The Norwegian Petroleum Directorate said Thursday there may be cost inhibitors to developing the remaining reserves on the Norwegian Continental Shelf.

"Cost trends in the petroleum industry pose a challenge, both for developing discoveries and for project commitments that can increase recovery from a field," NPD Director Jan Bygdevoll said in a statement.

NPD updated its reserve estimate for oil on the Norwegian Shelf, saying it believes reserves offshore could grow by 7.5 billion barrels over the next decade.

Bygdevoll said drilling costs have more than doubled and that could present a challenge for explorers looking to tap into Norwegian oil fields.

"This cost growth could threaten profitability of future projects," he said.

Nevertheless, NPD said it estimates oil production will remain stable. Less than 40 percent of the estimated reserves have yet to be formally discovered. More than 400 discoveries have been made since the first offshore find in 1967, and half of those are already in development.

So far, the Norwegian government estimates more than $500 billion have been invested in offshore developments.

Norway is the European leader in oil production. In January, the last full month for which data are available, production averaged about 1.6 million barrels per day.