Shale gas is produced from very fine-grained sedimentary rock (shales) that have low porosity and almost no permeability and are organic-rich. The gas is held on organic matter in the rock, in tiny pores between grains and in any fractures or small cracks present in the rock.
The shales have been buried, firmly packed/compacted and heated deep within the Earth’s crust. Heat converts the organic matter in the shale into gas.
Once it is formed, the gas is held on the organic matter and in between the fine grains within the shales. In this way, the shale is the source of the gas as well as the reservoir and trap for the gas.