Phenylnaphthalenes and polyphenyls in Palaeozoic source rocks of the Holy Cross Mountains, Poland

Source rocks from a marine depositional setting from Palaeozoic formations in the Holy Cross Mountains region (Midlands, Poland) were analysed for aromatics using capillary gas chromatography–mass spectrometry (GC–MS). The occurrence of two novel series of aromatic hydrocarbons in these sediments, namely phenyl derivatives of fused ring polycyclic aromatic hydrocarbons (PhPAH) and polyphenyls (PPh), was established. Furthermore, the methyl derivatives of these compounds were also present. The chromatographic behaviour of the triaromatic members of the series, i.e. two isomers of phenylnaphthalene (1-PhN and 2-PhN) and three isomers of terphenyl (o-TrP, m-TrP and p-TrP) was evaluated using authentic standards. The isomeric composition of the phenylnaphthalenes (PhNs) and terphenyls (TrPs) was found to depend on thermal maturity. In the lower maturity samples abundances of 1-PhN and o-TrP are higher. Increase in sample maturity is indicated by an increase in the relative abundance of 2-PhN as well as m-TrP and p-TrP. Three thermal maturity parameters of the organic matter based on the relative abundances of the PhN and TrP isomers are proposed: PhNR=2-PhN/1PhN, TrP1=p-TrP/o-TrP, and TrP2=(m-TrP+p-TrP)/o-TrP. In general their values positively correlate with the vitrinite reflectance (Ro) and MDR, while correlation of the other biomarker maturity parameters such as the Ts/Tm ratio are less apparent. The compounds above are believed to be geochemical products from unknown precursors. A potential geochemical process of formation for the o-TrP is proposed, and involves initial preservation of carbohydrates in sediments through sulfur incorporation, further dehydration, cyclisation and aromatisation to respective furan and/or thiophene derivatives, and finally reductive elimination of oxygen and sulfur in the furan and thiophene products, respectively.