According to the Pennsylvania Department of Conversation and Natural Resources (DCNR), “The Marcellus shale is an organic-rich black shale that was deposited in an oxygen-deficient marine environment during Middle Devonian time (~390 million years ago). Long known to be a source rock for many conventional oil and gas reservoirs in the Appalachian basin, it is now being explored as an unconventional reservoir. The formal name for this rock unit is the Marcellus Formation.”
