The giant Aldous Major South field is estimated to contain between 900 million and 1.5 billion barrels of recoverable oil equivalent (boe), or double the 400 to 800 million boe previously announced, Statoil said.
Along with the neighbouring Avaldsnes field, with which it is linked, it could be the third largest reserve of black gold ever found off Norway, containing between 1.7 and 3.3 billion boe.
"Aldous/Avaldsnes is a giant, and one of the largest finds ever on the Norwegian continental shelf," Tim Dodson, Statoil's executive vice president for exploration, said in a statement.
If the estimates are confirmed, Aldous/Avaldsnes would trail just behind Norway's biggest oil fields Statfjord, with 3.6 billion boe, and Ekofisk, with 3.4 billion boe.
Both of those fields were discovered some 40 years ago in the early days of Norway's oil era.
The discovery of Aldous Major South and Avaldsnes has reignited interest in the North Sea, where oil and gas production has declined sharply in recent years.
After hitting a peak in 2001, Norway's oil production has dwindled and the country is now the world's seventh-biggest oil exporter.
Last year, the Scandinavian country produced an average of 1.8 million barrels per day, just over half of what it did 10 years ago.
Aldous Major South is 40-percent held by Statoil, with Norwegian partners Petoro and Det norske oljesleskapet holding 30 and 20 percent respectively and Sweden's Lundin holding 10 percent.
Avaldsnes meanwhile is covered by a separate licence held 40 percent by Lundin, 40 percent by Statoil and 20 percent by Denmark's Maersk.