But the powerful explosions felt throughout a large swath of southeast England including London, 25 miles away, rattled nerves in a country still jittery over terrorism after deadly transit bombings in July killed 52 people and four suicide bombers.
The oil depot is near Luton Airport and some residents reported hearing an aircraft flying low overhead shortly before the first explosion at around 6 a.m. But police said there was nothing to suggest a plane was involved.
"All indications at this stage are that this was an accident," said Frank Whiteley, chief of police in Hertfordshire, the county where the depot is located. "However, clearly we will keep an open mind, as with all investigations, until we can confirm that for certain."
Al Qaeda and other terrorist groups have threatened to target fuel depots.
Police said only two of the wounded suffered serious injuries. More explosions were expected for several hours at the depot, which stores 4 million gallons of gasoline, diesel, kerosene and aviation fuel.
Total SA, the French company that owns the Buncefield Oil Terminal in a joint venture with Texaco, said the cause of the explosion was not immediately known.
Buncefield is the fifth largest of some 50 major oil storage facilities in Britain. The Total/Texaco reserves there account for about 5 percent of the country's oil supply.
Total said 400 tankers a day were loaded at the site.
BP also has a storage facility at the site, which was not damaged in the blast.
There were reports of panic buying at some gas stations, although companies said shortages were unlikely. The U.K. Petroleum Industry Association said it was working to meet demand from other distribution terminals.
Firefighters planned to use foam to stop the blaze spreading across the depot and adjacent industrial park. The 20 blazing tanks were being allowed to burn themselves out while firefighters and police watched.
"The fire is contained," Whiteley told a news conference in Hertfordshire. "Yes, it's big and it's going to burn for some time. But it is under control."
A dense pall of black smoke rose as high as 10,000 feet over the town of Hemel Hempstead and could be seen in satellite images, said Eddy Carroll, a forecaster at Britain's national weather forecasting office. The smoke also drifted over London, he said.
Whiteley cautioned that the cloud contained irritants that could cause coughing and nausea. People in the path of the smoke should stay indoors, he warned.
Hospital spokesman Howard Borkett-Jones said the smoke appeared to be of "low toxicity" but could cause chest tightness and coughing, especially for those people with asthma.
Forecasters were predicting a cold front for Monday that would bring rain to the region, washing the pollutants from the air and onto the ground.
"It's not clear to what extent it is going to be a serious pollution event," said Carroll, though he said burning compounds close to the site might pose a hazard.
Emergency workers wearing masks cordoned off the area around the oil depot, located about two miles from Hemel Hempstead. Authorities evacuated residents nearby as the plume of smoke moved slowly eastward.
Nearby Luton Airport remained open and flights were operating normally. A 15-mile stretch of the main north-south M1 motorway was shut after the blasts, causing severe traffic delays.
The blasts shook homes in Hemel Hempstead and many residents reported hearing a loud boom.
"Around 6 a.m. as we were sleeping, there was a mighty explosion — a thunderclap that woke me up," said Neil Spencer, 42, who lives less than a mile from the terminal. "It was fireball after fireball — truly amazing."
Duncan Milligan, who also lives in the town, said his house shook violently.
"I am about three miles from where the explosion took place but I can see flames high in the sky and smoke billowing everywhere. There is clearly a building on fire near the motorway and police and emergency services are everywhere," he said.
The explosion blew the door off its hinges at photographer Haris Luther's home.
"I thought the house had been hit by lightning," he said. "It sounded like an earthquake."
The Ramada Hotel in the town was evacuated after windows were shattered by the blast. Two guests received minor injuries from broken glass and were treated at the hotel, said Calum Russell, marketing director for Jarvis Hotels Ltd. All 187 guests had since returned to their rooms, Russell said.