A little-known deal related to an equally little-known yet highly productive oilfield has come to light as a graft investigation unfolds at oil giant China National Petroleum Corp. (CNPC). A businessman with strong ties to officials is behind the deal, which generated hundreds of million in annual income for its backers, despite little investment.

Workers at CNPC Changqing Oilfield's Wangtai Block in Wuqi County, in the northwestern province of Shaanxi, have speculated the oilfield would be linked to graft since a probe started to rock the company last year.

Business registration documents show that Niandai Energy was registered in October 2006 in Urumqi, in the northwestern region of Xinjiang. With 50 million yuan in registered capital, the company operated business ranging from technology consulting for oil and gas development to coal mining and logistics.

Niandai Energy was controlled by Cao Yongzheng, but in 2012 control was given to his brother, Cao Yongping.