Oil Tagging and Use of Biomarkers in Oil Pollution Monitoring
Biomarkers are compounds often hydrocarbons that are found in crude oils that are chemical signature of the organism from which they were produced. They are also found in other components like rock, soil and sediment extracts.
These biomarkers are often complex organic molecules whose chemical structures remained unchanged during the metabolic processes of diagenesis for oil generation.
This retention of the chemical structures becomes useful in the tracing of the crude oil source to molecules of the living organisms in which they were sourced. Biomarkers are often referred to as molecular fossils.
They also provide information of the transformation process, the deposition environment’s geochemistry, geological history and the migration path of the crude oil among others.
Typically, biomarkers in crude oil may lose the functional groups but retain virtually all the original carbon skeleton of the source materials making the structural similarity as factor explored for source apportionment.
Moreover, there are those biomarkers which are ubiquitous and prolific in many organisms and others that are only found in specific types of organisms. Hence, these are used as markers for source apportionment.
Some biomarkers only appear in parent organism after they have evolved. The presence of such biomarker compounds are used as marker for the diagnostic age of the crude oil. Additionally, some biomarkers are structurally stable in certain environments than others.
This type of biomarkers when abundant can be used to make inference on depositional or migration environments of the oil.
In some cases subtle chemical transformation from one structure to another may occur due to high temperature and the biomarker ratio to its transformed version used as a marker for maturity.
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Biomarkers may also contain heteroatoms like sulphur during early diagenesis under anoxic, marine condition; organic acids are a by-product of microbial degradation, and porphyrins containing nitrogen and metals are common biomarker in low maturity oils.
Common Biomarker Compounds
The following are some of the common biomarkers used in crude oil exploration and pollution monitoring and the implications of their presence in making inferences:
This pentacyclic triterpanoid can be further modified to produce wide array of biochemicals. Eukaryotic organisms use oxygen and synthesize tetracyclic steroids instead of triterpanoids. These are used to produce other biomarkers.
Squalene can be transformed into hopane. Hopane (compound II) is an indication that the oil is derived from a source rock deposited in an anoxic (low or no oxygen) environment and can also be used to indicate that the source rock carbonate in nature. The conversion process involved action of prokaryotic organisms.
The presence of these compounds in samples from oil pollution areas can be used as indicator of the source of the pollution and the biogeochemical process the pollutants have been subjected to.
Oleanane (compound III) is another biomarker that is synthesized and retains the main structure of squalene. Oleanane is an indication of higher, flowering plant as the raw material source and the source rock being aged source rock.
Steranes (compound V) are another class of biomarker that are ubiquitous in most oils and rock extracts. It is diagnostic for specific biota. Dinosteranes for example are produced by dinoflagellates, n- propylcholestane by chrysophyte marine algae.
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The value of the information gleaned from steranes is not from their occurrence but from their relative distribution to hopanes. For example, the ratio of hopanes- to-steranes can serve as an indicator of source.
Low ratios were found in marine sources, while high ratios were observed for lacustrine and terrigenous sources. This can be used to monitor the well and the field from where the oil pollution occurred.
R=H;CH3;C2H5;C3H7
