Forties at 50 years old

Five decades of learning the art of understanding the subsurface.

Forties Alpha Platform. From BP's Forties and forty commemorative volume.

Last year marked 50 years since the Forties field began oil production in the Central North Sea. It’s not only a milestone for the energy industry in the UK, but serves as a World-class example of the challenges of understanding the subsurface. This has made it an ideal case study to use to learn from and last Semester I transformed our University of Aberdeen third year 'Principles of Energy Geoscience' course to involve time travel to the early years of North Sea exploration and put the students in the driving seat to explore, drill and discover the Forties Field, giving them the tools they need to understand and de-risk geological subsurface data. Now as the world looks to decarbonise our energy system, we examined the role that depleted Fields such as Forties may play in carbon capture and storage and how knowledge of the subsurface can provide opportunities for geological energy storage solutions.

Students tracing 'Common Risk Segment' maps on the lab window to assess the distribution of reservoir, seal and source rocks in the North Sea subsurface. Photo shows the University of Aberdeen Science Teaching Hub.

History of the Forties Field

Where It Began: The Birth of a Giant

In September 1975, the first oil flowed from the Forties Field - officially opened that year by Her Late Majesty Queen Elizabeth II. But long before engineers and drilling rigs reached this milestone, the geologists had been hard at work understanding the rocks and fluids beneath the North Sea. The first methane gas discoveries were made in the Southern north Sea in the mid-1960s with the West Sole, Viking and Leman fields identified amongst others. But it was not until 1969 that the first oil was discovered. The Arbroath Field was the first UK oil discovery. Discovered by Amoco, this 102 mmbbl recoverable discovery targeted the deep water Forties sandstone of Eocene age in the Central North Sea and opened up a new hydrocarbon play or exploration activity. The giant Forties Field was discovered by BP the very next year in October 1970, about 110 miles (177 km) offshore from Aberdeen, Scotland.

Forties oil is pumped via pipeline to Hound Point, South Queensferry, for loading onto tanker and export around the world. View to the Forth Rail Bridge in the background (author's own).

The Geological Setting: Ancient Fans on the Sea Floor

The oil at Forties is found within the Eocene Forties Sandstone that was deposited around 55 million years ago. At that time, the North Sea was part of a deep basin receiving enormous volumes of sediment washed down from the then-uplifted Scottish Highlands. These sediments were transported into the deep basin by turbidity currents - high energy turbulent cascades of sand, mud and water that behave almost like underwater avalanches. These flows carve out channels and as they slowed, they spread out in a submarine fan pattern, building up thick layers of sandstone interbedded with mudstone. These sandstones make up the reservoir at Forties.

Not all rocks make good reservoirs. For that to happen, several geological conditions must be met:

• Porosity: The sandstone grains must pack in a way that leaves tiny spaces, or pores, between them. These spaces can hold oil.

• Permeability: The pores must be connected so oil can flow toward a well.

• Seal Rock: Above the sandstone, mudstones act like a lid, trapping the hydrocarbons and preventing them from escaping.

  
  

At Forties, the sandstones have excellent porosity and permeability, meaning they can hold and transmit oil efficiently.

A Geological Trap: The Anticline

Geology isn’t just about what rocks are made of — it’s also about how they’re shaped. In the Forties area, the sandstones were folded into a gentle dome-like structure called an anticline. Oil, being lighter than water, migrated upward through the pores in the rock during burial and was caught under the mudstone roof in this structural high. The mudstone above the Forties Sandstone seals the reservoir like a natural cap, forming a perfect geological combination.

The result of reservoir distribution and structural closure is a complex distribution of oil across the Field. The map below shows the distribution of channels and overbank deposits across the field structure (outlined in blue). Channels can clearly be seen on the seismic amplitude data. When compared to well data, it is clear that the best quality reservoir sands are contained within the channels, whereas overbank deposits show lower net-to-gross, porosity and permeability.

Maps across the Forties Field. Field outline in blue. Map on right shows the seismic amplitude at the top reservoir. Map on left shows the interpretation of seismic and well data and the clear distinction of deep water turbidite channels and overbank deposits. Adapted from Rose et al. (2020).

