shales

Shales are fine-grained sedimentary rocks composed mainly of clay minerals and silt-sized particles. They are typically fissile, splitting easily along planes of weakness into thin sheets, and often show laminations a few millimeters to centimeters thick. The mineralogy commonly includes illite, kaolinite, chlorite, and variable amounts of quartz, feldspar, and organic matter. Shale forms from the gradual lithification of mud deposited in low-energy environments such as deep lakes, continental shelves, and offshore basins, where clay and silt settle and compact over time. The texture is often smooth and can appear gray, greenish, or black when rich in carbonaceous material; black shales may be enriched in kerogen and are of particular interest in petroleum geology.

Shales have low permeability and high plasticity when wetted, enabling them to act as cap rocks or aquitards in sedimentary sequences, and they commonly occur atop sandstones or coals in regions of deposition. They are also a principal source rock for hydrocarbons if the organic matter is sufficiently preserved and mature. In unconventional petroleum geology, shale gas and tight oil are extracted from low-permeability shales using hydraulic fracturing and horizontal drilling. The related term oil shale refers to kerogen-rich shales that yield oil only after processing (pyrolysis).