Abundance of phytanyl and n-alkyl toluenes in petroleum from the Lower Saxony Basin (Germany) provides information on maturity, palaeosettings and sources

Abstract

Alkyl benzenes and toluenes are common in petroleum and shales. In addition to the series of n-alkylated benzenes and toluenes, the phytanyl toluene (and −benzene) is often abundant. In the present study, n-alkyl toluenes and phytanyl toluene were detected in more than 180 petroleum oils with different maturation histories and from two different source rocks, the Jurassic Posidonia Shale and the Cretaceous Wealden Shale (from the Lower Saxony Basin; Germany). A novel ratio of C₂₂-C₂₄ meta-substituted n-alkyl (combined in the ratio as “n”) to phytanyl (“iso”) toluenes is proposed as a thermal maturity indicator. Our data show that the relative proportion of phytanyl toluene to n-alkyl toluenes systematically decreases with maturity and therefore the “n/(n + iso)” toluene index increases. The relative change is due to both the processes of catalytic formation of n-alkyl toluenes and destruction of phytanyl toluene. The applicability of this ratio was confirmed using closed gold capsule pyrolysis maturation experiments with Posidonia Shale (for a maturation range from EASY%Ro of 0.78 to 1.4 %). The correlation is most pronounced in petroleum produced from the Posidonia Shale. For oils from the Wealden shales, the correlation is less clear due to the generally low phytanyl toluene contents. Comparisons of the “n/(n + iso)” toluene index with maturity-dependent biomarker ratios indicated its potential to record thermal maturity over a wide range. The occurrence of phytanyl toluene also appears to record a poorly understood (microbial) source, which was more abundant in the marine-euxinic depositional system of the Posidonia Shale than that of the brackish-marine Wealden shales.