Shale gas is defined as a fine-grained reservoir in which gas is self-sourced, and some of the gas is stored in the sorbed state. Sorbed gas is predominantly stored in the organic fraction - so organics are present. Shale gas is not just 'shale'.
Productive gas shales range from organic-rich, fine-grained rocks, such as the Antrim or Ohio Shale, to variable facies rocks, such as Lewis Shale.