Uncovering energy hidden in the cracks

Scotland's Highlands and Islands

The rugged beauty of Scotland's Highlands and Islands holds the promise of new oil

20 November 2018 by Julia Horton

The discovery of an alternative source of oil which could potentially be worth billions of pounds to the UK economy, and play a key role in helping the country to meet its spiralling energy needs, could be a good news story.

But there are rising calls to switch from dwindling supplies of fossil fuels to more environmentally-friendly options like wind power, to protect the planet and end people's reliance on limited energy sources before they run out. So any further exploration of oil is controversial.

Like the Arctic, the wild, rugged beauty of Scotland's Highlands and Islands also holds the promise of new oil. Unlike drilling in the pristine polar sea, however, this new oil source off Shetland has been found in existing fields, so tapping into it does not involve disturbing the environment to the same extent as it would in previously untouched areas.

Hidden in the cracks

Professor Bob Holdsworth

The fractured rock formations of the Hebridean hills form part of an ancient geological archipelago known as the Rona Ridge, much of which now lies hidden deep beneath the seabed west of Shetland.

It is almost 50 years since the industry first found evidence of oil in the broken 'fractured basement' rocks of the Rona Ridge. At that time, it was assumed that only small amounts of oil had seeped in from the well-known oil reservoirs in the surrounding sedimentary layers - and it was therefore not worth investigating further.

But they were wrong, as Durham University geologist Professor Bob Holdsworth and his team discovered after studying core samples taken during exploratory drilling in the 1970s. Bob's NERC-funded research was pivotal in allowing pioneering oil firm Hurricane to establish the UK's first basement oil field. Work on extracting an estimated billion barrels of oil is due to start there next year.

Explaining the work which led to Hurricane's Lancaster field - which is the UK's largest new oil field this century - Bob says:

In most reservoirs, oil is held in pores in sedimentary rock such as sandstone. Basement rocks, which are usually found beneath, are different because they have been buried at great depth in the Earth earlier in their history, making them denser, so oil can't move through them. The industry never believed that it would be commercially viable to extract oil from this kind of tightly-packed rock, so it was forgotten about in the search for more conventional reservoirs, often in untouched areas.

So it was many years before the samples were shared in detail. On closer inspection, Bob and his team found minerals and sandstone from over 70 million years ago that they didn't expect to be there. It was from the Cretaceous period, the time of the last of the dinosaurs, when Rona Ridge was above sea level.

As the core was taken from rock which was more than half a kilometre from the nearest sedimentary layer, we realised that the sand must have been washed in from above during the Cretaceous period along a network of deep fractures that also channelled oil in from rocks west of the Rona Ridge.

So, wherever you find fractures in the Rona Ridge basement which are filled with minerals and sand, you always find oil. Fractures alone are not enough - it is the minerals and porous sand that hold open the fractures for tens of millions of years.

Networks of oil

As these rocks are buried too deeply beneath the seabed to look at easily, Bob and his Durham University colleague, Professor Ken McCaffrey, had to use a different method to find out how connected the networks containing oil are. By studying the rocks in northwest Scotland, they demonstrated that fractures above sea level were very similar to those containing oil beneath the waves.

Using laser scans of the Hebridean hills they could predict the location and connectivity of fracture networks beneath the seabed, helping oil companies to target wells in the most likely locations.

International impact

While Hurricane prepares to begin exploiting basement reservoirs at the first UK field of its kind, a group of oil giants including BP plan what could be the UK's second basement oil field further along Rona Ridge in the heart of its existing eight-billion-barrel Clair Field. Bob's research also suggests that there are more basement reservoirs in other areas around the UK which could be exploited too.

The amount of oil which the UK and other nations could tap into is now known to be significantly greater than it was before the true nature of fractured basement reservoirs became clear. There are about 100 known fields of this kind worldwide, almost all of which were discovered by accident.

His research is already having an impact across the globe thanks to a spin-off company, also supported by NERC, which currently employs around half a dozen geoscientists at Durham advising more than 40 companies on how to tap into similar fractured reservoirs worldwide.

In transition

Acknowledging the concerns about exploiting fossil fuels, Bob says:

Everyone, including most international oil companies, agrees that we have to transition to more sustainable sources of energy - and soon - but the extraction of oil is a necessary evil for the next few decades in order to allow us to make that transition.

In my opinion, it makes good sense for the UK to exploit and control its own reserves rather than relying on oil from overseas - it's all about security of supply.