Deeper Insights into Oxygen-Containing Compounds of the Mandal Formation, Central Graben, Norway

摘要

Besides the kerogen composition, the amounts of generated bitumen play a major role when assessing the petroleum retention and expulsion behavior of a source rock. High-molecular weight (HMW) products dominate the source rock extracts during early stages of generation in the total organic carbon-rich, inefficiently expelling Mandal Formation. Such gas chromatography-unresolvable, bituminous compounds have not yet been structurally described. Based on 20 immature to peak-oil mature whole rock samples from different locations of the Central Graben, a compositional comparison of seven samples of different maturity stages is drawn to the excellently expelling Posidonia Shale, Germany, of similar maturity. Electrospray ionization negative Fourier transform ion cyclotron resonance mass spectrometry allows the investigation of the acidic heteroelemental interior of the in-source retained petroleum. Rather than the quantities of bitumen, its composition seems to be important for petroleum migration efficiency and fractionation. While Posidonia Shale extracts contain slightly higher proportions of NSO constituents than extracts of the Mandal Formation, they are dominated by lower polar nitrogen compounds. Instead, Mandal Formation extracts are strongly enriched in highly polar oxygen-containing (O-x) compounds (O-2-O-6) which are more aromatic but contain longer aliphatic chains than extracts of the Posidonia Shale samples, thus increasing their molecular size and the number of polar sites. In particular, it is the C-16 and C-18 aliphatic and C-20 aromatic homologues occurring in O2+ classes which most likely represent fatty and aromatic acids linked with additional oxygen-functional groups. We suggest that these features might be specific to the Mandal Formation of the Central Graben and are related to complex interactions of geological/palaeogeographic evolution, climate fluctuations, and biological input during Upper Jurassic times. Consequently, the compositional features of petroleum generated from the Mandal Formation-highly polar, large aromatic core structures with long aliphatic chains attached-control its physical properties and cause interaction with polar phases, such as the residual kerogen or clay minerals, and nonpolar phases in the source rock.