Energy Information Administration's "Energy Explained" series:

"Shale is a fine-grained sedimentary rock that forms from the compaction of silt and clay-size mineral particles and is easily broken into thin, parallel layers. Black shale contains organic material that can generate oil and natural gas and trap the generated oil and natural gas within its pores.

U.S. Geological Survey FAQ:

"Reports of hydraulic fracturing causing felt earthquakes are extremely rare. However, wastewater produced by the hydraulic fracturing process can cause “induced” earthquakes when it is injected into deep wastewater wells.