"The Canadian government approval is significant in that it shows an understanding of the importance of being able to attract capital in the industry," said Zahary of Sunshine Oilsands.

Peter Harder, president of the Canada China Business Council, which focuses on facilitating bilateral trade and investment, said CNOOC's investment signals the "maturing" of the relationship of the two countries. "This transaction demonstrates a high level of bilateral economic cooperation and speaks to the important role of foreign investment in Canada's energy strategy and the development of Alberta's oil sands," said Harder.

The examination of the acquisition by CFIUS, the US committee that reviews sensitive deals by foreign investors, had sparked speculation among industry experts of a possible denial, but the committee approved it last month.

Many related it to CNOOC's failed attempt for an $18.5 billion bid for California-based energy firm Unocal Corp.

"CFIUS views every transaction separately from a national security perspective and it would not be reasonable to extrapolate from this clearance that another attempt by CNOOC to acquire Unocal (which failed in 2005) would now be approved," said Carl Valenstein, a Washington-based partner with the law firm Bingham McCutchen LLP.

"We are definitely seeing increased Chinese investment in the US, but Chinese investments in critical infrastructure and technology (telecommunications and energy in particular) will continue to be carefully scrutinized by CFIUS and other US agencies," Valenstein added.

Actions defended

CNOOC's Li defended the company's investment - and those by China's State-owned enterprises - early this month at a global energy conference in Houston.

"We are supposedly doing this because we want to haul every barrel of oil back to China, or have other agendas rather than commercial reasons," the CEO said in his first public speech since signing the Nexen deal. "Everyone with knowledge of the global market system can easily figure out that is not true, simply because this notion does not make any commercial sense."