from Lower to Middle Jurassic sandstone formations within a fault block. The Snøhvit Gas Field and the Draugen oil field were discovered in 1984. The Heidrun oil field was discovered in 1985. The largest UK field discovered in the past twenty-five years is Buzzard, also located off Scotland, found in June 2001 with producible reserves of almost 64×106 m³ (400m bbl) and an average output of 28,600 m3 to 30,200 m3 (180,000–220,000 bbl) per day. The largest field found in the past five years on the Norwegian part of the North Sea, is the Johan Sverdrup oil field which was discovered in 2010, with further oil of the same field was discovered the next year. Total reserves of the field are estimated at 1.7 to 3.3 billion barrels of gross recoverable oil and Johan Sverdrup is expected to produce 120,000 to 200,000 barrels of oil per day. Production started 5 October 2019. It is one of the largest discoveries made in the Norwegian Continental Shelf. , the North Sea was the world's most active offshore drilling region with 173 active rigs drilling. By May 2016, the North Sea oil and gas industry was financially stressed by the reduced oil prices, and called for government support. The distances, number of workplaces and fierce weather push the 750,000 square kilometre (290,000 square mile) North Sea area to operate the world's largest fleet of heavy instrument flight rules (IFR) helicopters, some specifically developed for the North Sea. They carry about two million passengers per year from sixteen onshore bases, of which Aberdeen Airport is the world's busiest with 500,000 passengers per year. Licensing Following the 1958 Convention on the Continental Shelf and after some disputes on the rights to natural resource exploitation the national limits of the exclusive economic zones were ratified. Five countries are involved in oil production in the North Sea. All operate a tax and royalty licensing regime. The respective sectors are divided by median lines agreed in the late 1960s: Norway – Oljedirektoratet (the Norwegian Petroleum Directorate grants licences. The NCS is also divided into quads of 1 degree by 1 degree. Norwegian licence blocks are larger than British blocks, being 15 minutes of latitude by 20 minutes of longitude (12 blocks in a quad). Like in Britain, there are numerous part blocks formed by re-licensing relinquished areas. United Kingdom – Exploration and production licences are regulated by the