SALT LAKE CITY (AP) — Chevron Corp. said Monday it believes an electrical arc from a power line torched a quarter-size hole in a pipeline that spilled thousands of gallons of crude oil into a Salt Lake City creek over the weekend.

Rocky Mountain Power said the chain of events began with a tree limb that fell onto a power line late Friday. But the utility said it would be highly unusual for an arc to drill a hole in a buried pipeline by traveling through a metal fence post.

Chevron officials said the fence post was anchored within inches of the 3-foot-deep pipeline and could have acted like an electric torch, puncturing a hole in the pipe's metal casing.

"We think of this as a one-in-a-million event," Mark Sullivan, manager of Chevron's oil refinery, said Monday at an oil-coated city pond. He said the systems in place for detecting pipeline leaks never imagined "this very unusual circumstance."

Sullivan said Chevron estimates that 33,000 gallons of crude spilled into Red Butte Creek through the heart of Salt Lake City, twice the figure previously offered by the Salt Lake City Fire Department. But the company doesn't know when the leak started.

The shorting of the power line that Chevron blames for causing the arc occurred at 9:19 p.m. Friday, but Chevron says it wasn't notified of the spill for about another nine hours, until 6:52 a.m. Saturday.

It wasn't until Sunday morning that residual amounts of oil stopped leaking from the ruptured pipeline, Sullivan said.

Much of the crude was collected from a pond in a city park where it coated hundreds of Canada geese and ducks. It also suffocated trout in Red Butte Creek, according to residents.

The creek drains into Jordan River and, eventually, Great Salt Lake, but Sullivan said tests show no oil has reached the lake, a crucial migratory stop for bird species that feast on the lake's freshwater margins.

Adding to the unlikely circumstances of the leak was the size of the hole in the pipeline that brings crude from the oil fields of western Colorado and eastern Utah.

"I was really surprised to hear this was a quarter-size hole," Salt Lake City Mayor Ralph Becker said Monday.

Chevron officials said the pipeline was pressurized and could have spewed 33,000 gallons through the tiny hole before the leak was stopped.

Rocky Mountain Power spokesman David Eskelsen confirmed that a 40-minute power outage originated at the leak site but said the utility wasn't certain that could set off a chain of events that resulted in the oil spill.

"There's no scenario we understand that would produce the kind of damage that was suffered," he said.

Eskelsen said he had no information about the fence post that reportedly acted like an electric torch and guards a spot where overhead power lines go underground. Chevron said no fence post should have been anchored within inches of a crude oil pipeline.

Eskelsen said he couldn't immediately confirm the fence belonged to the utility or explain the purpose of the fence. He expected to have more information on that later Monday.

Chevron spokesman Dan Johnson planned to take reporters to the origin of the leak, but reporters didn't get closer than about 50 yards. Johnson said the tour was overruled by Chevron superiors or fire officials as a safety risk.