TEHRAN, March 17( Xinhua)--The National Iranian Oil Company( NIOC) on Sunday signed contracts worth 13 billion U.S. dollars for the development of six oilfields with a number of domestic contractors, Shana News Agency affiliated with the Iranian Oil Ministry reported. The signing ceremony was held in the Iranian capital Tehran and attended by Iran's First Vice President Mohammad Mokhber, Oil Minister Javad Owji, and NIOC's Managing Director Mohsen Khojastehmehr, the report said. The contracts pe