oa N-Alkyl and Phytanyl Toluenes as Markers for Maturity and Source of Petroleum in the Lower Saxony Basin (N Germany)

Alkyl benzenes and toluenes are common constituents in thermally matured shales, kerogens and petroleum. In addition to the series of n-alkylated benzenes and toluenes formed by catalytic kerogen cracking, especially contents of the single isoprenoid phytanyl toluene (and-benzene) are often elevated. Sources of these compounds and the geochemical information from their occurrence are still under discussion and a subject of paleoreconstruction- and applied petroleum research (e.g., Solli et al., 1980 ; Sinninghe Damsté et al., 1993 ; Moura et al., 2019 ). Recently, for example, enhanced concentrations of phytanyl toluene in rocks of the Permian-Triassic boundary, a critical interval in Earth History, have been demonstrated ( Grotheer et al., 2017 ). In the study presented here, variations in contents of n-alkyl toluenes and phytanyl toluene were analysed for more than 100 oils of two different petroleum source rocks, the Lower Jurassic Posidonia Shale and the Lower Cretaceous Wealden shales, both from within the Lower Saxony Basin (Northern Germany). Based on this dataset, a ratio of C22–C24 meta-substituted n-alkyl toluene to phytanyl toluene was developed and its changes with maturity and source rock characteristics are discussed. The ratio had to be based on combined abundances of alkylated toluenes in both, the aliphatic and aromatic fractions, as due to the intermediate polarity, the compound class eluted into both fractions with differing phytanyl/alkyl toluene ratios.