The blaze had been controlled and there was currently no secondary environmental or security risk.
The explosion occurred at around 1:50 p.m. at the Jinlingshihua Nanjing Refinery, a subsidiary of Sinopec Group, the official Xinhua news agency said, adding that no casualties had yet been reported.
Xinhua said fire fighters had been dispatched to battle the flames and pictures carried by state media showed thick black smoke rising into the sky above an industrial complex.
The blaze had been controlled and there was not currently a secondary environmental or security risk, the company said on its official microblog.
Deadly accidents are relatively common at industrial plants in China, and anger over lax standards is growing after three decades of swift economic growth marred by incidents from mining disasters to factory fires.
China has vowed to improve safety at such facilities. President Xi Jinping has said authorities would learn the lessons paid for with blood after chemical blasts in the port city of Tianjin on Aug. 12 last year killed more than 170 people.