unconventional natural gas
Also: unconventional gas
Natural gas (mostly methane) produced from low-permeability reservoirs such as shales or "tight" (impermeable) sandstones. The reservoir permeability is usually enhanced by hydraulic fracturing of the rock, a process better known as "fracking".
All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.