China's proposed tariffs on United States petroleum imports, part of a mounting trade war between the two countries, would crimp sales to the shale industry's largest customer, adding new pressure on U.S. crude prices, energy executives and analysts said in interviews this week. China has said it would slap a 25 percent tariff on imports of U.S. crude, natural gas and coal on July 6 if Washington went ahead, as planned, with its own tariffs on Chinese goods that day.