Tonen Chemical, an arm of Exxon Mobil's Japan unit, shut its 491,000 ton-per-year naphtha cracker in Kawasaki, south of Tokyo, from around noon on Thursday due to problems, an industry report said.      

Japanese oil research firm RIM Intelligence Co also said the cracker may not be able to restart for at least a week.      

A spokeswoman at TonenGeneral Sekiyu KK, a subsidiary of Exxon Mobil, declined to comment. Tonen Chemical is a petrochemical unit of TonenGeneral Sekiyu.