State regulators report that Marcellus Shale natural gas production in Pennsylvania topped 3 trillion cubic feet in 2013. That's more than double the previous year's production, and the energy equivalent of more than 500 million barrels of oil.

The Department of Environmental Protection reported the production figures. The data are submitted by energy companies and sometimes contains errors, but the report of 3.1 trillion feet for total 2013 production is very close to independent estimates.

The Marcellus Shale is a gas-rich formation deep underground that extends across several states, but most of the current production is in Pennsylvania.

The U.S. Department of Energy estimates that the Marcellus Shale now provides about 18% of the nation's natural gas, but that figure includes some West Virginia production.