Synthetic crude oil that is extracted from oil shale by pyrolysis, or destructive distillation.
The oil obtained from oil shale cannot be refined by the methods that have been developed for crude oil, however, because shale oil is low in hydrogen and contains large amounts of nitrogen and sulfur compounds. To be made usable, shale oil must be hydrogenated and then chemically treated to remove the nitrogen and sulfur, a process too expensive to make shale oil commercially competitive with crude oil. See also kerogen , petroleum .