarbons (e.g., Gibson, 1998). We can calculate the (supportable) hydrocarbon column height by relating the capillary pressure of the fault rock, fluid densities and height above free water level.
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 (Figure 2). The damage zones may be wider when a fault changes dip with depth or if there are linking fault segments (Fossen, 2010, Figure 8.25).
Figure 3. Permeability contour maps (pressure decay permeameter, Corelabs) of a slabbed sandstone sample containing cataclastic deformation bands (Hopeman Sst) at 1 cm and 5 mm grid resolution. Source: Ogilvie & Glover, (2001).
References
Edwards, H.E, Becker, A.D, Howell, J.A. 1993. Compartmentalisation of an aeolian sandstone by structural heterogeneities: Permo-Triassic Hopeman Sandstone, Moray Firth, Scotland. From North, C.P. & Prosser, D.J. (eds), 1993, Characterization of Fluvial & Aeolian Reservoirs. Geological Society Special Publication No 73, pp 339 - 365.
Fossen, H. 2010. Structural Geology. Cambridge University Press.
Gibson, R.G. 1998. Physical character and fluid-flow properties of sandstone-derived fault zones. In: Coward, M.P, Daltaban, T.S& Johnson, H. (Eds) Structural Geology in Reservoir Characterization, Geol. Soc. Lond. Spec. Publ. 127, 83-97
Ogilvie, S.R. Glover, P.W.J. 2001. High Resolution Petrophysical measurements of Deformation Bands in Sandstones. Physics and Chemistry of the Earth, Part A, 26, 1-2, 27-32.