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

Partially open fractures

What is a cemented fracture ?

A fracture that now contains a mineral fill or cement. When fluids travel along fractures, cements can precipitate out into the fractures. Calcite is a common fracture cement but many minerals form cements. For example, fluorite can be found within the deformation bands in Fig 1 (a) and anhydrite fills the lowermost fracture in Fig 1 (b).

http://www.ogilviegeoscience.co.uk/blog/2022/12/4/natural-vs-induced-fractures-in-rock-core

Fig 1. (a) Fluorite cemented fracture network in the Permian Hopeman Sandstone, UK. (b) Deformation bands in Rotliegende sandstone from the Southern North Sea. The lowermost (thickest) fracture has been reactivated and cemented by anhydrite.

What are the consequences of a completely “filled” fracture ?

Most likely it will impede flow and hinder the productivety of a fracture network, as the permeability and storage capacity (porosity) is significantly reduced.

What about a partially cemented fracture ?

The fractures (in sandstone) from the Clair Field (West of Shetland, UK) in Fig. 2 appear to have some cement i.e., they are partially filled. In areas of the field, this has been beneficial in preserving fracture aperture during hydrocarbon production. Otherwise, the fractures may close.

Fig. 2. Fractured sandstones from the Clair Field, West of Shetland, UK (From Barr et al. 2005). Note the partial fill on the right hand most fracture in (b).

Further examples of partially filled fractures are given in Fig. 3. Sometimes the aperture is obvious (Fig 3a) other times we can just pinpricks along the extent of the fracture (Fig 3b).

Fig. 3. (a) Partially open fracture in dolomite, Middle East (b) partially open fracture in sandstone core

This together with fracture surface roughness (where asperities can prop open the fracture even as the normal stress increases) may explain why pressure drawdown does not always result in closed fractures. If the fracture is reactivated, these asperities may collapse (depending upon the rock) and new points of contact taken up, creating new apertures. In strong rocks such as igneous rocks (e.g. granite) these asperities will be less likely to collapse.

Can we see partially filled fractures on image logs ?

Yes, but it is valuable to have a core calibration for any image log interpretation. On the image on Fig. 4, note how the aperture appears to be thickest at [1] but thins towards [2]. This could indicate a partially filled fracture. Rock core in this well would be most useful to calibrate this interpretation i.e., can we pick the same structure in core that is demonstrably a partially filled fracture or perhaps the fact that partially filled fractures dominate the core is sufficient calibration.

Fig 4. Electrical Resistivety image where a partially open fracture is interpreted.

In conclusion..

Cemented fractures will usually impede fluid flow in a hydrocarbon or geothermal reservoir and would likely negatively impact production. However, a small amount of cement can act as a natural propant, keeping fractures open during production and is therefore beneficial to the field development. These are most confidently identified on core and it is valuable to use core (if available) to calibrate image log interpretations.

References

1. Barr, D, Savory, K.E., Fowler, S.R, Arman, K, McGarrity, J.P. 2005. Pre-development fracure 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.

DEVEX fieldtrip to Hopeman Sandstone 30th May 2024

A highlight of the year so far for Ogilvie Geoscience has been a trip to the Hopeman Sandstone in Morayshire, UK as part of the DEVEX 2024 conference.

Led by Steven Ogilvie and sponsored by Craig Lindsay of Core Specialist Services. We also had the pleasure of the company of David Longstaff from Elgni Museum who pointed out numerous geological structures and reptile footprints.