Kerogen composition and origin, oil and gas generation potential of the Berriasian Wealden Shales of the Lower Saxony Basin

• Published in: International journal of coal geology 2021-10, Vol.246, p.103831, Article 103831
• Main Authors: Froidl, Felix, Littke, Ralf, Grohmann, Sebastian, Baniasad, Alireza, Böcker, Johannes, Hartkopf-Fröder, Christoph, Weniger, Philipp
• Format: Article
• Language: English
• Subjects: Berriasian; Gas generation; Hydrogen generation; Kerogen structure; Pyrolysis; Wealden; XRD; XRF
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2021.103831

This study evaluates the organofacies and petroleum generation potential of kerogen of Early Cretaceous Wealden Shales in the Lower Saxony Basin, Germany. Methods applied include open-system-and Curie-point-pyrolysis, attenuated total reflectance infrared spectroscopy, x-ray diffraction and x-ray fluorescence spectroscopy. The geochemical results give a comprehensive elemental characterization of the Wealden source rocks and indicate an anoxic to oxygen-depleted lacustrine depositional setting in a rather warm climate. The kerogen composition is characterized by high abundance of aliphatic compounds with many alkylated low molecular weight oxygen-bearing compounds, but only a minor aromatic contribution. Differences between organic matter-rich lamalginite facies A and Botryococcus-bearing, less organic matter-rich facies B occur. For both facies, the total hydrocarbon generation potential (Rock-Eval HI) of facies A is higher than that of facies B. TOC-normalized cumulative hydrocarbon gas yields generated during pyrolysis-GC are similar for both facies and account for only 8–16% of the total hydrocarbon generation potential. In contrast, the cumulative yield of molecular hydrogen generated during open-system pyrolysis of facies B is twice as high than for facies A. Overall Wealden Shales generate 55 mg HC per g rock on average, they generate roughly 6 mg H2 per g TOC. Although these H2 yields are rather low compared to other organic-rich rocks or coals with aromatic kerogen structures, this study shows that detecting H2 during open-system-pyrolysis is important for the understanding of kerogen transformation processes during pyrolysis in both natural and laboratory settings. •Advanced, detailed geochemical characterization of the Wealden Shale.•An anoxic-suboxic lacustrine depositional environment and warm paleo-climate persisted.•Kerogen composed mainly of aliphatic and alkylated oxygen compounds.•GOR of Wealden shales pyrolysates range from 0.05 to 0.16.•H2 yields demonstrate that geological HC yields are underestimated by up to 6 mg H2 per g TOC.