By Michael Erman and David Ljunggren

NEW YORK/OTTAWA (Reuters) - Canadian authorities have approved the acquisition of Nexen Inc by China's CNOOC Ltd, a source familiar with the matter said on Friday.

The ruling follows months of debate over how much of Canada's energy sector should be controlled foreign oil companies. Investors have watched developments closely.

CNOOC has offered $15.1 billion to buy Nexen, which has Alberta oil sands assets and offshore operations in the North Sea, Gulf of Mexico and Nigeria.

Canada's Industry Ministry said it would hold a media briefing at 4 p.m. (2100 GMT) on Friday with information embargoed until 5 p.m. Prime Minister Stephen Harper will make a statement at 5:15 p.m, the government said.

Industry Canada did not reveal the topic for the announcement. The source spoke on condition of anonymity because the matter was not yet public.

CNOOC's takeover of Nexen was overwhelmingly approved by Nexen shareholders in September, but the government delayed approvals while it drafted a long-promised update to the rules governing investments by state-owned foreign companies.

It also had to deal with the qualms of some of its own members over whether companies from the communist country should be allowed to buy up Canadian energy assets.

($1=$0.99 Canadian)

(Additional reporting by Solarina Ho, Euan Rocha and Alastair Sharp in Toronto; Writing by Jeffrey Jones; Editing by Frank McGurty, Bernard Orr and Tim Dobbyn)