An essential part of the petroleum system that consists of fine-grained sediments (shales or carbonates) that have been deposited in a variety of environments, including deep water marine, lacustrine, and deltaic, which can yield gaseous or liquid hydrocarbons upon generation, expulsion, and migration to a reservoir rock. Source rocks are categorized as potential, effective, and spent according to its state of generation. Unconventional plays target the source rock for oil and/or gas production.