Three years after spending $15 billion on an ambitious bid to revitalize a troubled oil-sands project in a northern Canadian swampland, one of China’s largest state-controlled oil companies has run out of gas. “The deal has turned out to be a bit of a dud for them,” said Gordon Houlden, a China expert at the University of Alberta. The Nexen acquisition ranked as China’s biggest-ever overseas takeover, surpassed only recently by China National Chemical Corp.’s pending acquisition of Switzerland’s Syngenta SA. In buying Nexen, Cnooc aimed to use it as a beachhead in North America after the failure of a controversial 2005 bid for Unocal, now a unit of Chevron Corp. CVX 0.05 % “They went in with their eyes wide open thinking there were things they could do to improve Nexen,” said Greg Stringham, an industry consultant who until recently was in charge of oil sands at […]