UPDATE 1-Nexen takeover boosts China CNOOC's Q3 oil output 18 pct
* CNOOC says Q3 output up 17.8 pct
* Increase largely due to contribution from Nexen, which CNOOC acquired in Feb
* CNOOC says 2013 production target remains unchanged (Adds comments from chief financial officer, details, context)
HONG KONG, Oct 24 (Reuters) - China's top offshore oil explorer CNOOC Ltd said on Thursday its production rose 17.8 percent in the third quarter, boosted by output from Canadian energy firm Nexen which it acquired for $15.1 billion earlier this year.
CNOOC, once an investor darling for its high-growth profile, has been struggling to boost its own production over the past few years as domestic fields age.
The company has invested in advanced technology to drill in deep-sea areas off the Chinese coast and expanded into unconventional energy such as oil sands and shale in North America.
It also acquired assets including Nexen, the biggest ever overseas corporate takeover by a Chinese company.
CNOOC said in a stock exchange filing that production jumped to 1.034 million barrels of oil equivalents (boe) in the quarter from 87.8 million boe a year earlier. Excluding Nexen's contribution of 16.1 million boe, CNOOC's output for the period was down slightly.
CNOOC's 338-348 million boe production target for this year remained unchanged, Chief Financial Officer Zhong Hua said.
"We are able to meet the target," he told reporters. The figure, excluding Nexen's contribution, would amount to just a 2 percent increase from 2012's output.
Keen to boost overseas acquisitions, CNOOC joined an international consortium which earlier this month won the right to explore and develop Brazil's biggest oil field, Libra.
Zhong said CNOOC would pay about $700 million for its share in the project, but declined to give further details.
CNOOC and China National Petroleum Corp, parent of the country's dominant oil and gas producer PetroChina , will have a 10 percent interest in the consortium, which also includes Brazil's state-run oil firm Petrobras, France's Total SA and Anglo-Dutch Royal Dutch Shell Plc.
CNOOC said its unaudited revenue hit 57.35 billion yuan in the July-September period, up 17 percent year-on-year as its average oil selling price in the third quarter increased 1.5 percent to $106.26 per barrel.
Capital expenditure for the third quarter, excluding Nexen, climbed 18.2 percent to 17.7 billion yuan as the company kicked off more oil and gas development projects. Nexen's capital expenditure reached 4.7 billion yuan in the period, it said.
Average realised natural gas prices fell 6.9 percent to $5.43 per thousand cubic feet. Zhong attributed the drop to the lower prices of gas produced by Nexen.
(Reporting by Charlie Zhu; Editing by Miral Fahmy)