Chinese energy company Nexen, a unit of CNOOC, is preparing to pull out of the United States amid rising trade tensions, Reuters reports , citing three unnamed sources with knowledge of the company’s plans.Canada-based Nexen became part of CNOOC back in 2013, with the Chinese company paying over US$15 billion. The acquisition gave it a presence in the Gulf of Mexico, including a 25-percent holding in the Stampede field, operated by Hess Corp. and 21 percent in Shell’s Appomattox field. Stampede has recoverable reserves estimated at 300-350 million barrels . . .