Molecular Indicators of Organic Matter Sources
Due to their specific structures, inherited with minimal alteration from biochemicals (lipids) in organisms, the biomarkers are particularly useful in identifying the types of organic matter contributing to source rocks.
Lipids are among a wide range of molecules produced by organisms. They are effectively insoluble in water, but soluble in many organic solvents, and include various fats, waxes and phospholipids, as well as steroids and hopanoids. They generally (though not always) contain functional groups such as alcohol or acid functions.
Lipid functionality.
Although many types of lipids are common to a wide range of organisms, individual specific structures can be restricted to certain groups or even a specific species. Consequently, lipids can contain quite detailed information regarding their biological origin, and for this reason they have been termed 'geochemical fossils'.
However, for this information to be preserved in the geological record and through to the oils generated from source rocks, they must undergo minimal alteration to their structure during burial and maturation. Double bonds and functional groups such as acid and alcohol functionalities are reactive, and generally are lost, but the hydrocarbon skeleton tends to be preserved relatively intact. Thus, the resulting saturated hydrocarbon biomarkers do preserve some source-specific information.
The diagenetic reactions that convert biologically-derived lipids to geologically-stable hydrocarbon biomarkers are complex (, 1982), but the precursor-product relationships should be reasonably well understood in order for biomarkers to be unambiguously related to known biological precursors, and thus to act as markers for specific sources of organic matter.
In general, land plant derived organic matter is gas-prone, whilst algal and bacterial organic matter is oil-prone.
References
Mackenzie, A.S., Brassell, S.C., Eglinton, G. & Maxwell, J.R. (1982). Chemical fossils - the geological fate of steroids. In: Science vol. 217 pp. 491-505.
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