CHESWICK, Pa. - A natural-gas well exploded Friday, killing two workers, leaving a third man remarkably uninjured, and sparking a smoky fire.

A team of oil- and gas-well firefighting experts was flying from Texas to help put out the fire, which was still burning Friday afternoon.

A three-man crew, employed by a maintenance contractor, was working on oil tanks at the well in Indiana Township when the blast occurred at 9:50 a.m., rocketing one of the tanks more than 70 yards into the woods, said Robert Full, Allegheny County's emergency director.

The cause has not been detailed in full, but state officials believe "people were welding at the site and there was an explosion and the well caught fire," said Helen Humphries, a spokeswoman for the state Department of Environmental Protection.

The resulting fire spewed thick, black smoke and scorched the workers' truck, melting the tires. About 200 firefighters and rescue personnel responded, fighting the fires with thick, heavy foam.

Firefighters were hindered by the site's remote location, about 15 miles northeast of Pittsburgh, and the fact that access could be gained only by a single, steep gravel road. As a result, firefighters had to carry between 3,000 and 4,000 feet of hose, Full said.

Temperatures topping 90 degrees also caused problems, and three firefighters had to be taken to hospitals for heat-related ailments.

Still, Full said the fact that the fire was still burning Friday afternoon was a good sign.

"As long as the well is burning, we know that the gas is being consumed," Full said.

A team from Wild Well of Texas was flying to Pittsburgh to help put the fire out. Depending on their efforts, the blaze could be put out by Saturday, Full said.

Full said the two workers who died had not been identified, nor was their employer. They had been hired by the well's owner, Monroeville-based Huntley & Huntley Inc., to do maintenance work, Full said.

The well, dubbed Murray Heirs No. 6, is shallow - about 3,500 feet deep - and was drilled in May 2008, Humphries said. It was considered to be a producing well, she said.

The department won't be able to determine the environmental impact until after the fire is extinguished, Humphries said.

DEP Secretary John Hanger said in an e-mail message that the well is not part of the lucrative Marcellus Shale, a Pennsylvania rock formation that drillers began tapping about two years ago.

As of 2007, Pennsylvania had more than 65,000 producing oil and gas wells, most drilled no deeper than 3,000 feet.