Pennsylvania regulators may be vastly underestimating the amount of methane emitted from the state's oil and gas production sites, in part because of an emphasis on unconventional shale wells, an environmental group's analysis found.

The state's producers emitted an estimated 522,400 tons of methane in 2015, according to calculations from the Environmental Defense Fund, or EDF, compared with the Pennsylvania Department of Environmental Protection's reported 112,100 tons.

A sizable chunk of the emissions measurement gap comes from Pennsylvania's focus on measuring methane releases from unconventional wells. The state agency's data set does not include conventional production wells, which the EDF estimated accounted for 268,900 tons of emissions in 2015.

While Pennsylvania's gas and oil production is heavily skewed toward unconventional development, conventional wells have been known to emit more gas, the EDF noted, pointing to a study released in 2016 that took site-level measurements at 35 well pads in southwestern Pennsylvania and northern West Virginia.

The EDF based its calculations on production data from Drillinginfo's information on well types and totals, plus an array of studies and federal data on emissions rates.

The Department of Environmental Protection has a methane emissions reduction strategy in motion. The department in early 2017 proposed permitting updates that would apply to unconventional gas well sites, along with other infrastructure downstream of the wellheads. Pennsylvania Gov. Tom Wolf has voiced his support for the plans to cut methane releases, noting that curbing emissions protects the air and prevents industry waste.

The state expects to finalize its methane rules in late March.

"We are pleased that the Wolf administration continues to promise bold action on methane. Pennsylvanians have known for quite some time that unchecked oil and gas pollution is making air quality and climate change worse, and now we have new insight as to just how severe and urgent this problem is," Andrew Williams, the EDF's director of regulatory and legislative affairs, said in a statement.

The Pennsylvania agency said it was still reviewing the EDF's analysis, and it underscored ongoing efforts to curb emissions in the state. Department spokesman Neil Shader said that "addressing methane pollution is a key priority for the Wolf administration."

He did not comment on the reasons for the difference between the EDF's and the agency's emissions totals.