State and local officials said an explosion early Tuesday at Chevron Appalachia LLC’s Lanco 7H well in Greene County, PA, was one of the worst the state has seen in modern history and the largest accident yet at an unconventional well in the state.

The Pennsylvania Department of Environmental Protection (DEP) said the cause was likely a blowout, but neither it nor Chevron Corp. could say exactly what caused the blast, which sent one person to the hospital and left another unaccounted for late Tuesday afternoon.

“We haven’t seen anything of this magnitude,” said Greg Leathers, director of the Greene County Emergency Management Agency. “No one else is hurt that we know about or can confirm at this time.”

Emergency management officials were called to a site in Dunkard Township, about 70 miles south of Pittsburgh and near the West Virginia border in southwest Pennsylvania, after Chevron reported the blast at 6:45 a.m. EST. Shortly after arriving, the Pennsylvania State Police established a half-mile perimeter around the site and local responders were “letting the fire burn,” Leathers said.

“Chevron decided to keep everyone away for safety reasons and a lack of training,” he added. “None of the local first responders are really trained to deal with this, and that’s why they’re bringing in the experts.”

Chevron dispatched Houston-based Wild Well Control, which offers engineering services and specializes in fighting such blazes. The company, a subsidiary of Superior Energy Services, has an office about an hour away in Canonsburg, PA.

“Chevron has initiated its emergency response procedures,” said Kent Robertson, company upstream spokesman, in an email. “Chevron personnel immediately responded to the fire and called in assistance from Wild Well Control. Chevron’s primary concern at this point is to contain the fire and ensure the safety of its employees, contractors and the surrounding community.”

Wild Well Control was expected to arrive at the site early Tuesday afternoon to try and extinguish the fire. Township officials said about 20 people were working at the site at the time of the explosion and that 18 had been accounted for shortly after the explosion occurred. Initially, they said, first responders had a difficult time accessing the site, which is in a rural area at a safe distance from the public.

John Poister, a spokesman for the DEP’s southwest regional office, said his agency was sending an emergency response team to the site on Tuesday afternoon to get people out of the cold and to establish a line of contact with the central office in Harrisburg, the state capital. A local team was sent earlier in the day to monitor the air quality in the area because the well was burning off heavy amounts of natural gas, he said.

“At this point, we are out there observing,” Poister said. “This is a very extensive well fire, very large, and there wasn’t any active fighting of it early on.”

Three well blowouts have occurred in Pennsylvania in recent years, Poister added, while there have been another five accidents involving minor well control problems. Two people were killed a few years ago when a conventional well blew out, but the other incidents led to only minor injuries, he said.

“Judging by the pictures I’ve seen and some of the television footage, this is among one of the most serious incidents we’ve seen,” Poister said of Tuesday’s explosion.

Chevron has a significant footprint in the Marcellus Shale, where it has about 714,000 net acres under lease, according to company documents and calculations by NGI’s Shale Daily. It has 40 permits to drill in Dunkard Township, in a part of the state where operators have flocked to produce liquids-rich gas.

In its fourth quarter earnings report, Chevron said production increased in the Marcellus, but stopped short of breaking down those numbers. It also added 800 million boe to its proved reserves last year, in large part due to its assets in the Marcellus, the company said.

State records show that there are two other wells on the same pad as the Lanco 7H where the blast reportedly originated. Like in Ohio and West Virginia, a number of trade organizations in the state have worked to train first responders by underwriting programs to teach them how to deal with accidents at well sites.

The state’s impact fee also allocates $1.5 million to the Office of the Pennsylvania Fire Commissioner for similar programs and better equipment. Despite those efforts, state and local officials said the size of the fire proved too much for their personnel on scene Tuesday, forcing them back to wait for Wild Well Control.