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According to Reuters, the blast at the Lanco 7H well near the West Virginia border occurred at about 6.45am local time yesterday, with the gas-fuelled fire continuing throughout the day.
Wild Well Control was called in to contain the fire.
While one worker was taken to hospital for minor injuries, all reports so far indicated that another worker remained missing as night fell.
"Chevron's primary concern at this point is to contain the fire and ensure the safety of its employees, contractors and the surrounding community," Chevron spokesman Kent Robertson reportedly said via email.
Pennsylvania Department of Environmental Protection spokesman John Poister told Bloomberg that before the accident the contract workers "were lowering pipe into the well hole" to help connect the gas into their existing system.
Crews reportedly began fraccing the horizontal well around March 15 last year.
