Department of Environmental Protection spokeswoman Kathy Cosco said the unidentified victim was working alone at the time.
The worker was attempting to transfer brine from a tank into a truck, she said. What sparked the explosion is unclear and will be the focus of the state's investigation, already under way.
Brine is the salty, chemical-laced water that gas drillers use to blast fissures into shale, allowing the gas to migrate up for collection. Some of the chemicals used in the brine can be flammable.
Pittsburgh-based EQT didn't immediately return a telephone or email message.
Cosco said the DEP wasn't sure what time the blast happened because no other workers were present.
But Mary K. Singleton, who lives about a mile away, said her house shook at 6:45 a.m.
"I heard an explosion but I didn't know what it was. You never know around here," she told The Associated Press.
"I got up and made sure the house was all right, but couldn't see anything. I didn't know what's going on," she said. "I asked my husband, 'you suppose it was a meteor?'"
Corky DeMarco, executive director of the West Virginia Oil and Natural Gas Association, said EQT is calling the fatality an "industrial accident," not an explosion, and he's never heard of brine exploding.
"To the best of my knowledge, brine is not flammable," he said. Accidents involving brine are typically spills, he said, "so that, I don't understand."
Brine is 99.5 percent water and sand, and drillers typically do not add potentially flammable chemicals, DeMarco said. What flows back up from a well is mostly salty water, and any chemicals are diluted.
"Contrary to what some people like to say," he said, "we don't use diesel fuel or any of those kinds of additives that would be flammable."
EQT is one of the largest exploration and production companies in the Appalachian shale gas fields, with drilling rights to more than 3.5 million acres in West Virginia, Pennsylvania, Kentucky and Virginia.
EQT's website says it has proven reserves of more than 32 trillion cubic feet equivalent, and it's grown those reserves by more than 120 percent over the past five years.
EQT says it plans to spend $1 billion in well development this year.
Associated Press writer John Raby contributed from Charleston.