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Deformation bands & later open fractures, Clair Field

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Location/Geology

The Clair Field is located some 70 km to the West of the Shetland Isles, the nothernmost UK islands (Figure 1). The reservoir is naturally fractured Devonian age sandstones. Some wells depend upon fractures for providing reservoir permeability, whereas in other parts of the field, natural fractures provide a (permeability) assist to an already high matrix permeability. Fractures can be directly observed in rock core; an example from well 206/8-8 [1] is given in Figure 2.

Open fractures form later than deformation bands

The sandstone core in Figure 3 has been cut in a high angle well so some core re-orientation would be required but we can assume that way up is bottom to top for our purposes. The sandstone is pervasively fractured; not by open fractures though ! The interpreted red set are small faults/deformation bands which usually serve to compartmentalise reservoirs and can negatively affect well production. This is because they often have much reduced porosity/permeability compared to the host rock. Note how these form conjugate sets often with 60 degree dips. There are open fractures (green) also called joints which cross-cut these early formed fractures and provide a permeability assist to the already high matrix permeability (Type III of Nelson classification of fractured reservoirs). These joints are vertical as predicted by Anderson’s model of faulting.

Figure 1. Location of Clair Field, West of Shetland. From Ogilvie et al. (2015).

Figure 2. Open fractures in cored sandstones from well 206/8-8 (a) shear fracture showing some bedding displacement (b) the fracture on the right hand side has some mineral fill. Barr et al (2007).

Figure 3. Deformation bands (red) and open fractures (green) in sandstone core from 206/8-14Z, Clair Field.

https://largeimages.bgs.ac.uk/iip/index.html?id=20111220/S00158762

British Geological Survey © UKRI

Key takeaways

  • Open fractures can provide essential permeability or assist (matrix) permeability in even where deformation bands are present

  • This is because the deformation bands may have formed early during burial and are later cross-cut by open fractures (joints).

References

Barr, D, Savory, K.E., Fowler, S.R, Arman, K, McGarrity, J.P. 2007. Pre-development fracture modelling in the Clair Field, west of Shetland. From Lonergan, L, Jolly, R.J.H, Rawnsley, K and Sanderson, D.J. (eds) Fractured Reservoirs. Geological Society Special Publications, 270, 205 - 225.

Ogilvie, S.R., Barr, D, Roylance, P, Dorling, M. 2015. Structural Geology and Well Planning in the Clair Field. In: Richards, F.L, Richardson, N.J., Rippington, S.J., Wilson,R.W. & Bond, C.E (eds). Industrial Structural Geology: Principles, Techniques and Integration. Geological Society London Special Publications, 421.