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Low-molecular-weight-aromatic hydrocarbons (LMWAHs), like alkylbenzenes (ABs), alkylnaphthalenes (ANs) or alkylphenanthrenes (APs) are ubiquitous constituents of petroleum and sedimentary organic matter (OM). LMWAHs can be formed by geosynthetic processes, resulting in a host of alkylated, dealkylated and isomerised components. The main diagenetic processes are isomerisation and trans-alkylation reactions (methyl shift, methyl transfer) and these have been shown to be highly sensitive to thermal maturity. Some AHs are also associated with specific microbial and land plant sources, especially in sediments and crude oils of relatively low maturity, where the major isomers may be directly formed from natural products. In the present work, we report d¹³C of individual LMWAHs (ABs, ANs, APs) in crude oils and sediments of varying age, facies type and thermal maturity in order to understand the influence of both thermal maturity and source on the d¹³C of LMWAHs.

Overall , d¹³C of LMWAHs range from -23‰ to - 30‰ whereas, by comparison, d¹³C values of n-alkanes range from -27‰ to -28‰. For most of the oils studied, d¹³C of AHs show a ¹³C enrichment as the degree of methylation decreases. This positive trend is observed for all the aromatic groups investigated, ABs, ANs and to a lesser extent APs. Interestingly, the thermal maturity is also reflected in d¹³C of individual AHs. The immature oils show the greatest enrichment with decreasing degree of methylation. These observations enable us to have a closer look at the fate of alkylated LMWAHs during diagenesis. Methyl groups bound to AHs nuclei largely take part in methyl-shifts and methyl-transfer reactions between the different aromatic components within oil, thus forming a ‘methyl pool'. With time and increasing temperature, the above reactions could lead to the formation of thermodynamically more stable compounds such as methane. Methyl groups can be removed from the ‘methyl pool' leading to a decrease in the degree of methylation of AHs. Isotopically, it is thought that the lighter methyl groups are more likely to undergo these reactions than the isotopically heavier methyl groups because the activation energy for ¹²C containing molecules is lower than that of molecules containing ¹³C. Thus it is suggested that the isotopically lighter methyl groups will be preferentially removed from the ‘methyl pool' as thermal maturation increases, thus leading to an isotopically depleted ‘methyl pool'. We propose an isotopic fractionation associated with the methyl transfer mechanisms via the ‘methyl pool' which would be directly related to thermal maturity.

Attempts have been previously made to classify oils to their geological age and origin. Most of the previous studies have used molecular characteristics and d¹³C of bulk parameters (e.g. d¹³C of saturates and aromatics). Previous applications of bulk stable isotopes for constraining the geologic ages of crude oils and their respective petroleum basins have only been met with limited success. This is a consequence of the large range of bulk d¹³C values for crude oils from any specific time interval. In general, oils tend to become enriched in ¹³C with decreasing geological age. Those changes are thought to be independent of the source rocks from which the oils are derived. In the present study, we have investigated the d¹³C of individual aromatic hydrocarbons to obtain insight into geological age, petroleum basin and source. d¹³C of individual aromatic hydrocarbons from five crude oils of varying ages and from different WA basins have been determined. d¹³C of individual end-members of land plant-derived and algal-derived PAHs reflect age and source (terrestrial versus marine). A refined data set has been generated and a new tool for establishing age has been obtained.

Pathways leading to CH₄ generation involving C_18- products during thermal degradation of 1,2,4-trimethylbenzene i.e. a model compound representative of methylated monoaromatic hydrocarbons of oil have been established in collaboration with IFP. Pyrolysis experiments (395 to 450°C, 3 to 648h) were performed and all pyrolysis fractions were recovered. Products up to C₁₈ were individually identified and quantified by GCMS and GC to develop a mechanistic kinetic model of 122 reversible reactions involving 47 species and more than 200 kinetic parameters. The model was then used to compare the relative contributions of specified CH4 generation pathways under laboratory conditions (high temperatures-short reaction times) and geological conditions (low temperatures-short reaction times).This is the first time in that a mechanistic model is validated for conversions of the reactant as high as 70%. We have also established a lumped kinetic model for basin simulation without any controversy on the reaction scheme when extrapolated to geological condition. Furthermore, the isotopic fractionation formalism has been implemented in the model to improve prediction of d¹³C(CH₄) in deeply buried reservoirs where thermal degradation of oil occurs.