A World Class Discovery

Author Halfdan Carstens

The first oil was sampled in September last year. It took several wells and one full year of detailed analyses to prove that the discovery may contain more than 2.6 billion barrels of oil.

The cores smell beautiful. They are saturated with black oil in an excellent reservoir. Porosities are between 25 and 30 percent and the permeability is several darcy. Statoil’s Sigrid Borthen Toven (North Sea Exploration Manager) and Silje Fekjar Nilsson (Team Leader Utsira High) have both been involved in the drilling of Aldous Major South that proved at least 800 MMbo. Source: Halfdan Carstens

A World Class Discovery

The discovery was announced last year. Oil had been found at about 2,000m in the Upper Jurassic Draupne Formation in licence 501. The operator, Lundin Norway, said the reservoir in Avaldsnes probably contained between 100 and 400 million barrels of recoverable liquids. In other words, it could be huge, by Norwegian standards. And it was of course good news, not only for Lundin and its partners, Statoil and Maersk, but for the entire Norwegian petroleum community who were in desperate need of encouragement following many disappointments in the last decade.  

Nevertheless, most petroleum geologists working the North Sea were taken by surprise, for at least two reasons. Firstly, the presence of a decent trap was not at all obvious from the seismic data, and secondly, and possibly more importantly, the closest mature source rock is in the deep Viking Graben, down below the Utsira High, a basement ridge separating the Viking Graben from the Stord Basin, the latter having only immature source rocks.  

”How could oil possibly migrate through basement rocks, or, also quite strange, could the oil have gone the long way all around the high?” Those were the questions asked by experts who had a long time ago given up on this area.  
 

The two wells 16/2-6 and 16/2-8 proved that Avaldsnes and Aldous Major South, respectively, belong to the same field. Aldous Major North, shown in yellow, is separated from the southern part by a major fault and does not contain a commercial discovery, as was proven in September by 16/2-9.

Good News – Then Bad News

Another crucial question was whether the discovery continued into the neighbouring licence to the west, operated by Statoil with Lundin as one of three partners. The seismic indicated two separate highs, one in each licence, but there was certainly no clue to whether the two were in pressure communication.  

The answer came last August. And for the second time this year petroleum exploration made front cover stories in Norwegian national newspapers – the first time was with the 250-500 MMbo discovery of Skrugard in the Barents Sea (GEO ExPro, Vol. 8, No. 3). The reason was simple. It had been proved that Aldous Major South was in communication with Avaldsnes, meaning that the reserve estimate for the two fields had grown to between 800 and 1,200 MMbo. If the northern continuation, Aldous Major North, was also flush with oil, the accumulation could possibly be as large as 1.5 Bb of recoverable oil.  

Well, it only took some six weeks to disprove the latter possibility. The Statoil well on Aldous Major North was quite disappointing, leaving a lot of question marks, but certainly not an increased reserve figure for the entire Aldous structure. All the same, the combined Avaldsnes/Aldous Major South was still huge and it is still the largest discovery in Norway since the 1980s.  

Everybody’s eyes were now on Lundin who in the meantime had drilled two appraisal wells on Avaldsnes. The company had promised an updated reserve estimate during autumn following interpretation of the well results. On the last day in September, in the early morning, the Swedish company, with a largely Norwegian crew, once again took the petroleum community by surprise. No fuss, however, just a modest press release stating a few numbers.  

”The Avaldsnes discovery is estimated to contain gross contingent resources of between 800 million and 1.8 billion barrels of recoverable oil,” it said.  
 

Second to None

That’s a lot of oil. In fact, if the high number is proved right, Avaldsnes will by itself rank as the sixth largest discovery on the Norwegian continental shelf, ever. But we have now already learned that Avaldsnes and Aldous Major South, despite being two separate structural highs, are connected with the same oil water contact, pressure regime, oil type and reservoir.  

Statoil as operator for Aldous Major South has announced an estimated recoverable resource range from 400 to 800 MMbo. The combined Avaldsnes/Aldous Major South discoveries therefore have a resource range estimate of 1.2 to 2.6 Bb of recoverable oil.  

Again, if the high number is correct, we are looking at the third largest discovery on the Norwegian continental shelf, only trailing Statfjord and Ekofisk, which each contained some 3.5 Bb when they were discovered in 1972 and 1969, respectively. Sceptics should note that the contingent resource range has been independently audited by Gaffney, Cline & Associates.  

Therefore, we are now talking about a world class discovery. So far it is the biggest find this year all over the globe. It is second to none!
 

Lateral Continuity

Hans Chr. Rønnevik, VP Exploration in Lundin Norway, has been given the credit for revisiting the greater Utsira High due east of the deep Viking Graben. Lundin surprised the petroleum community for the first time in 2007 when it made the Luno discovery in the midst of several dry wells. Source: Halfdan Carstens

The resource increase of Avaldnes is due to the positive results from appraisal wells as well as the Aldous Major South discovery well. All the wells have been cored through the reservoir section for direct reservoir properties analysis, age dating, log calibrations and analysis of the depositional system of the reservoir sands.  

The Avaldsnes discovery well 16/2-6, drilled last autumn, and first appraisal well 16/3-4, drilled this summer, were both successfully tested and proved the high lateral continuity and productivity of the main Upper Jurassic reservoir sandstones. The reservoir has confirmed excellent properties in all wildcats and appraisal wells. The appraisal wells have also significantly upgraded the thickness and properties of the main prime Volgian Upper Jurassic reservoir which has had a material impact upon the revised recoverable resources.  

Appraising the reservoir will continue to narrow the recoverable resource range and to assist with the development planning strategy. One well is planned to be drilled in the south-western part of the Avaldsnes discovery in the fourth quarter 2011, while three further appraisal wells will be drilled in 2012.  
 

A Mature Province

From an exploration viewpoint it is certainly worth noting that we are not talking about a frontier province. The Utsira High is in the middle of a mature petroleum province that has been explored since 1965.  

In fact, the area of the discovery was once part of licence 001, the very first licence offshore Norway. It was awarded to Esso Exploration, and the second well drilled in Norwegian waters, in 1966, resulted in an oil discovery on this acreage. It was later developed into the oil field Balder, that went on stream in 1999 producing from a 450 MMbo reservoir.  

Since then several discoveries have been made, with Grane due east of Balder as the largest with some 750 MMbo of recoverable oil.  

Both Balder and Grane are only 30 km north of the Aldous/Avaldsnes discovery. We also know that Norsk Hydro and Elf almost made it when, in 1976, they missed the structure by only 400 metres.  

In other words, the story behind Avaldsnes is a classic illustration how mature petroleum provinces may very well hide giant fields.  

The cores smell beautiful. They are saturated with black oil in an excellent reservoir. Porosities are between 25 and 30 percent and the permeability is several darcy. Statoil’s Sigrid Borthen Toven (North Sea Exploration Manager) and Silje Fekjar Nilsson (Team Leader Utsira High) have both been involved in the drilling of Aldous Major South that proved at least 800 MMbo.
 

The two highs, Aldous Major South and Avaldsnes, which were explored as two separate prospects with two separate wells, have proven to belong to the same accumulation with the same oil water contact, pressure regime, oil type and reservoir. So far the highest estimate for recoverable oil is 2.6 billion barrels. Source: Statoil