POLYNUCLEAR AROMATIC-HYDROCARBONS (PAH) AS INDICATORS OF THE SOURCE AND MATURITY OF MARINE CRUDE OILS

Whole oil GC-MS was used to characterize selected polynuclear aromatic hydrocarbons (PAH) in a suite of marine crude oils derived from sourc e rocks deposited under different conditions. The selected PAH consist primarily of 2- and 3-ring aromatic hydrocarbons, including naphthale nes (to C-4), fluorenes (to C-3), phenanthrenes (to C-4) and dibenzoth iophenes (to C-3), as well as several 4- and 5-ring compounds. Concent rations of the selected PAH range from 2294 to 129,170 ppm and typical ly comprise between 1% and 20% of the total C-12+ aromatic fraction. P AH compositions in all source types are dominated by naphthalenes. The se compounds comprise between 41.9% and 88.9% of the total PAH measure d. 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 i n the paralic oils (< 2.1% of total PAH). These differences, which ref lect differences in elastic content, Eh-pH, and availability of reduce d sulfur in the source rock depositional environment, are captured in the ratio DBT/P, calculated as the sum of all dibenzothiophenes relati ve to phenanthrenes. DBT/P is greater than 1.0 in most carbonate-sourc ed 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 de gree 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 C-3 or C-4, while paralic-sourced oil s generally exhibit a lower degree of alkylation in which the C-1 or C -2 isomers predominate. Siliciclastic-sourced oils are intermediate, w ith alkylation maximizing between the C-2 and C-3 isomers. We propose that these alkylation trends reflect differences in the PAH precursor moieties in source kerogens and/or variations in the thermal history o f the source types, i.e., milder thermal history for the carbonate oil s relative to paralic and siliciclastic oils. Copyright (C) 1996 Elsev ier Science Ltd