CNOOC's headquarters in Beijing, China. Reuters

Years after being humiliatingly rebuffed in a lucrative offer for Unocal, Chinese state-owned energy corporations are again ready to take a major bite out of the North American energy market. This time, their chances look much brighter.

In the largest foreign investment deal ever assembled by a Chinese company, the China National Offshore Oil Corporation or CNOOC (NYSE: CEO) announced Monday that it would purchase Nexen Inc. Toronto: NXY), a Canadian oil and gas company, for $15.1 billion.

A tougher North American financing environment, growing acceptance of Chinese capital internationally, and smoother relations between Canada and China in recent years make the deal more likely to go forward than CNOOC's failed $18.5 billion offer for Unocal in 2005.

CNOOC's acquisition of Nexen would provide it lucrative holdings of undeveloped but highly valued plots for shale gas and oil sands (300,000 acres of the former in northern British Columbia, and 650,000 acres of the latter in Alberta's Athabasca oil region). Industry analysts believe Nexen's reserves, the equivalent of some 900 million barrels of oil in proven and 1.122 billion in probable reserves, would be enough to increase CNOOC's total reserves by some 30 percent and its total output by 20 percent.

Nexen's holdings in British Columbia for shale gas are thought to be particularly seductive. CNOOC's public release of its proposed arrangement agreement with Nexen on Monday said the area could hold enough natural gas to double the Canadian company's proven reserves.

While most of CNOOC's crude oil production is located in the Bohai Gulf region of northeastern China, much of its natural gas is taken from overseas and western portions of the South China Sea. The company produced 79.8 million barrels of oil equivalent in the first quarter of 2012, a full 5.6 million barrels less than in the same period of 2011. It had $75.5 billion in revenue in 2011 and made $8.8 billion in profits, ranking 101st on the Global Fortune 500. CNOOC had $7.8 billion in revenues in first quarter of 2012, compared with $7.2 billion over the same period in 2011.

Nexen largely produces oil and gas from offshore facilities outside Canada, where it has only 28 percent of its assets. Nexen overseas operations are concentrated in the British North Sea (where it is the second largest producer), as well as the Gulf of Mexico and Nigerian coast -- a good fit for CNOOCs offshore focus.

Shale gas projects and oil sand extraction could also give China more access to technologies in a growing field where it has little know-how, but which has already helped to decrease gas prices in the U.S. and create heady predictions of an energy revolution there.

CNOOC has promised to keep Nexen's management and employees unchanged and will make an additional listing on the Toronto Stock Exchange. Calgary would remain the head office of Nexen's operations in the Americas. In addition, CNOOC is offering $27.50 per share, though prices late last week still only equated to around $17 (news of the deal pushed Nexen's share price up nearly 52 percent on Monday to almost $26).

While publications like China Daily say that CNOOC will absorb Nexen's $4.3 billion in debt, documents released by CNOOC itself on Monday said that amount would remain outstanding.

The deal comes after Ottawa has vigorously courted Chinese investment to Canada, eager to attract new capital in order to offset global financial instability. Chinese companies have invested $12.8 billion in Canada since 2009, and more than half of their overseas investments last year ended up in America's northern neighbor.

The Canadian Broadcasting Corporation estimates that roughly one-fifth of the Canadian oil sands have already been purchased by Chinese state-owned companies, though it did not specify whether that fraction refers to geographic area or productive value. Canada's oil sands are operated by numerous smaller companies, leaving less worry that a single foreign company could take over the area at a stroke, but perhaps also making it easier for larger Chinese state-owned companies to enter and buy out smaller companies piecemeal.

In fact, the CNOOC-Nexen deal is not a shot out of the blue. Both entities are already extremely well connected. When CNOOC purchased OPTI Canada last year for $2.1 billion, it became partners with Nexen at Long Lake in the oil sands region. In late 2011, a joint venture deal gave CNOOC stakes in Nexen drilling operations in the Gulf of Mexico.

Although both companies have now agreed to the deal, it still awaits government approval in Canada and the U.S.

CNOOC CEO Li Fanrong told the Wall Street Journal on Monday that Canada is one of the best destinations for [Chinese] overseas investment. He added that CNOCC would actively work to promote approvals from governments of Canada, U.S., and China.

Canadian rejection of the deal, especially after the government endorsement of Chinese investment, is thought to be unlikely. Prime Minister Stephen Harper made a name for himself in previous years opposing the Chinese government on human rights issues and democratic reform, but has recently warmed to Beijing and more aggressively pushed for economic links and investment from China. Energy experts expect Canadian approval to put pressure on the Keystone Pipeline project, largely by raising fears in U.S. policymakers that China is increasingly influential over a major source of American oil.

Chinese rejection of the deal would be almost unthinkable.

But that leaves American opposition to the lease holdings Nexen has in the Gulf of Mexico. American officials have approved CNOOC deals in Wyoming and Texas (where it has 33.3 percent stakes in Chesapeake Energy's oil and shale leasings) as well as exploration blocks in Alaska. Even in an election year with two candidates eager to bash China, energy experts are doubting the Nexen deal will meet strong opposition in the U.S.

The last deal of comparable size made by a Chinese company occurred in 2008, when the China Aluminum Corp. took a 12 percent stake in Rio Tinto for $14.3 billion.

CNOOC has captured headlines over the past year because of a number of other geopolitically and environmentally sensitive issues. In June 2011, an oilfield owned by CNOOC, but operated by ConocoPhillips, in the Bohai Gulf gained notoriety for a 140,000 gallon oil and drilling mud leak (a small incident, since by comparison the BP oil disaster in 2010 in the Gulf of Mexico leaked 200 million gallons). ConocoPhillips and CNOOC eventually reached an agreement to pay the Chinese government $160 million in January in a deal largely dismissed by industry insiders as a means to place blame on the foreign company. ConocoPhillips agreed in April to offer a further $191 million for environmental rehabilitation projects for the area.

In late June, CNOOC angered Vietnam by offering to auction off oil blocks to international bidding in the South China Sea. The blocks, close to the Vietnamese shore and within its claimed exclusive economic zone, are nevertheless within China's own maritime sovereignty claims on the South China Sea.

CNOOC's major overseas projects in 2012 also included the Missan oilfield, in southern Iraq, near the border with Iran.