Det norske, with a 20 percent stake in the giant Johan Sverdrup oil field, the biggest North Sea find in decades, reported an operating loss of 552 million crowns (50.2 million pounds) for the quarter against expectations for a 346 million crown loss and a 518 million crown loss a year ago.

Exploration costs in the quarter amounted to 426 million crowns, above an average forecast for 301 million crowns. Exploration costs rose more than threefold from the previous quarter and included costs of two dry wells.

The firm, which is controlled by billionaire Kjell Inge Roekke, said it expected to drill 5 to 7 exploration wells in the next twelve months.

On Oct 15 Det norske finalised its acquisition of Marathon Oil Corp's Norwegian business for $2.1 billion.

The deal transforms Det norske from an explorer with a heavy capital budget to a major producer with significant cashflow, but it also increases the firm's exposure to the oil price.

"Sensitivity to oil price fluctuations has increased after the acquisition of MONAS (Marathon Oil Norge), as demonstrated with the recent volatility in the oil market," the company said in a statement.

Shares in Det norske are down around 39 percent over the last three months, underperforming a 13.7 percent decline in the Thomson Reuters Global Oil & Gas Index.

The firm trades at 5.2 times forward earnings - a discount to the peer median of 10.7, Thomson Reuters data showed.

(Reporting by Stine Jacobsen; Editing by Balazs Koranyi)