An oil well blowout in Wyoming prompted 50 residents to evacuate their homes amid concern that a spewing cloud of natural gas could explode.

Gas continued to erupt from the ground Wednesday after the blowout Tuesday afternoon five miles northeast of Douglas in east-central Wyoming. Witnesses told television station KCWY-TV they could hear the roaring gas from six miles away.

Authorities were concerned about the possibility of an explosion or fire before the well could be plugged with drilling mud. No workers were injured.

"I'm hoping this thing doesn't catch on fire because they have a good chance of salvaging the rig and the well bore," said Tom Doll, supervisor of the Wyoming Oil and Gas Conservation Commission, which oversees oil and gas drilling in the state.

The gas leak was diminishing Wednesday and officials were optimistic that workers with the Houston-based well control company Boots & Coots would plug it soon, Doll said.

Air samples were normal but the company asked 67 residents who live within 2.5 miles to voluntarily evacuate. Fifty of those residents left and 17 opted to remain at their homes, according to officials with the well operator, Oklahoma City-based Chesapeake Energy.

"We have our specialized equipment in there," company spokeswoman Kelsey Campbell said. "They are in there and are working to contain the well."

Although the well was relatively close to Douglas, population 6,100, the area immediately nearby was mostly undeveloped rolling prairie.

The leak occurred after the well was drilled and while steel casing was being installed down the well hole. Oil-based drilling mud spewed from the ground, along with the gas, but was mostly being contained to the drilling site, according to Chesapeake.

An oil and gas commission inspector happened to be at the site hours before the blowout occurred.

"If there was something that was odd in terms of the activity, he would have known about it," Doll said. "There was nothing going on outside of the normal routine."

The well was targeting the Niobrara Shale, a deep oil-bearing formation beneath eastern Wyoming, northeastern Colorado and western Nebraska where drilling has increased over the past two years. The Chesapeake well had been drilled to its full depth and length of close to 18,000 feet vertically and horizontally, Doll said.

Oil wells commonly produce gas but other oil wells recently drilled in the area haven't been particularly gassy, he said.

Well blowouts have been relatively rare in Wyoming despite thousands of oil and gas wells being drilled in the state over the past decade. Blowouts are more common in the South where the geology is much different than in the Rockies.

A 2006 blowout 40 miles east of Yellowstone National Park prompted the evacuation of 25 homes until the well could be plugged a couple of days later. The blowout caused underground pollution and lingering concern about well water contamination.