Related Clues:

• Mud, after compaction
• Something fracked in fracking
• *Sedimentary rock
• Source of gas market includes hydrogen
• Type of rock formed from igneous or sedimentary rocks
• Fracking target
• Fuel oil source
• Rock that's fracked
• Fossil-yielding sedimentary rock
• Say nothing, given drink that may be full of gas!
• Rock that's 'fracked'
• Oil-yielding mineral
• Badly singes rock
• Rock from which slate is formed
• Easily cleaved rock