Shale Gas : Origins

Basically, Shale gas is natural gas – primarily methane – found in shale formations, some of which were formed
300-million-to-400-million years ago during the Devonian period of Earth’s history. The shales were
deposited as fine silt and clay particles at the bottom of relatively enclosed bodies of water.

Some of the methane that formed from the organic matter buried with the sediments escaped into sandy rock layers adjacent to the shales, forming conventional accumulations of natural gas which are relatively easy to extract. But some of it remained locked in the tight, low permeability shale layers, becoming shale gas.