The state oil giant's move in Canada this time also triggered debates on whether Canadian oil and gas reserves should fall under foreign state control.
Investors and business people supported the deal, which they said will benefit Canada's oil and gas industry by giving it the much-needed capital.
The acquisition proposal was approved by a majority of Nexen shareholders in September, nearly two months after it was brought forward by CNOOC.
However, even after the purchase, CNOOC may still encounter political troubles in Europe for Nexen's assets there, said Shen Yan, general manager of merger and acquisition at CCID Consulting Co., Ltd..
Moreover, CNOOC has somewhat overbid for the deal and has to take over 4.3 billion U.S. dollars of debts from Nexen, making profitability a key aspect to improve after the transaction, Shen said.
Spectacular images of erupting volcanoes