Where is the Buchan Field ?

In the Central North Sea about 150 km NE of Aberdeen, near the tip of the offshore extension of the Highland Boundary Fault (HBF). Well productivety is through open fractures formed during rift-related uplift of the Buchan Horst [1].

Figure 1. Location of the Buchan Field, offshore from Aberdeen.
https://www.spe-aberdeen.org/wp-content/uploads/2020/10/Wednesday-Jersey-Oil-Gas.pdf

The only non carbonate fractured reservoir in the N Sea ?

If not, then its probably the main clastic fractured reservoir. The others are carbonates e.g., chalks in Ekofisk, Valhall and Eldfisk and dolomites in the Southern N Sea. Buchan has recently undergone re-development by operator Jersey Oil & Gas..

https://www.spe-aberdeen.org/wp-content/uploads/2020/10/Wednesday-Jersey-Oil-Gas.pdf

What do the fractures look like ?

There are some photos of individual fractures (from well 21/01-6) in the above pdf. I’ve included photos of core sticks from BGS (from well 21/01-6) in Fig 2. The fracture density is not that high, maybe 2-3 fractures/m although they appear to be oil stained (Fig 3).

Fig 2. Fractured channel sands in well 21/01-6. The fractures are oil stained.
*British Geological Survey © UKRI*
https://largeimages.bgs.ac.uk/iip/index.html?id=20121005/S00136556

Fig 3. Open fractures (oil stained) in channel sandstones. The left hand piece has numerous rip up clasts of clay. The curved scratches are saw marks and not geological.
*British Geological Survey © UKRI*
https://largeimages.bgs.ac.uk/iip/index.html?id=20121005/S00136556

Are all the fractures open ?

No, there are deformation bands present in the more crumbly, high porosity sands (Fig 4).

Fig 4. Deformation bands in the good quality bar sand deposits in 21/01-6.

Conceptual model

That of fracture corridors or fractures around seismic scale faults (Fig. 5). This is an intergrated interpretation using outcrop data and Production logs (PLTs).

Figure 5. Fractured hanging wall of a small fault in Devonian Sandstones, Arbroath, Scotland.

References

[1] Benzagouta, M.S, Turner, B.R, Nezzal, F, Kaabi, A. 2001. Reservoir Heterogenieties in Fractured Fluvial Reservoirs of the Buchan Oilfield (Central North Sea). Oil and Gas Science and Technology – Rev. IFP, Vol. 56, No 4, pp 327 – 338.

[2] Wynn, T, Saundry, E. The Buchan Field, Blocks 20/5a and 21/1a, UK North Sea. Geological Society, London, Memoirs. Volume 52. Pages 679 - 690