Compositional differences in biomarker constituents of the hydrocarbon, resin, asphaltene and kerogen fractions: An example from the Jet Rock (Yorkshire, UK)

Soluble and insoluble fractions of the Jet Rock sedimentary organic matter have been subjected to hydropyrolysis to evaluate their biomarker content. Hydropyrolysis is the temperature-programmed pyrolysis of organic matter in an open system fixed bed reactor under high hydrogen pressure in the presence of a sulfided molybdenum catalyst. The hydrocarbon products contain fossil lipids and biomarkers including high carbon number homologues and compounds with stereochemical configurations that are sensitive to exposure to high temperatures. Distinct differences exist between the concentrations and composition of biomarkers present in the different organic matter fractions. The sterane carbon number distribution, the dominant hopane carbon number and the proportions of tricyclic terpanes and extended tricyclic terpanes are different in each fraction, so biomarker parameters measured for different fractions are not directly comparable. The samples are of early oil window maturity and yet the quantities of biomarkers covalently bound into the kerogen fraction are of equal magnitude to the quantities that are present in the extractable aliphatic hydrocarbon fraction. However, the quantities of biomarkers in the polar resin and asphaltene fractions are two orders of magnitude lower and these two organic matter fractions contain biomarkers that are compositionally very different from the bulk of the biomarkers present in the total sedimentary organic matter. Isorenieratene derivatives were found in all fractions, indicating that green sulfur bacteria were present during sedimentation and that water column anoxia extended into the photic zone periodically if not continuously during the deposition of the Jet Rock. (c) 2005 Elsevier Ltd. All rights reserved.