It's expected to take around 40 days to tap into a Barents Sea prospect estimated to hold at least 200 million barrels of oil equivalent, Lundin Petroleum said.

The Norwegian subsidiary of the Swedish energy company said it started drilling an exploration well in the Neiden prospect, located in the southern waters of the Barents Sea.

The company said drilling should take approximately 40 days. The reserve is estimated to contain about 204 million barrels of oil equivalent.

Lundin made a discovery in the Barents Sea in the Alta prospect, about 100 miles from the Norwegian coast, in June.

The company estimated the discovery contained between 125 million and 400 million barrels of oil equivalents. The Norwegian Petroleum Directorate, the regulator that confirmed the discovery, placed the reserve estimate at between 85 million and 315 million barrels of recoverable oil and 175 billion and 600 billion cubic feet of recoverable natural gas.

Luding revised its 2015 production guidance lower to 32,000 barrels of oil equivalents per day as a result of less than expected output from offshore Norway. Revenue for the first half of the year was $279 million, down nearly 40 percent year-on-year.