Polynuclear aromatic hydrocarbons (PAH) as indicators of the source and maturity of marine crude oils
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abstract
Whole oil GC-MS was used to characterize selected polynuclear aromatic hydrocarbons (PAH) in a suite of marine crude oils derived from source rocks deposited under different conditions. The selected PAH consist primarily of 2- and 3-ring aromatic hydrocarbons, including naphthalenes (to C4), fluorenes (to C3), phenanthrenes (to C4) and dibenzothiophenes (to C3), as well as several 4- and 5-ring compounds. Concentrations of the selected PAH range from 2294 to 129,170 ppm and typically comprise between 1% and 20% of the total C12+ aromatic fraction. PAH compositions in all source types are dominated by naphthalenes. These compounds comprise between 41.9% and 88.9% of the total PAH measured. The greatest difference between oils lies in the relative abundance of dibenzothiophenes. Their abundance rivals that of naphthalenes in the carbonate oils (up to 41.7% of total PAH) but is exceedingly low in the paralic oils (< 2.1% of total PAH). These differences, which reflect differences in clastic content, Eh-pH, and availability of reduced sulfur in the source rock depositional environment, are captured in the ratio DBT/P, calculated as the sum of all dibenzothiophenes relative to phenanthrenes DBT/P is greater than 10 in most carbonate-sourced oils but ranges between 0.08 and 0.18 in the paralic-sourced oils. Siliciclastic-sourced oils are intermediate, with values ranging from 0.14 to 0.87 (average 0.40). All oils show a predominance of alkylated PAH homologues over the unsubstituted parent, however, the maximum degree of alkylation (the most abundant alkylated homologues within any given PAH series) varies. Carbonate-sourced oils show a high degree of alkylation, often maximizing at C3 or C4, while paralic-sourced oils generally exhibit a lower degree of alkylation in which the C1 or C2 isomers predominate. Siliciclastic-sourced oils are intermediate, with alkylation maximizing between the C2 and C3 isomers. We propose that these alkylation trends reflect differences in the PAH precursor moieties in source kerogens and/or variations in the thermal history of the source types, i.e. milder thermal history for the carbonate oils relative to paralic and siliciclastic oils.
