Fractures

Fractured Sandstone, Buchan Field, N Sea

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

Deformation bands & later open fractures, Clair Field

Deformation bands & later open fractures, Clair Field

Deformation bands (permeability reducing) and open fractures (enhancing) can co-exist in a sandstone reservoir but owing to timing (the d-bands form early during burial), the open fractures can still assist or provide essential reservoir permeability.

Cemented fractures: good or bad news ?

Cemented fractures: good or bad news ?

Cemented fractures are often bad news for hydrocarbon production but a partially filled fracture network can be benefical..

Natural Fractures in Mudstone

Natural Fractures in Mudstone

Pink (Devonian age) mudstones on the bank of the River Tay (Perthshire, UK) are intensively fractured - including joints and larger through going fractures Here we look at some of their characteristics including some splaying geometries.

Identifying Fracture Swarms

Identifying Fracture Swarms

Fracture swarms are concentrated zones of open fractures, over which there is little or no offset. Although there are excellent outcrop examples, these structures can be difficult to identify in the subsurface. An integrated static-dynamic approach is best to successfully identify fracture swarms.