A type of sedimentary rock composed chiefly of a combination of silt and clay. Shale may contain kerogen, a type of organic matter that yields oil and gas. Shale formations are best known for shale oil, a type of unconventional oil. Deposits of shale are found in many areas around the world, and large areas of the United States, Russia, Argentina, Libya, and China are known to have shale oil and gas reserves.