A quantitative study of aromatic hydrocarbons in a natural maturity shale sequence—the 3-methylphenanthrene/retene ratio, a pragmatic maturity parameter

The aim of this study is to illustrate the use of multivariate statistical methods in identifying potential maturity parameters. A study of the quantitative aromatic hydrocarbon results from a natural maturity sequence through a thick (> 3000 m) organic lean shale followed by multivariate statistical treatment of the data was performed. The study series consists of low TOC (ca. 0.5 wt.%) and Rock-Eval S2 (generally below 1 mg HC/g rock) shales showing a continuous maturity increase from Tmax values of 410°C to ca. 490°C. This trend is mirrored by a concomitant decrease in the hydrogen index from values of around 150 mg HC/g TOC to ca. 40 mg HC/g TOC. Based on the production indices and GC MS analysis of the saturated hydrocarbon biomarkers no indications of migrated hydrocarbons or major facies variations were found in the studied samples. In essence this shale sequence represents a near perfect natural maturity series. The 3-methylphenanthrene/retene ratio shows an exponential increase with maturity and is proposed as a very sensitive maturity indicator extending into the late oil window. This maturity ratio has since been successfully applied to both source rock extract (kerogen type III, II/III and II) and oil studies.