Trace elements
Applications
The results of trace metal analysis can be applied to correlation of oils and determining hydrocarbon loss during biodegradation, as the trace metals are considered highly conservative.
Perhaps the most useful information is obtained from Ni and V, which provide clues to the depositional environment of an oil’s source rock, particularly when coupled to S content.
Sample requirements
Solvent-free total rock extract (EOM), solvent-free SARA fractions, topped/untopped oil and pre-extracted kerogen concentrate can be analysed.
Analytical procedure
The APT set up involves analysis by Inductively Coupled Plasma Atom Emission Spectrometry (ICP-AES). The liquid samples are dried to powder and introduced to a plasma at ~8000°C. The intensity of emissions in the 180–770 nm range is recorded and converted into concentrations.
Potential problems
Normally, V:Ni ratios in source rocks do not seem to be affected by post-diagenetic processes. However, if there are large amounts of porphyrins present, which chelate Ni and V, there can be a significant maturity-related influence on V:Ni ratios, because of the changes in metal chelation. Such examples are relatively rare, however.
References
Lewan M.D. (1984) Factors controlling the proportionality of vanadium to nickel in crude oils. Geochimica et Cosmochimica Acta 48, 2231–2238.