Natural gas has been rising toward areas of low pressure since it formed in the Marcellus Shale, which
is part of the reason for the existing fractures in the unit. Natural gas has
reached near-surface layers along fracture networks where the Marcellus Shale (and other shales and limestones) comes within several hundred feet of the surface. Some of these seeps were sources for natural gas for
towns during the 19th century. This is why methane can also be found rising from some abandoned, uncapped wells