Oil explorers are neglecting their old hunting grounds in the North Sea, threatening to deepen an expected slump in Norway's output in the middle of the next decade, the country's industry regulator warned.

Norway's oil and gas production, which peaked in 2004, is already forecast to fall again after 2025 as old fields decline. But that estimate includes resources yet to be found, meaning without new discoveries it could be even lower. This year's record drilling campaign in frontier areas of Norway's Arctic yielded disappointing results. It also masked a greater threat: exploration in the North Sea, the heart of the country's offshore industry, fell to an 11-year low.