Biomarkers are complex organic molecules whose chemical structures can remain largely unchanged during the processes of diagenesis and oil generation. This often allows them to be traced back to the original molecules in once living organisms. For this reason they are often called molecular fossils.
The distribution of the various saturate and aromatic biomarkers present in geological samples can be used for oil to oil, and oil to source rock correlations. If the source rock is unavailable it is still possible to infer certain source rock characteristics such as source input, depositional environment, thermal maturity, geological age and lithology for example, from the analysis of the biomarkers in crude oil.
Some biomarkers are prolific in many organisms while others are only found in specific types of organisms and hence these can be used as markers of source input. Additionally, some biomarkers do not appear until their parent organism has evolved, and therefore the presence of these compounds can be used as an age diagnostic marker. Similarly, some biomarker structures are more stable in certain depositional environments than others and hence their abundance can be used to infer depositional environments. Also, subtle chemical transformation from one chemical structure into another can occur due to thermal stress and hence the ratio of these biomarkers can be used as a maturity marker.
About Biomarker Analysis
Biomarkers are generally minor components of a crude oil or rock extract and highly selective and sensitive methods of analysis employing gas chromatography – mass spectrometry (GC-MS) are necessary for their measurement. Typically a crude petroleum sample is separated into a saturate and aromatic fraction prior to biomarker analysis and then both fractions are analyzed independently. This provides greater resolution with less interference and allows for the determination of biomarker concentrations down to the low ppm and ppb levels. It is typical for a single analysis to monitor several hundred biomarkers. If additional selectivity is needed to resolve more complex biomarker distributions a tandem mass spectrometry technique (GC-MSMS) is used to analyze and distinguish biomarkers further.
These compounds have molecular structures based on the carbon skeleton of diamond and are highly stable. They are often used to monitor and quantify the thermal cracking of oil to gas. Additionally they are used to determine if a high maturity petroleum charge has been mixed with a lower maturity petroleum charge.
We use gas chromatography – mass spectrometry to measure C8 through C11 alkylbenzenes in crude oils. This technique is used in oil-oil correlation studies such as reservoir compartmentalization and production allocation. Consequently, alkylbenzene analysis can distinguish families of oils that may be hard to distinguish using other techniques such as high resolution gas chromatography.
Application & Data Examples
M/Z Ion 192 spectrum displayed using the ChromEdgeTM PeakView software