Shale

Shale is a fine-grained sedimentary rock composed primarily of clay minerals and very small particles of silt. It is typically fissile, meaning it splits easily along thin, laminated layers called bedding planes.

The mineral makeup is dominated by clay minerals such as illite, kaolinite, and smectite, with varying amounts of quartz, feldspar, and organic matter. In some shales, particularly black shales, organic matter is abundant.

Shales form from the compaction of muds in quiet water settings, including lakes, floodplains, deltas, and low-energy marine basins. The fine clay and organic-rich layers can preserve fossils and show lamination.

Because of their fine grain and low porosity, shales typically have low permeability; fluids and gases do not move easily unless there are fractures or thermal maturation.

Shale is widely important as a source rock for hydrocarbons. Organic-rich shales can generate oil or gas during maturation; hydraulic fracturing is used to enhance permeability in shale reservoirs, enabling the production of shale gas and shale oil (tight oil).

Black shale denotes high organic content; gray and other colors reflect mineralogy and oxidation state. Some shales are calcareous or carbonaceous.

Shale is distinct from mudstone and claystone; shale is fissile and laminar, whereas mudstone is typically non-fissile.