Oil Shale
Oil shale is a fine-grained sedimentary rock rich in solid organic matter called kerogen, which yields liquid hydrocarbons when heated.
Oil shale is a fine-grained sedimentary rock rich in solid organic matter called kerogen, which yields liquid hydrocarbons when heated. It is not oil itself, and it is different from "shale oil" or tight oil, which is liquid oil that is trapped in shale and produced by drilling and fracturing.
🪨 Geology Guide Specimen: Open Oil Shale (Kerogen Marl) in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 6 subgroups of Sedimentary, and test your eye in LithoClade.
Kerogen: What Is the Organic Matter?
Kerogen is a waxy, insoluble residue of algae and other organic matter. In oil shale it has not been buried and heated enough to turn naturally into oil, so it must be heated in a retort or in the ground to convert it. A rich oil shale can contain as much as a few tens of percent kerogen.
Green River Formation: A Lake Full of Fossils
The Eocene Green River Formation of Colorado, Utah and Wyoming holds some of the world's largest oil shale deposits. It formed in ancient lakes whose fine yearly layers, called varves, preserved fish, insects and plants in great detail, and Fossil Butte in Wyoming is famous for them.
Energy and Environment: Why Is Development Contested?
Producing oil from shale takes a lot of energy and water and releases much carbon dioxide, so it is more expensive and more carbon-intensive than conventional oil. Estonia burns oil shale for power and chemicals, while other countries have kept deposits mostly in reserve. See fossil fuels and the carbon cycle.
Oil Shale at a Glance
| Property | Value |
|---|---|
| Type / variety | Kerogen-Rich Lacustrine Laminated Shale |
| Composition | Fine Dolomitic/Clayey Matrix + 15–40% Solid Algal Kerogen Organic Matter |
| Geology Guide group | Sedimentary Rocks (Clastic, Chemical & Biogenic) |
| Rock Classification | Fine Clastic (Mudrocks & Shales) |
| Mohs hardness | Mohs 2.5–3.5 |
| Texture / habit | Finely varved chocolate-brown to tan seasonal laminations |
| Density | 1.80–2.30 g/cm³ |
| Lustre and cleavage | Waxy-to-dull brown; thin splinters can be ignited with a match |
| Geological setting | Stratified Eocene lakes (e.g., Green River Formation) and marine platforms |
| Diagnostic feature (Geology Guide) | Finely laminated chocolate-brown lakebed rock rich in solid algal kerogen; seasonal varve layers are <1 mm thick and often preserve complete fossil fishes and insects. |
| Organic component | Kerogen |
| Famous deposit | Green River Formation, western United States |
See Also
- Fossil Fuels — the energy context of oil shale
- Black Shale — a related organic-rich mudrock
- Carbon Cycle — where buried organic carbon fits