Zones of Deformation Bands
We recently looked at some key features of deformation bands in sandstones and why they matter in geothermal energy and hydrocarbon projects.
They usually have considerably lower permeabilities than their host sandstones, acting as barriers to flow and potentially sealing columns of hydrocarbons.
Deformation band contained joints
I often see that joints are contained within zones of deformation bands. The first time I came across this was through work by Tindall & Eckert [1] on the Navajo Sandstone in Utah (Fig. 1).
Figure 1. Deformation band contained joints in the (weaker) Navajo Sandstone, Oak Creek Canyon, Utah (Tindall & Eckert, 2015).
How do the joints form ?
Deformation bands have high compressive strength and can create a mechanical anisotropy in the rock - for this reason, younger joints can be concentrated in these zones of deformation bands [1]. Note how the joints in Fig. 1 terminate at the contact between the sandstone (host rock) and the zone of stronger deformation bands. Another example is given in deltaic sandstones on the floor of Kirkmaky Valley, Baku (Fig. 2); the deformation band-contained joints are coloured red on the interpretation and the individual deformation bands are green. There are also background joints which have likely formed earlier.
Figure 2. Map view of a zone of deformation bands with contained joints, in deltaic sandstones on the floor of Kirkmaky valley, Baku, Azerbaijan. The deformation bands represent conjugate strike-slip shear bands similar to those shown on Figure 1.
Figure 3. Deformation band contained joints in the Permian Hopeman Sandstone (Moray, UK). Note how some joints enter the host rock.
If the deformation band zone is not aligned parallel to the extension direction at the time of jointing, the contained joints are unlikely to be perpendicular to this extension direction [1]. Compare this to when we often report joints in relation to bed thickness (mechanical stratigraphy) where joints are confined in stiffer and thinner layers and are perpendicular to the far field extension direction. Local stresses (at the boundary between mechanical layers, where strength contrast is important) therefore, play a key role in the mechanics of these deformation band confined joints [1]. Some joints enter the host rock in the example in Fig 3, suggesting that the mechanical contrast to the deformation band zone is not as high as that in Fig 1.
Compromising the seal/barrier potential of deformation bands ?
Joints are more likely to have aperture and therefore permeability than deformation bands - therefore, could they compromise the sealing/barrier potential of these zones of deformation bands ? I’ve worked a field where deformation bands demonstrably impacted oil production by reducing effective permeability but I wondered if concentrated joints could be responsible for at least some of the mud losses observed. Would the subsurface equivalents of the above examples close as a result of hydrocarbon production ? Unless there is roughness/cement present to act as a natural propant ? Perhaps these contained zones are too local anyway to have any real material impact upon field performance, but extensive enough to alter the stress distribution ?
Key takeaways
Deformation band contained joints form due to local stresses at the boundary between mechanical layers where there is a strength contrast
Such mechanical stiffness contrasts can result in significant stress reorientations.
These joints are unlikley to have a large impact upon hydrocarbon recovery but are extensive enough to alter the stress distribution.
References
[1] Tindall, S & Eckert, A. 2015. Geometric and mechanical-stiffness controls on jointing in cataclastic deformation bands. Journal of Structural Geology 77, 126-137.