Moving ice may keep a Royal Dutch Shell petroleum drill ship away from a Chukchi Sea prospect for several days, a Shell Alaska spokesman said Tuesday.

Curtis Smith said at midday that a massive ice pack heading toward the Burger Prospect had slowed from 0.5 knots to 0.2 knots — about 1/4 of a mile per hour — and remained 10 to 12 hours away.

“Depending on conditions, it could be a few or, potentially, several days before it’s safe enough to resume drilling,” he said in an email response to questions.

The prospect is 70 miles off the coast. The Noble Discoverer drill ship has moved 30 miles south and will remain there until the ice — 30 miles long, 12 miles wide and up to 82 feet thick — has passed and is unlikely to change course and return.

The ship began preliminary work Sunday on an exploratory well — the first drilling in U.S. Chukchi waters since 1991 — but was forced to stop hours later when ice headed for the ship was spotted more than 100 miles away.

Shell drilling protocol calls for the vessel to disconnect from its eight anchors when ice approaches.

Environmental groups, which have challenged Arctic offshore drilling in lawsuits, said the interruption underscores the dangers of drilling. Smith said the company is taking a cautious approach.

The company spent $2.1 billion on Chukchi Sea leases in 2008 and more than $4.5 billion overall on Arctic offshore drilling, including the Beaufort Sea.

The state of Alaska hopes oil from offshore drilling will eventually refill the trans-Alaska pipeline, which now runs at less than one-third capacity. Federal officials estimate Arctic waters hold 26 billion barrels of recoverable oil and 130 trillion cubic feet of natural gas.

Interior Secretary Ken Salazar announced Aug. 30 that Royal Dutch Shell PLC would be permitted to begin preparation work at the Chukchi site even though the company’s spill response barge has not been certified. The company was authorized to drill narrow pilot holes 1,400 feet below the ocean floor and roughly 4,000 feet above a petroleum reservoir.

Smith said the response barge, which was refurbished in Bellingham, Wash., has been undergoing sea trials and was scheduled for more inspections Tuesday.

The company’s drilling permit calls for Chukchi drilling to end Sept. 24, a month before seasonal sea ice is expected at the prospect. The company has applied for a two-week extension but has heard no word on the request, Smith said.

Shell’s second drill ship, the Kulluk, is in the Beaufort Sea waiting to move onto an offshore prospect after the fall bowhead whale hunting season. Whalers from Nuiqsut have used three of their four strikes, Smith said, and Kaktovik whalers have used one of three allowed under subsistence hunting rules. Both were hunting Tuesday.

The Kulluk, an ice-reinforced drill ship, is authorized to drill into hydrocarbon zones up to Oct. 31, Smith said.