A Symbol of the North Sea Industry

50 years of production later and the Forties Field is the second largest oil field on the UK Continental Shelf (after Clair). At peak production in 1978, the field provided around a fifth of the UK's oil requirements alone with 500,000 barrels per day. Production was on a steady decline until BP sold the Field to Apache in 2003. Apache immediately revaluated the Field and unlocked a further 800 million barrels of oil. As a result, the Field's life has been significantly extended. Over 120 production and injection wells later and today it continues to produce 10,000 barrels per day. Apache plan to cease production in 2029 and move to an extensive decommissioning project to remove the six platforms and extensive sub-sea infrastructure that have brought these resources from the Forties reservoir to the forecourt.

What can we learn from Forties?

A World-class case study for subsurface uncertainty

It’s not been plain sailing for Forties with wide ranges of oil in place estimations through the life of the field as new data (or new operators) have come in. This makes it an excellent case study for teaching geoscience and engineering students the uncertainty involved in understanding the subsurface. In September 2025, exactly 50 years since the Forties started flowing oil, I took my class back in time to 1969 following the discovery of the Arbroath Field and posed the question- where do we drill next? This provided an opportunity to generate a gross depositional map to reconstruct what the North Sea looked like 55 million years ago when the Forties reservoir sandstone was laid down. These maps were generated using information from a small number of wells drilled in the area in 1969 and then used to assess the likely chance of encountering the various aspects of the petroleum system: reservoir; seal; and charge. The resulting ‘common risk segment’ maps allowed us to identify a fairway of high chance of success of finding a hydrocarbon accumulation.

Results of the play mapping exercise. Students used depositional environment maps to generate risk maps (common risk segment maps) for each of the aspects of a petroleum play (reservoir, seal and source). These were stacked to identify the region with the highest chance of success for finding il within the Forties Reservoir.

The next step was to calculate the potential value of oil within the Forties anticline using assumptions based on data at the nearby Arbroath Field, and assuming that the structure was filled to spill. Estimated inputs to to HCIIP equation (hydrocarbons initially in place) were used to estimate the volume of oil in the structure. Following class discussion on the hefty size of our potential profit(!), we decided to drill the Forties Discovery Well and refined out economic assessment based on the results from the well. The discovery well, and four subsequent appraisal wells, determined that the structure was not filled to spill, and that reservoir properties varied across the structure, but that the accumulation was still extremely profitable. These calculations led to discussion on how we can estimate subsurface properties with such limited information, and how we can capture and communicate uncertainty through applying ranges of possible outcomes.

50 years on and over 100 wells later, we have still only directly sampled less than 0.000006% of the Forties reservoir and the success of the Field relies on geoscientists and reservoir engineers being able to make predictions on how the reservoir will flow between wells where data is absent. communicating this uncertainty and risk is a key aspect of working with subsurface data.

Legacy of a Field

Since production began, the Forties Field has produced billions of barrels of oil, forming the backbone of the UK’s offshore oil industry and supporting decades of economic activity. Its geology continues to be studied because it’s a classic example of deep-marine turbidite reservoirs — one of the world’s major types of petroleum reservoirs, and now similar deports are being assessed for their potential for use as carbon capture and sequestration stores.

The course concludes with a theoretical exercise assessing the viability of the Forties Field to be repurposed for carbon capture and storage. Despite challenges in CO2 supply, the nearby Acorn Project has identified the potential to use similar reservoir sands as a CO2 store at their East Mey extension. Here, Paleocene Lista Sandstones were deposited but the same turbidity processes as the Eocene Forties Sandstone. The Mey site is waters filled with no oil or gas making it an attractive store, however well penetrations are sparse making our understanding of how injected CO2 would flow uncertain. Learnings from data-rich sites like the Forties Field can help reduce risk in proposed CCS store sites.

Map of the proposed Acorn CCS project. From theacornproject.uk

Fifty years on from first oil, the Forties Field remains far more than a production milestone in the UK’s energy history. It is a living laboratory for understanding the challenges of subsurface interpretation, the value of data integration, and the importance of embracing uncertainty in geoscience decision-making. From its discovery in the early days of North Sea exploration to its mature phase as a data-rich producing field, Forties demonstrates how geological insight underpins both commercial success and technological innovation. As the UK transitions toward a lower-carbon energy system, the lessons learned from Forties — in reservoir characterisation, risk communication, and long-term subsurface management — are directly transferable to emerging applications such as carbon capture and storage. In this way, Forties’ legacy is not only defined by the oil it has produced, but by the knowledge it continues to provide for the next generation of geoscientists and the future of subsurface energy solutions.

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