Biological markers (biomarkers) are complex molecular fossils derived from biochemicals, particularly lipids, in once–living organisms. Because biological markers can be measured in both crude oils and extracts of petroleum source rocks, they provide a method to relate the two (correlation) and can be used by geologists to interpret the characteristics of petroleum source rocks when only oil samples are available. Biomarkers are also useful because they can provide information on the organic matter in the source rock (source), environmental conditions during its deposition and burial (diagenesis), the thermal maturity experienced by rock or oil (catagenesis), the degree of biodegradation, some aspects of source rock mineralogy (lithology), and age. Because of their general resistance to weathering, biodegrada-tion, evaporation, and other processes, biomarkers are commonly retained as indicators of petroleum contamination in the environment. They also occur with certain human artifacts, such as bitumen sealant for ancient boats, hafting material on spears and arrows, burial preservatives, and as coatings for medieval paintings.

Biomarker and non-biomarker geochemical parameters are best used together to provide the most reliable geologic interpretations to help solve exploration, development, production, and environmental or archeological problems. Prior to biomarker work, oil and rock samples are typically screened using non-biomarker analyses. The strength of biomarker param-eters is that they provide more detailed information needed to answer questions about the source-rock depositional environment, thermal maturity, and the biodegradation of oils than non-biomarker analyses alone.