Evaluating transformation of marine kerogens from Rock-Eval measurements: B. Confirmation of a scaled thermal maturation path by natural samples

Kerogen transformation with thermal maturation is often assessed using open system Rock-Eval pyrolysis. The decrease in hydrogen index (HI, mg HC/g TOC) with increase in Tmax of kerogen can be viewed as a maturation path that describes kerogen transformation. However, natural heterogeneity causes the initial properties (i.e., HI_initial and Tmax_initial) of immature kerogens to vary. This heterogeneity, as well as instrument variability, can lead to distinct maturation paths on the classical HI-Tmax plot, especially when comparing sulfur-lean (Type II) and sulfur-rich (Type II-S) marine kerogens. For these reasons, a scaled maturation path (HI_relative-ΔTmax) was previously formulated based on laboratory maturation data. The HI_relative-ΔTmax maturation path considers the HI_initial and Tmax_initial of each kerogen, shows their relative change with thermal maturation, and reveals two distinct maturation paths for Type II and II-S kerogens. In this contribution, natural maturation data for Type II and II-S kerogens were compiled and used to test the applicability of the HI_relative-ΔTmax maturation path. The compilation is based on literature data for 16 datasets of natural thermal maturation, comprising eight different source rocks, including the Barnett, New Albany, Monterey, Ghareb (Ein Zeitim), Nordegg, Upper and Lower Eagle Ford, Bakken and Posidonia formations. As long as the immature HI_initial and Tmax_initial can be adequately estimated, the data for most of these source rocks fall within the expected kerogen type on the HI_relative-ΔTmax plot, supporting the usefulness of the scaled maturation paths. Moreover, the scaled HI_relative-ΔTmax maturation paths minimize the impact of the heterogeneity of different kerogens and instrumental variability that can affect the measured Rock-Eval parameters. Hence, the HI_relative-ΔTmax scale is a tool to evaluate the transformation ratio of marine kerogens better.