The worst Nigeria offshore oil spill in more than a decade has been contained before reaching the West African nation’s coast, officials with Royal Dutch Shell PLC said Monday, less than a week after one of its lines bled crude into the Atlantic Ocean.

An investigation into how the spill of less than 40,000 barrels — or 1.68 million gallons — happened remains ongoing, though company officials acknowledged workers only discovered the leak after seeing a sheen of crude in water surrounding its Bonga offshore oil field.

Meanwhile, Shell officials say the company will clean up another spill it discovered while containing its own, highlighting how prevalent pollution remains in oil-stained Nigeria after more than 50 years of production.

“We can undeniably say we traced our oil … and stopped it,” said Cliff Pain, who manages the Bonga operation for a Shell subsidiary.

Shell organized a helicopter flight Monday for journalists to see the Bonga field controlled from a large ship as opposed to a stationary rig, about 75 miles off Nigeria’s coast. There, waters appeared free of the oil sheen as ships continued to patrol along the underwater lines linking the vessel to oil fields and transfer buoys for filling tankers.

The leak discovered Dec. 20 came from a break in a flexible line about 360 meters out from the vessel that sends oil to tankers, Pain said. While the vessel has a variety of gauges to check pressure on the line, it wasn’t until daylight broke that workers noticed a sheen surrounding the Bonga vessel, he said.

It takes about 25 hours to fill a waiting tanker with 1 million barrels of oil from the vessel, Pain said. That means the leak could have spewed for hours before being noticed.

At its height, Shell statistics show the sheen spread across about 350 square miles, matching an estimate earlier issued by an independent watchdog group called SkyTruth. Nigerian government officials previously said the spill only affected an area a third that size

Using ships and aircraft, workers spread chemical dispersants to break up the oil, which also evaporated in the region’s warm water and air, said Steve Keedwell, a Shell employee who helped oversee the cleanup operation. Shell ultimately stopped the sheen about 11 miles before it made landfall, Pain said.

However, workers then discovered a separate oil spill around the mouth of a river in Delta state, said Mutiu Sunmonu, Shell’s Nigeria country chairman. Sunmonu said samples of the oil showed it came from a different source, though the company would clean it up as well.

“When I sighted it myself, my initial reaction was anger, but I told myself: ‘You know, you just cannot afford to be angry, just deal with it,”‘ Sunmonu said.

The Nigerian group Environmental Rights Action, which monitors spills around Nigeria’s oil-rich southern delta, has blamed Shell for the new spill. Nnimmo Bassey, the group’s executive director, could not be immediately reached for comment Monday night.

Shell operates the Bonga field in partnership with Italy’s Eni SpA, Exxon Mobil Corp., France’s Total SA and the state-run Nigerian National Petroleum Corp. It produces about 200,000 barrels of oil a day around 10 percent of production in Africa’s most populous nation. The field remains shut down and Shell officials offered no estimate Monday of when production could resume at a field vital to Nigeria’s government finances.

Nigeria, an OPEC member nation producing about 2.4 million barrels of crude oil a day, is a top supplier to the United States. However, pollution from spilled oil stains its Niger Delta region, with crude lapping against beaches and leaving a black ring around creeks in an area about the size of Portugal.

Some environmentalists say as much as 550 million gallons of oil poured into the delta during Shell’s roughly 50 years of production in Nigeria a rate roughly comparable to one Exxon Valdez disaster per year. Many blame Shell and foreign companies working in Nigeria for the pollution. However, Shell in recent years has blamed most of its spills on militant attacks or thieves tapping into pipelines to steal crude oil, which ends up sold on the black market or cooked into a crude diesel or kerosene.

Talking with journalists, Sunmonu acknowledged that the limited spill, open ocean and favorable weather had helped Shell quickly contain the spill. If it had been on land, the oil could have sunk into the soil, remaining there for years, he said.

It also would have pushed Shell into negotiations with village elders to clean up the spill, something it often contracts other companies to handle. Many view the company with hostility after its years in the delta, and its employees remain targets of kidnap gangs and militants.

“You don’t have communities to contend with,” on the ocean, Sunmonu said.