Billionaire Zhang Zhirong is at the centre of a probe involving China's biggest overseas acquisition. A company controlled by the 43-year-old mainland-born entrepreneur who founded Hong Kong-listed shipbuilder China Rongsheng Heavy Industries is the subject of an insider-trading complaint filed by the US Securities and Exchange Commission. The SEC obtained an emergency US court order on Friday to freeze the assets of several share traders, including Zhang, for alleged illegal trading in connection with a US$15.1 billion takeover offer by state-owned China National Overseas Oil Corporation (CNOOC) for Canadian energy company Nexen. The SEC alleged that Rongsheng has a 'strategic co-operation agreement' in terms of doing business with CNOOC. Besides Rongsheng, which Zhang chairs as a non-executive director, he is founding chairman of Glorious Property Holdings, a Hong Kong-listed property developer. Forbes estimated Zhang's wealth at US$2.6 billion as of March. Zhang could not immediately be reached for comment, but a Rongsheng spokesman said yesterday: 'As the filing involves the private affairs of one of the non-executive directors, it is not appropriate for Rongsheng to make further comment. Normal operations of the group will not be affected.' Zhang owns a controlling 47.75 per cent stake in Rongsheng, and 68.39 per cent of Glorious Property, according to the Hong Kong stock exchange website. On May 15, Rongsheng, with CNOOC, completed China's first 3,000-metre deepwater pipe-laying crane vessel, according to Rongsheng's website. Last October, Rongsheng signed a contract with Shanghai Northsea Shipping, where CNOOC is a shareholder, to build a shuttle tanker, the website said. 'The company will take part in the [Chinese] government's South China Sea Exploration Strategy, and provide state-owned oil majors like CNOOC with equipment,' it said. In a press statement, the US securities watchdog alleged that Well Advantage, a British Virgin Islands firm wholly owned by Zhang, along with some unknown traders stockpiled shares of Nexen based on confidential information before the deal became public on July 23. That day, CNOOC announced it would bid for Nexen, which is traded on the New York Stock Exchange. 'Well Advantage and these other traders engaged in an all-too-familiar pattern of misusing inside information to place extremely timely trades and profit handsomely from their illegal acts,' Sanjay Wadhwa, deputy chief of the SEC Enforcement Division's Market Abuse Unit, said in the statement. Well Advantage bought more than 830,000 US shares of Nexen on July 19 and had an unrealised trading profit of more than US$7 million based on Nexen's closing price on the day of the announcement, the SEC alleged. The unknown traders used accounts in Singapore to buy more than 676,000 Nexen shares in the days preceding the announcement, and sold nearly all the stock once the announcement was made on July 23 for 'illicit profits' of US$6 million, the SEC said.