Nexen Inc said on Friday that shareholders would vote next month on whether to approve the $15.1 billion friendly offer for the Canadian oil producer made last month by China's CNOOC Ltd after more than two months of negotiations.

Nexen said in a statement it would hold a special meeting of shareholders on September 20 to vote on the richest foreign takeover ever by a Chinese company, if completed.

The move is the most ambitious foray by resource-hungry China into North American energy since a 2005 attempt to buy US-based Unocal for $18.5 billion was thwarted by a political backlash in the United States.

Chinese companies have been among the most aggressive in targeting assets around the globe to help feed demand in the world's second-biggest economy.

According to details included in a proxy circular filed with regulators, CNOOC's July 23 offer of $27.50 per Nexen share came after the Canadian firm rejected two earlier bids.

Nexen said in its circular it first became aware in February that a Chinese state-controlled oil company might be interested in making a acquisition offer for the company. In April, prior to any offer being made, it retained Goldman Sachs and RBC Capital Markets as financial advisers to review possible responses to any bid.

CNOOC made its initial approach to Nexen while that review was taking place. On May 17, Barry Jackson, Nexen's chairman, and acting Chief Executive Kevin Reinhart met with CNOOC's vice-chairman Yang Hua and the head of its international operation in Vancouver, when the Chinese firm made its first, unspecified offer for Nexen.

That bid was rejected by Nexen's board, as was a second offer, again unspecified in the circular, made on July 3.

"The Board determined that it would not support a transaction with CNOOC at the price specified in CNOOC's July 3, 2012 letter but would consider supporting a transaction if a higher price was offered," Nexen said in its proxy.

It took another meeting by Jackson and Yang, held in London on July 10, to come up with CNOOC's final offer. That $27.50 per share bid, a 61 percent premium to Nexen's share price, was approved by the board on July 22.

Along with shareholder approval, the takeover must also be approved by the Chinese, Canadian and US governments.

Nexen shares were down 1 Canadian cent to C$25.53 in mid-afternoon Toronto Stock Exchange trading.