Shale: A very fine-grained, clastic sedimentary rock that forms when mud, silt, and clay-size mineral particles are consolidated and compacted into relatively impermeable layers. Shale is characterized by a finely laminated or stratified structure and is the most abundant sedimentary rock. Shale can contain varying amounts of organic matter, so it has the potential to become a hydrocarbon source rock. The pore size of shale reservoirs ranges from nanometers to micrometers. Hydrocarbons in some shale reservoirs can be extracted using hydraulic fracturing.
Shale Gas: Natural gas produced from wells that are open to shale formations. Shale is a fine-grained, sedimentary rock composed of mud from flakes of clay minerals and tiny fragments (silt-sized particles) of other materials. The shale acts as both the source and the reservoir for the natural gas. See natural gas.