The acquisition will increase CNOOC's oil production by about 20 percent, which was between 341 million and 343 million barrels oil equivalent in 2012, and reserves by 30 percent, said CEO Li.

"The move certainly helps CNOOC become more geographically diversified," said John Licata, founder and chief energy strategist at New York-based Blue Phoenix Inc, an independent energy-research company.

"But now that the company gains a footprint in new areas, management needs to explore how best to build upon existing relationships to further enhance and foster even more business opportunities in these regions."

"CNOOC has the opportunity to roll up its sleeves and maximize Nexen's ability to break down heavy oil through technological advancement," he said.

"CNOOC needs to beef up exposure to clean fuels, especially since coal is still rather dominant in China, so while the Nexen deal makes sense, the company needs to not make the same mistakes as the US and become too focused on one or two energy sources."

The largest overseas acquisition by a Chinese company was first announced in July. Canadian regulators approved it in December.

In February, the Committee on Foreign Investment in the United States (CFIUS), which reviews sensitive deals by foreign investors in the country, signed off on the deal's part involving Nexen's assets in the Gulf of Mexico.

"The one-year acquisition process was filled with hardships," CNOOC Chairman Wang recently told the Shanghai Securities News.