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

Abstract

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.