As is so often the case, the discovery of the Forties Field, the first major oil field in the UK North Sea, was serendipitous. In late 1969, while BP geologists were still trying to understand the – to our eyes now, very primitive - seismic shot over their Central Graben acreage, their Sea Quest drilling rig became available. Since the Forties structure was the only one over which a site survey had been shot, it seemed better to drill there than leave the rig idle. In October 1970 well 21/10-1 hit a 119m high column of good quality oil - the Forties Field, with recoverable reserves estimated at the time to be 1.8 Bbo, had been found.
In early 1969, oil companies were becoming disillusioned with the prospects of finding oil in the North Sea. Nine wells had been drilled in the UK sector of the Central North Sea, but with little success. The early explorers had been surprised and excited when the first seismic lines across the Viking Graben, in the centre of the area, had shown significant Tertiary basins, over 3,000m deep, rather than the extension of the Palaeozoic fold belts of Scotland and Norway which they had expected. But they were disappointed as good Lower Tertiary reservoir sands proved illusive, while poor results from the Cretaceous meant a British Ekofisk – the first giant Norwegian field, which is reservoired in the chalk – seemed unlikely to be found.
In fact, as Myles Bowen from Shell is quoted as saying (Moreton, 1995) “in May 1969 the view was that all the worthwhile gas fields in the Southern North Sea had been found, while the search for oil in the north was doomed to failure.”
Then a small field, called Montrose at the time, but now known as Arbroath, was discovered in the middle of the Central North Sea, and although deemed only just commercial, prospects for North Sea oil looked up.
1st UK giant field
BP had obtained its acreage in Blocks 21/9 and 21/10, at the northern end of the Central Graben and about 180km east of Aberdeen, in the 2nd UK Licensing Round in 1965. As the BP interpreters continued reviewing the seismic, they realised that they could map a large Palaeocene four way dip closure. When this was tied to a nearby well with minor oil shows on Block 22/6, drilled a few months earlier by a Gulf/Shell consortium, they saw what looked like a large thickness of potentially oil bearing sands in the anticline.
And, in October 1970, that is exactly what they found. After a five well appraisal programme over 1970 and 1971, BP was able to announce that the Forties field, at a depth of 2,130m sub seabed, covered an area of 93 km2, much larger than initially estimated, and contained in-place reserves of 4.6 billion barrels of good quality, 37° API oil.
Interestingly, the major prize of the Forties field nearly belonged to another company, as Richard Hardman explains in the UK Oil and Gas fields Millennium Geological Society Memoir, in an article entitled ‘lessons from oil and gas exploration in and around Britain’. He points out that BP were not the only company to have identified an interesting structure on Blocks 21/9 and 21/10. “The BP geologists concerned with the prospect did not try to run economic cases as they felt their knowledge was too sketchy to be meaningful. There is no doubt that if economics had been the determinant, BP would never have drilled the Forties discovery. In contrast, it transpires that Shell geologists had correctly analysed the Forties anomaly as containing over one billion barrels, but they were so afraid that their management would think them ridiculous that when they were considering farming-in to the block with BP, they arbitrarily reduced the reserve calculation to ‘over 200 million barrels’, (not likely to be economic at the very low oil prices at the time), and management decided not to participate.”
Having found this giant field, BP then set about planning the first major oil field development in the OK sector of the North Sea, a task which took several years. Four fixed steel platforms were installed on Block 21/10 in 1974 and 1975, with a total capacity for over 100 wells, and a system capable of handling 600 Mbpd. Development drilling began in June 1975 and production started on 12thSeptember 1975, with the field, named Forties after the sea area in which it lies, officially inaugurated by Queen Elizabeth II in November 1975. A fifth platform was added on the south-eastern extension of the field in 1985, where production started in 1987. The oil is produced using peripheral water injection, enhanced by pressure from the underlying natural aquifer.
Sands draped over basement high
The main reservoir in the Forties field, and in a number of other major North Sea discoveries, is the Upper Paleocene, Late Thanetian Forties formation. Prior to the deposition of these sediments, the North Sea Basin was largely cut off from major oceanic currents, before the opening of the North Atlantic resulted in a rise in sea level. The Forties formation was deposited over a wide area at this time, 55 million years ago. It is a mixed sand and mud turbidite fan complex, which flowed south-eastwards, bringing sediments eroded from the uplifted Scottish Highlands to the north-west, forming a submarine delta trapped in what is now the Moray Firth.
The Forties Formation is usually split into two lithologically distinct sequences; an upper, predominantly sandy sequence, deposited as a submarine fan, which contains most of the oil found in the formation and a lower interbedded sandstone and shale sequence, deposited as feeder channels on the sea floor.
The actual reservoir at the Forties field consists of several stacked sandstone bodies, where the primary targets are the channel sandstones, which are separated by poorer quality thin interchannel facies. The average porosity of these reservoir sands, which have a gross thickness at the Forties field of 350m, is 27%, permeability is 400 mD and water saturation 23%.
As the turbidites flowed down into deeper water, they were deposited on top of the underlying structures, which in the Forties field formed a Jurassic-aged basement high, over which the sands were draped, thinning slightly at the apex of the feature. This formed the simple dome in which hydrocarbons, migrating upwards from the Jurassic Kimmeridge Clay buried deep within the underlying sediments, could be trapped. This rich mature source rock originated from kitchens to the south and north-east, where the main phase of expulsion started in the Middle Eocene, 50 million years ago, and continued to the present day. The field is structurally simple, with only limited faulting, post-dating major episodes of faulting in the North Sea.
The reservoir is sealed by the overlying conformable mudstones of the Sele Formation, deposited after a regional rise in sea level at the end of the Palaeocene.
Still going strong
When production from Forties commenced in 1975, it was predicted that the field would stop producing by the early 1990’s. In 1990, it was expected to be shut down by the year 2000. Yet now, forty years since it was first discovered, the Forties Field is still producing, the recoverable reserves have increased by 35% since the start of production, even though the area of the field as defined by the oil-water contact has remained approximately the same.
Plateau production of 500 Mbopd was reached in 1978, which lasted until 1981, declining to 77 Mbopd in 1999. At this point the field had already produced 2.5 Bbo and still had nearly 60 producing wells. With production down to 35 Mbopd, BP controversially sold the Forties field in 2003. It is such an iconic field for UK North Sea exploration that some commentators likened this to ‘selling off the family silver’.
The purchaser, Apache, initiated an intensive re-evaluation of the field and found a further 800 million barrels. By undertaking various efficiency measures (see GEO ExPro Vol 7, no 1, p. 8), and installing new equipment, it has brought new life to the field. Forties is now producing 70 Mbopd and is expected to be still pumping oil for the next twenty years.