RIO DE JANEIRO — Nearly 110,000 gallons of oil may have spilled into the Atlantic Ocean because of a leak at an offshore Chevron drilling site, Brazil's environmental protection agency said Friday.
Officials think between 8,400 to 13,800 gallons of oil leaked each day from Nov. 8 through Tuesday, Ibama said in a statement on its website. Chevron had said that only 16,800 to 27,300 gallons in total leaked into the ocean.
Officials are still investigating the cause of the leak, which has been almost entirely contained, but the Ibama statement said it was a result of drilling.
An official at Brazil's Federal Police, which has opened an investigation into the spill, said Chevron "drilled about 500 meters (1,640 feet) farther than they were licensed to do." The official, who agreed to discuss the matter only if not quoted by name, said that information came from a person with knowledge of the drilling.
The leak occurred at a drilling site about 230 miles (370 kilometers) northeast of Rio de Janeiro.
Rio state Environment Minister Carlos Minc said earlier he was sure the leak was larger than Chevron estimated and he called for more transparency from the company.
"We can't trivialize this," he told the Globo TV network. "It's really serious and we don't yet know all the consequences."
Marine life in the area of the spill will be affected by the leak, Minc said, adding that whales are migrating from north to south through the spill area.
The oil slick, which was moving away from the coast, grew to 11 miles (18 kilometers), Ibama said. Most of the oil was concentrated around the drilling rig in a layer about 3 feet (1 meter) thick.
Chevron said "current estimates place the volume of the oil sheen on the ocean surface to be less than 65 barrels."
The company said it has 18 ships working on a rotating basis to collect oil off the surface and monitor the slick.
The drilling contractor for the well is Transocean Ltd., the owner of the Deepwater Horizon rig that oil company BP PLC was leasing at the time of last year's Gulf of Mexico oil spill, the largest in U.S. history and one that dwarfs the Brazilian leak. At its peak, BP's Macondo well was spewing more 2 million gallons a day.
Chevron said cementing operations were taking place so the well off Brazil is plugged. ANP, Brazil's national petroleum agency, said in a note on its website that "the first stage of cementing, to permanently abandon the well, was successfully completed." The regulator said the success of permanently plugging the well would be known "in the coming days."
ANP also said underwater footage showed that a "residual leakage flow" was continuing, but that "the oil slick continues moving away from the coast and is being dispersed, as desired."
Fabio Scliar, head of the Federal Police's environmental affairs division, which is investigating the case, said those responsible would be held accountable.
"There is no doubt that a crime occurred. The spill comes from the drilling activity. What interests me now is to find who is responsible," Scliar was quoted as saying by the newspaper Folha de S. Paulo.
The oil is believed to be coming from seep lines in the seafloor near the well and not from the well itself. Natural seeps are common around the world – perhaps the most well known in the U.S. is the La Brea Tar Pits in the heart of Los Angeles – and are often used by oil companies during undersea exploration to determine where a good prospect for oil drilling may be.
Natural seeps are usually so small in volume they don't cause a nuisance beyond producing the periodic tar ball that washes up on a beach.
But problems with drilling a well nearby can exacerbate the seeps and cause greater flow of oil, which can be hard to control, said George Hirasaki, a Rice University engineering professor who was involved in the Bay Marchand oil containment effort for Shell off Louisiana in the 1970s.
"Anytime there is movement of fluids, even if it didn't go to the surface of the well, the internal flow could result in the fluid going somewhere else," Hirasaki said. "It could move laterally at the same depth or increase the flow rate of natural seeps that are connecting to the surface."
Investigators will want to look at whether the weight of the mud being used during the drilling and abandonment operations was sufficient to contain the pressure inside the well, and they will also want to see whether drilling too deep caused problems in a geopressure zone beneath the seafloor, experts said.
Ed Overton, a Louisiana State University environmental sciences professor, said that to truly control the leak could be difficult.
"If you have this stuff oozing up through the ground you don't have a mechanism for control," Overton said. "If something started that to leak, that would worry me a lot more than a leak around the well. You'd have to drill a relief well and intercept that ooze."
People familiar with last year's BP oil spill off Louisiana know about relief wells.
BP spent four months drilling a relief well that it used to pump cement under the area that was spewing oil into the Gulf of Mexico, and sealed the leak permanently.
Experts said that while there are many physical differences between the BP spill and the Chevron spill, the main common thread is the slow flow of information and different explanations for what happened and the severity of what happened.
"There's a pretty long track record of all the people involved in spills underestimating at least initially the size of the spills," Overton said. "I would suspect they literally don't know, so they are trying to figure out."
The Chevron leak is smaller than those Brazil has seen in the past.
In 2000, crude spewed from a broken pipeline at the Reduc refinery in Rio de Janeiro's scenic Guanabara Bay, spewing at least 344,400 gallons into the water. Just a few months later, more than 1 million gallons of crude burst from a pipeline state-controlled oil company Petrobras into a river in southern Brazil.
Brazil's worst oil disaster was in 1975, when an oil tanker from Iraq dumped more than 8 million gallons of crude into the bay and caused Rio's famous beaches to be closed for nearly three weeks.
Associated Press writer Bradley Brooks reported this story in Rio de Janeiro and Harry Weber reported from Atlanta, Georgia.