In addition, the Chinese company will gain high-quality shale gas blocks covering 300,000 acres in Canada, a hugely significant asset for a country which already owns largest shale gas reserves but which is thirsty for acquiring share gas exploration know-how.

However, Yang said this does not mean that CNOOC will shift its shale gas development focus from China to foreign countries.

"Our overall $4 billion investment in the overseas shale gas sector over the past year is mainly market and profit-driven," he added.

According to company figures, overseas reserves accounted for 29 percent of its total by the end of 2011 and 21 percent of total output was from outside China.

"Thus, overseas assets have become an important part of the company," said Yang.

Liao Na, the vice-president of Shanghai-based energy consultancy ICIS C1 Energy, said that the Nexen deal will help CNOOC further strengthen its dominant role in offshore oil exploration, but it's the North Sea content of the deal which is particularly significant in that it will lead to China gradually increasing its participation in the international oil-pricing market, given the importance and size of its new assets in the area.

However, it will not contribute much to the weakest part of CNOOC's operations, its downstream businesses, she added.

CNOOC is not the only Chinese company that now has interests in the North Sea region, off the Scottish northeast coast.

In December, China's largest refiner China Petrochemical Corp, or Sinopec Group, acquired a 49 percent stake in the UK subsidiary of the Canada-based Talisman Energy Inc for $1.5 billion, marking its first foray into region's oil and gas business.

Talisman's UK subsidiary is headquartered in Aberdeen, and has interests in 51 oil and gas fields in the region which hold probable reserves of 489 million barrels of oil equivalent, of which 95 percent is crude oil.

Lin Boqiang said that deal will help Sinopec gain valuable offshore oil exploration experience which will improve its technology levels as it continues to compete for further interests in China's offshore oil exploration market, currently dominated by CNOOC.

Yang Hua insists Sinopec presents little threat to CNOOC, given its lead in the country's technology, services, offshore oil exploration and production sectors.

As Chinese companies have continued to expand their overseas assets, there has also been widespread opposition, especially to the ambitions of its acquisitive State-owned companies.

In December, as regulators mulled over the Nexen deal, the Canadian government released an updated foreign investment framework that established new rules for takeovers of State-owned enterprises, which served notice that no foreign takeovers of Canadian oil companies would be permitted in the future, except in "exceptional" circumstances.

However, Liao from ICIS C1 Energy said that it is inevitable that Chinese oil companies will continue to buy more overseas assets.

A recent report from accounting and consulting firm Deloitte Touche Tohmatsu shows that China's appetite for foreign energy and resource assets remains strong.

In 2005, just 10 overseas energy and resources deals were announced by China.

In 2008, that number had grown to 34, and during the first nine months of 2012, 39 such transactions were announced, it said.

Over the past year, China's outbound mergers and acquisitions reached a record high as companies and investors continued to seek opportunities abroad, especially in the energy and resources sectors.

Data from Dealogic Inc released at the end of last year showed that China's total of 35 outbound M&As in the oil and gas industry had reached $34.5 billion in value.

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Previous reports

CNOOC-Nexen deal expects to close on Feb 25

CNOOC, Nexen deal wins US approval

NDRC approves CNOOC's acquisition of Nexen

Canada OKs CNOOC's purchase of Nexen

CNOOC, Nexen reapply for US deal approval

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