Drilling was stopped as a "precautionary measure," and drilling will resume when the ice, which measures about 30 miles (48 kilometres) by 12 miles (19 kilometres), has moved on, Shell Alaska spokesman Curtis Smith told The Associated Press in an email. The delay could be a couple of days or more, Smith said.
Sunday's start of Arctic Ocean drilling came after years of lobbying by Shell and plenty of bitter opposition from environmental groups, which say oil companies have not shown they can clean up an oil spill in ice-choked water.
"It's the first time a drill bit has touched the sea floor in the U.S. Chukchi Sea in more than two decades," Shell Alaska vice-president Pete Slaiby said in a prepared statement Sunday.
The oil giant was given permission last month to begin preliminary work on an exploratory well. The company's oil spill response barge has not been certified, but Shell is authorized to drill pilot holes that do not descend into oil reservoirs.
Shell's Burger Prospect is 70 miles (112 kilometres) off the northwest coast of Alaska.
Shell officials began monitoring the vast piece of ice Sunday when it was 105 miles (168 kilometres) away, Smith said. The decision to halt drilling was made Sunday. At noon Monday, the drill ship was still detaching from eight massive anchors. Smith said he did not know how far away the ice was at that time. The ice was moving at .5 knots, or .575 mph.
"We're using satellite images, we're using radar images, we're also using onsite reconnaissance to watch this ice so there are no surprises," Smith said.
"Part of working in ice is having the ability to temporarily relocate," Smith added, saying changing winds could blow the ice back even after it passes. "You never want to stop operations when your crews and your equipment are working smoothly, but this is what it means to work safely in the Arctic."
Shell has spent upward of $4.5 billion for Arctic Ocean drilling but had been thwarted by environmental lawsuits, regulatory requirements and short open-water drilling seasons.
Environmental groups that a spill of the magnitude of the Deepwater Horizon blowout in the Gulf of Mexico in 2010 would be catastrophic in a region hammered by climate warming and home to endangered or threatened marine mammals such as bowhead whales, polar bear and walrus.