In line with Harper's sentiment, the ministry also said Friday that future decisions would be subject to strict new rules on investment in Canada by government-owned foreign companies. (A delay in drafting the revisions had been cited as the reason for holding up a decision on the Nexen acquisition.)

CNOOC has said it will absorb about $4.3 billion in Nexen's debt as part of the deal.

The Chinese company has for years signaled its desire to buy energy assets abroad. It attempted to acquire US-based Unocal Corp in 2005 for $18.5 billion but later abandoned that bid under pressure from political leaders in Washington citing national-security concerns.

In 2011, Nexen and Beijing-based CNOOC jointly invested $2 billion to develop oil sands on Alberta's Long Lake, a partnership that set the stage for the takeover. CNOOC's first investment in Canada was a 17 percent stake in oil sands developer MEG Energy Inc in 2005.

The Nexen deal raises CNOOC's proven reserves by 30 percent, the company has said. It currently has about nine years' worth of proven reserves based on current production.

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