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Rattray could hold the last great North Sea oil reserves

Vulcanism was supposed to have scuppered the oil potential of a large area of the central North Sea, but academics say it may be worth another look

North Sea operators have been urged to look again at old assumptions, after fresh analysis of historic seismic data revealed a swathe of the basin may yet hold hydrocarbon reserves.

The Rattray volcanic province in the central North Sea remains largely unexplored, despite being located in one of the world's most prolific-though now mature-regions for oil and gas production.

Drillers have given Rattray a wide berth in the past, because it was believed that basalt lava flows erupting within an area of 7,000 km2 (2,700 sq miles) from volcanoes active 165mn years ago had formed magma chambers. These would have prevented the existence of large oil and gas deposits today, the geological argument ran.

But analysis by a team from the University of Aberdeen's School of Geosciences, along with colleagues from Heriot-Watt in Edinburgh and the University of Adelaide, indicates that the lava emerged not from volcanoes, but from fissures. That process would not have led to the creation of magma chambers and thus would not rule out the existence of hydrocarbons deposits.

The new geological model was attained by combining 3D seismic data donated by Oslo-listed Petroleum Geo-Services (PGS) with further well data.

"There is a huge area under there that hasn't been looked at in detail for a long time, because of the previously incorrect geological model," says Nick Schofield, who worked on the research at Aberdeen University. "That is not to say that exploration would not be challenging, but technology is constantly improving and there are still big discoveries being made in the North Sea, as we've recently seen in the Central Graben and Viking Graben areas."

Lucy King, a North Sea upstream analyst at consultancy Wood Mackenzie, believes the new research suggests the area-around 130 miles (209km) north-east of Aberdeen-could be similar to hydrocarbons-rich areas, including Rosebank, now operated by Norway's Equinor, one of the largest undeveloped fields on the UK continental shelf (UKCS). But more work needs to be done.

"Volcanics are particularly tricky to map on seismic. The next step will be to acquire broadband seismic data across the area to gain a better resolution of the formations present beneath the volcanics," she says. "It is seismic like this that exposed the likes of Cambo and Rosebank in amongst the volcanics in the west of Shetland."

Wells to test the potential are still a few years away, while North Sea operators, the Oil and Gas UK (OGUK) trade body and the Oil and Gas Authority (OGA) all insist it is too soon to reach solid conclusions on the findings. But, given the UK industry's brief to "maximise economic recovery", some companies may be interested in taking a look at the area's potential sooner rather than later.

A long delay could push up costs of exploiting any commercial reserves in Rattray to prohibitive levels, if potentially useful infrastructure associated with maturing fields in the region are decommissioned beforehand.

Surrounded by discoveries

The Rattray province is surrounded by major installations at varying life stages. To the west is Cnooc of China's mighty Buzzard field and Golden Eagle, which started production in 2015. To the south is the Forties field-the oldest and biggest on the UKCS, which has been producing since 1975 and whose life has been extended further by its operator, US independent Apache, who bought the field from BP in 2003.

The Britannia field, operated by US independent ConocoPhillips, and Chevron's recently redeveloped Alder field, are within the confines of the Rattray province and highlight both the possibilities and further challenges to acting on the promising new analysis. Both operators have put their North Sea assets up for sale as they look to reinvest proceeds into US shale, putting investment decisions on their historic asserts on the UKCS on hold.

Meanwhile the Buchan field, on the fringes of Rattray, has ceased production and its platform has been removed.

The OGA has begun a new "area plan" to spur operators to look at currently unlicensed acreage and work together on an asset plan that will ensure an estimated 150-300mn barrels of oil in undeveloped discoveries are not left in the ground.

King says that while the Rattray volcanics are within a "very well-established area" of the UKCS, "it won't be without risk-any potential wells will have to drill through over 1km of volcanics".

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