|Warning: bad jokes ahead.|
Q. What do organic-rich mudstones and Muhammad Ali have in common?
A. Both used to be gaseous clay.
|Cassius Marcellus Clay|
Should you wish to make this into a REALLY niche shale gas joke, one of the biggest shale gas plays in the world is the Marcellus Formation, in the Appalachians of Pennsylvania. And what perchance, was Muhammad Ali's original middle name? Why, it was Marcellus!
[Please note that although I thought this joke highly original, many others have apparently used the term 'gaseous clay' already, such as Viz, MTV, and a musical outfit named Boomish.]