The occurrence and distribution of phenylated aromatic hydrocarbons and terphenyls during heating experiments of marine and lacustrine source rocks in a semi-open system

Abstract

This study aimed to explore the presence and distribution of phenyl substituted polycyclic aromatic hydrocarbons (Ph-PAHs) and polyphenyls in one Middle Devonian marine carbonate sample from the Western Canada Sedimentary Basin and one Cretaceous lacustrine shale sample from the Songliao Basin by artificially heating them in a semi-open system. GC-MS analysis of residual extracts and expelled oils showed no trace of phenylnaphthalenes, terphenyls, phenylphenanthrenes and binaphthyls in original extract and low temperature pyrolysates, but these compounds emerged at high temperatures, indicating a pyrogenic origin. Compared to the lacustrine shale, the marine carbonate consistently exhibited higher concentrations of these compounds that are likely controlled by the depositional environment. The main reaction mechanisms involve consecutive reactions of free radicals, with phenylation of unsubstituted aromatic hydrocarbons being the predominant reaction under high thermal stress. While oxidized organic matter may facilitate the formation of Ph-PAHs and polyphenyls, they can be formed from any type of organic matter. The presence of metal oxidants was found to be unnecessary, as high temperatures alone were sufficient to promote the formation of these compounds. Thus, their occurrence indicates abnormal heating regardless of the maturity of the host rocks. Phenylnaphthalenes were produced earlier than other Ph-PAHs and polyphenyls. While stable Ph-PAH and terphenyl isomers increase with simulation temperature, thermal maturity parameters derived from phenyl-PAHs and polyphenyl are insensitive to maturity except for p-TrP/m-TrP, which increases almost linearly with temperature in Brightholme shale. Ph-PAHs and polyphenyls were found to be less thermodynamically stable than conventional PAHs. Under extremely high thermal stress, they lose hydrogen atoms and transform into more stable condensed PAHs. The study provides valuable insights into the occurrence of high abundance phenyl-aromatic and polyphenyl compounds in sedimentary organic matter, enhancing our understanding of these complex organic molecules.