A small amount of oil spilled into Fidalgo Bay late Friday from a barge unloading at the Shell Puget Sound Refinery in Anacortes.

The Washington State Department of Ecology said midday Saturday that only 20 gallons had spilled, and most of it stayed on the Crowley Maritime barge. Only about 5 gallons reached the water, contained by booms set in place before the unloading began.

“No shorelines or wildlife were impacted,” the department said on Twitter.

The spill occurred at about 11:30 p.m. Friday as a barge was transferring about 5 million gallons of crude oil from Alaska to the refinery. A sheen was visible on the water within a containment boom covering an area about the size of a tennis court, and no oil had been observed outside of the boom.

The spill, according to an image the Ecology Department posted on social media, happened at the end of a long pier extending into Fidalgo Bay, where an Alaska North Slope crude-oil blend was being unloaded for refining. Shell said crews spotted oil coming from the barge and later identified the source as a pressure-relief valve, which was secured, stopping the spill.

State regulations require booms be set in place before oil is transferred off a vessel. “In this case, it was pre-boomed, as it’s supposed to be,” said Ty Keltner, Ecology’s communications manager for spill response.

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A second boom was set up around the containment area, and a skimmer vessel was sent to clean up the spilled oil, according to Shell. By midday Saturday, that was nearly complete, according to the department.

No injuries were reported, and air quality around the spill remained normal, according to Shell.

The National Response Center, Marine Spill Response Corporation and the U.S. Coast Guard were also responding, according to Ecology and Shell.

The Shell plant, opened in September 1958, was originally owned by Texaco. It processes on average 145,000 barrels, or about 5.7 million gallons, of crude oil a day from central and western Canada, as well as Alaska. It produces gasoline, fuel oil, diesel, propane, jet fuel, butane and petroleum coke, as well as nonene and tetramer, chemicals used in plastics, according to the company.

About 500 people work for Shell in Skagit County.