Chevron spokesman Trip Oliver said the fire was reported about 6:45 a.m. at the Lanco 7H well in Dunkard Township, near Bobtown, about 50 miles south of Pittsburgh.
Oliver said one person was reported hospitalized and another was unaccounted for.
State Department of Environmental Protection officials say the fire may burn for days, delaying efforts to determine its cause.
Department of Environmental Protection spokesman John Poister said the well is located in a rural area where no homes or schools are located nearby. He said a DEP team at the site found no signs of threats to public health, noting that the fire was burning off volatile organic compounds in the gas. However, the fire was burning so hot that DEP crews have had to stay about 300 yards away, he said.
Poister said the agency now suspects the explosion and fire occurred on the surface of the well and suspect the fire wasn't caused by an explosion inside the well itself, what is known as a blowout. A blowout results from a rapid increase in oil or gas pressure that ruptures a well. He said further investigation will be needed.
Poister said of the missing worker that everyone "just hopes for the best."
Chevron had previously completed drilling and hydraulically fracturing, or fracking, the well and was in the final stages of using steel pipe to hook it up to a pipeline distribution network for production, he said.
State police established a half-mile perimeter around the well.
"We want to find out how this happened and why," Poister said, adding that it was the first serious Marcellus Shale well explosion in western Pennsylvania.
The Marcellus Shale formation lies under large parts of Pennsylvania, West Virginia, Ohio and other neighboring states; it's currently the country's most productive natural gas field.
Poister said well blowout experts from Houston-based Wild Well Control arrived at the Pittsburgh airport around noon and headed to the site to work out a plan to extinguish the fire. The Texas company did not immediately respond to a request for comment. It said on its website that Wild Well responds to more than 85 percent of blowouts in the U.S.