Kerogen is aged and degraded carbon that was once assimilated by living organisms and is non-extractable using organic solvents; this includes, for example, the majority of molecules found in coal. In contrast, the fraction that is soluble in organic solvents is called bitumen. Kerogen represents about 90% of the organic carbon in sediments.

Kerogen can be classified by its source material. Algal kerotin derives mainly from algae, liptinitic kerogen mainly from plankton (plus some contribution from algae), and humic kerotin mainly from higher plants.