"We will investigate the incident with responsibility and give timely reports," another Sinopec statement said.
The Qingdao Environmental Protection Bureau said barriers had been set up to contain the oil as it spread into the sea, but that a mixture of gas and oil from a storm sewer exploded and caught fire over the sea.
More than 3,000 square meters (32,000 square feet) of sea surface was contaminated, the city government said.
Authorities said the oil seeped into underground utility pipes, which could have been a factor in the blasts, but they did not elaborate. They assured the public that the explosions did not affect any petrochemical plants or military facilities in the seaside district and that air quality remained good after the disaster.
About 18,000 residents were evacuated in the wake of the blasts, and power was restored to all but two residential neighborhoods, authorities said.
Of the 136 people hospitalized, 10 were in critical condition, the city government said.
The Beijing News cited a resident surnamed Gao as saying he was driving past Qingdao's Huangdao district when he felt the force of the blasts, and then realized the ground in front of him had fractured. The air was pungent, many cars on both sides of the road were overturned and there was dark smoke rising in the distance, he said.
"It felt like an earthquake, and I was dumbstruck," Gao said, adding that there was chaos on the street as people ran, panicking, in all directions.
Authorities ruled out terrorism but the incident remained under investigation, it said.
President Xi Jinping urged local officials to go all out in finding missing people, treating the injured and finding the cause of the accident, state TV broadcaster CCTV said.
It was China's second-deadliest industrial accident of the year, behind a chicken factory fire in June in Jilin that killed 121 people.