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Fractured Basement Reservoirs

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What makes Basement Reservoirs different ?

Basement reservoirs consist of igneous and metamorphic rocks (Fig. 2) that are unconformably overlain by sedimentary rocks [1]. These rocks usually have little/no matrix porosity and permeability. Instead, the fracture network provides almost all of the storage (porosity) for hydrocarbons and permeability to deliver the hydrocarbons to a well. They are therefore Type 1 reservoirs (of Nelson) meaning that description/understanding the fracture network is critical to development success. Given the often high connectivity of the fracture network, production interference between wells is common as was the case in tonalite reservoir of the Lancaster Field, West of Shetland.

In contrast to more conventional fractured reservoirs, the source rock usually doesn’t underlie basement reservoirs. In the White Tiger Field in Vietnam (Fig. 1, [2]) the uplifted basement block has been brought into contact with Jurassic source shales.

Weathering and Fractures

Many of these rocks have been subject to extensive weathering if the reservoir fault blocks have been uplifted and stayed high for a significant time. This can be deduced by onlapping of more recent sediments onto these fault blocks on seismic data. In turn, fracture apertures and resulting porosities can therefore be bigger than in more conventional fractured reservoirs.

A more recent development is the Lancaster Field, West of Shetland, UK. As with the Vietnam example there are 3 main fractured zones (Fig. 1, 3)

  • True Basement (red) has not been affected by weathering

  • Exposed zone (dark green), often has wider (aperture) fractures than other zones due to repeated exposure to the elements. Often provides best porosity of the 3 zones.

  • Granite wash (light green on top of the fault block), a regolith layer often with very variable properties.

Fig 1. Conceptual model of a fractured basement reservoir in White Tiger field, Vietnam [2]. Note the fluid leak (black arrow) towards Miocene sands draping the structure.

Fig. 2. True fractured basement in NW Highlands of Scotland. Analog for Lancaster Field, West of Shetland. These fractures are iron stained in places and cross cut the foliation direction (across the page).

As with more conventional fractured reservoirs, basement fractures can be fault/fold related and also related to regional tectonics (Figs 1 - 3).

Fig. 3. Conceptual model of structural elements for Lancaster Field showing fault related and regional fractures, West Shetland [1]. Not shown are the 3 vertical zones of fracturing outlined in the text.

KEY TAKEAWAYS

  • Basement reservoirs are usually metamorphic and igneous rocks overlain by sedimentary rocks.

  • They usually have negligible matrix porosity and permeability. Instead, the fracture network provides most, if not all of the reservoir porosity and permeability.

  • Therefore, these are Type 1 reservoirs of Nelson where accurate description of the fracture network is vital to development success.

References

  1. Bonter, D. A & Trice, R. 2019. An integrated approach for fractured basement characterisation: the Lancaster Field, a case study in the UK. Petroleum Geoscience.

  2. Hung, D. N., & Hung, V. L. 2004. Hydrocarbon geology of Cuu Long Basin–offshore Vietnam. AAPG International Conference, Barcelona, Spain, September 21–24, 2003.