By Siyi Liu and Chen Aizhu SINGAPORE, Dec 15 (Reuters) - The volume of Venezuelan oil already headed to China before the U.S. seized a Venezuelan tanker last week, plus a glut of crude in storage and weak demand, will limit ​the near-term impact of the move in the Chinese market, traders and analysts said. Exports from the South American producer have ‌fallen sharply since the U.S. seized a tanker off Venezuela's coast and imposed new sanctions on shipping firms and vessels doing business with it, with ...