Norway's oil and gas sector faces another two years of falling investment as the industry's downturn lasts longer than expected, a lobby group forecast on Monday.

A 50 percent drop in crude prices to around 57 per barrel since mid-2014 has lead oil firms to tighten spending and postpone or cancel projects.

Investments in 2017 are expected to fall by seven percent to 143 billion Norwegian crowns (16.93 billion) from 154 billion crowns in 2016 and decline further to 131 billion in 2018, the Norwegian Oil and Gas Association estimated in a report.

In 2015, the oil and gas industry accounted for 15 percent of Norway's gross domestic product, for around 20 percent of the state's total revenues and 39 percent of Norwegian exports, according to the country's oil ministry.

Last year's report predicted that 2017 would be a turning point for renewed investment growth. However, a lower level of costs and less optimistic price expectations mean the present view is that capital spending will level off over the next few years, the Oil and Gas Association said.