The pipeline ruptured and leaked for about 15 minutes onto a street and into the sea before it was shut off. Hours later, as workers cleaned up the spill, the oil caught fire and exploded in two locations, the city government said.
Sinopec's expansion of petrochemical projects has met with resistance from members of the public, and Friday's blasts will likely add to growing concern about safety and environmental risks.
Sinopec apologized Saturday for the explosions, which ripped slabs of pavement and overturned vehicles. It said it would investigate and "give timely reports."
About 18,000 residents were evacuated following the explosions, according to the official Xinhua News Agency.
It was China's second-deadliest industrial accident of the year, after a chicken factory fire in June in northeastern Jilin province that killed 121 people.