China's state-controlled oil producers are targeting struggling energy companies and projects world-wide, opening up production that might not otherwise happen.

In the latest deal, Cnooc Ltd. last week said it would buy one of Canada's largest oil producers for $15.1 billion. If the acquisition of Nexen Inc. passes regulatory hurdles in Washington and Ottawa, it would join more than $50 billion in overseas oil-and-gas deals completed by Chinese companies since 2009, according to data provided by securities firm Jefferies Hong Kong Ltd. That is in addition to tens of billions of dollars in loan-for-oil programs in Venezuela, Brazil, Kazakhstan and elsewhere.

Including Nexen's output, a conservative tally of publicly available data on foreign production for major Chinese oil companies would be in the neighborhood of the total output of Norway, the world's No. 15 crude-oil producer.

In many cases, China's oil producers have targeted struggling and underfunded oil-and-gas operations abroad, helping to support supplies amid tight credit markets and a natural-gas glut in North America. Nexen Chief Executive Marvin Romanow resigned in January amid production-related setbacks around the world, including at the company's Long Lake oil-sands project in Canada.

Analysts said Chinese capital investment has helped put downward pressure on energy prices even as demand from China and other parts of the developing world skyrockets.

'Projects are being developed that wouldn't be if China wasn't in the game,' said Laban Yu, head of oil-and-gas research at Jefferies.

Mr. Yu cited China Petrochemical Corp.'s recent $2.5 billion investment in five U.S. shale blocks owned by Oklahoma-based Devon Energy Corp. The Chinese company, known as Sinopec, owns just a 33% stake in the blocks but is responsible for funding about 80% of the exploration activity.

In other cases, Chinese capital has helped boost output of existing projects. Sinopec said in May that its Addax Petroleum unit, acquired in 2009 for $7.2 billion, had boosted crude-oil output to 8.4 million metric tons from 6.8 million tons in 2009. At the time of the takeover, Geneva-based Addax's crude output was declining because of low reserves and a lack of drilling rigs, the company said.

Sinopec in 2010 acquired 40% of the Brazilian assets of Spain's Repsol YPF SA for $7.1 billion. The deal provided funding for the Spanish company's oil-and-gas exploration and production off Brazil. Since then, the companies have announced discoveries in the Campos, Espírito Santo and Santos basins off shore.

Chinese investment also has spurred supply in regions where few companies had dared tread. The country has emerged as a competitor in Sudan's oil industry despite conflict there.

The world's oil-supply growth is expected to keep pace with demand next year, the International Energy Agency has said, largely because of new supplies from Canadian oil sands, U.S. shale oil and Brazil's Campos and Santos basins─all of which China is heavily involved in.

Cnooc and other Chinese oil companies are under government orders to make their operations more international. Beijing is concerned that domestic oil and gas production is stagnating and that initiatives to develop unconventional reserves like shale gas in western China and deep-water reserves in the South China Sea remain years away. The government also hopes to reduce its dependence on the Middle East.

Overseas output has compensated for a domestic lag. Sinopec has said it plans to double its share of oil output from overseas projects to more than 50 million metric tons by 2015 from 22.9 million tons last year. PetroChina Co., the listed arm of China National Petroleum Corp., the country's largest oil and gas producer, has said it would spend $60 billion this decade to boost overseas oil production.

PetroChina, whose combined overseas and domestic crude-oil output surpassed that of Exxon Mobil Corp. last year, has said its overseas oil-and-gas production more than doubled since 2007 to 120.8 million barrels last year. The figures don't include output from politically sensitive countries like Iran and Sudan, where operations are controlled by state-controlled CNPC.

Cnooc's acquisition of Nexen would mark an uptick in Chinese involvement in North America, where China has been relegated mostly to serving as a financial backer to U.S. and international oil companies. Nexen holds sizable assets in the Gulf of Mexico, and a successful acquisition would further open the door for Chinese companies to operate in North American oil and gas fields alongside established energy giants like Exxon Mobil.

Analysts have said that acquiring U.S. assets by way of a third country is a less politically dicey way to make U.S. inroads than buying American companies outright.

'Gaining an operating role in the United States via the acquisition of a Canadian company is a smart approach,' said Erica Downs, who analyzes Chinese energy policy at the Brookings Institution in Washington. 'If Cnooc was attempting to buy an American energy company for $15 billion, I would be much less optimistic about the transaction getting a green light.'

美国布鲁金斯学会(Brookings Institution)研究中国能源政策的专家邓丽嘉(Erica Downs)说，通过收购加拿大公司来获得一个参与美国石油业务的角色是聪明的做法。她说，如果中海油试图直接以150亿美元收购一家美国能源公司，那我对于该交易会获得绿灯放行就远没有这么乐观了。