Joints breaching the sealing fault ?

Deformation band contained joints

As deformation bands (Fig. 1) often have considerably lower permeabilities than their host sandstones, they can be seals or barriers to hydrocarbons, water etc. 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.

the mechanics

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].

COMPROMISING THE SEAL ?

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 but I wondered if concentrated joints could be responsible for at least some of the mud losses observed. There’s certainly evidence of deformation band concentrated joints in the deltaic sandstones on the floor of Kirkmaky Valley, Baku (Fig. 2). BUT, these joints would likely close as a result of production unless there is roughness and/or cements (as props) present. However, these zones may be extensive enough to alter the stress distribution.

Figure 1. Deformation band contained joints in the (weaker) Navajo Sandstone, Oak Creek Canyon, Utah (Tindall & Eckert, 2015). Such mechanical stiffness contrasts can result in significant stress re-orientations.

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 discussed in the US by [1].

I’ve had a look at deformation band zones (closer to home) in the Hopeman Sandstone (Moray Firth, UK) to see if I could observe a similar phenomenon. There are some indications of this in Fig. 3, although I need to take another look to see how widespread it is. These joints may be related to stress release post-dating the formation of deformation bands.

Figure 3. Compound zone of deformation bands in the Hopeman Sandstone, Inner Moray Firth, UK. There is a higher joint density in the d-band zone (cf. to host sandstones) but this needs more work. Again looking down i.e., not a cross-section view.

Reference

[1] Tindall, S & Eckert, A. 2015. Geometric and mechanical-stiffness controls on jointing in cataclastic deformation bands. Journal of Structural Geology 77, 126-137.