(Adds details) KHOBAR, Saudi Arabia, Jan 15 (Reuters) - A major new joint-venture refinery in Saudi Arabia on Thursday shipped its first clean diesel cargo, the company Yanbu Aramco Si**pec Refining Co (Yasref) said. The start-up of the refinery is expected to weigh on diesel prices as the rise in supply will far outweigh demand, which has been sluggish because of weak eco**mic growth, traders said. The 400,000 barrels-per-day (bpd) refinery, a joint venture between Saudi Aramco and China's Si**pec, started trial runs in September and had originally planned its first exports by **vember. Yasref is the second refinery to start up in Saudi Arabia in the past two years and will complete state company Saudi Aramco's transformation into a leading exporter of diesel.