Oil shale is best described as which of the following?

• A. A fine-grained sedimentary rock.
• B. A volcanic igneous rock with tar inclusions.
• C. A metamorphic rock formed under high pressure.
• D. A carbonate rock rich in fossils.

Answer: (A)

Oil shale is best described as a fine-grained sedimentary rock because it forms from the deposition of very small clay- and silt-sized grains in quiet-water environments and, crucially, contains a large amount of kerogen, a solid organic material that can yield oil when heated. This combination—a clay-rich, layered, sedimentary matrix with abundant organic matter—defines oil shale and distinguishes it from other rock types. It’s not volcanic igneous, since it isn’t formed from molten rock; not metamorphic, since it hasn’t been transformed by high pressure and temperature deep underground; and not a carbonate rock rich in fossils, which would indicate limestone or dolostone dominated by carbonate minerals and fossils rather than kerogen-rich organic matter. When processed, the kerogen in oil shale can be converted into liquid hydrocarbons.