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Abstract

Xanthone and its alkylated homologues were determined to be present in 64 of 69 investigated oils from offshore Norway (Central Graben, Viking Graben, Haltenbanken). This is the first description of xanthones in crude oils. These compounds were identified by comparison with authentic standards by coinjection, based on mass spectra and relative retention times on two different GC columns. The elution order of the four methylxanthones was established as 1-4-2-3 by using a BPX-5 column. About 2/3 of the oils contain concentrations of xanthone lower than 5 [mu]g/g oil, but some oils are clearly enriched in the parent compound. The highest amount of xanthone in the sample set was 38 [mu]g/g oil. The relative abundance of xanthone, the sum of the methylxanthones and the sum of the C2-xanthones is mainly controlled by maturity. Partitioning processes may effect changes in the distribution of methylxanthones as observed for a biodegradation sequence from the Gullfaks field. Molecular dynamics calculations support the observation of a better preservation of the shielded isomers (1- and to a lesser extent 4-methylxanthone) in the oil phase compared to the non-shielded isomers (2- and 3-methylxanthone). The ratio of these two different isomer groups may be useful as an indicator of secondary migration distances, as demonstrated for an oil sequence from the Tampen Spur and Haltenbanken oils. However, biodegradation could also cause an increase of the shielded isomers relative to the non-shielded isomers due to sterical hindrance by the methyl groups restricting access to the oxygen functionalities. The origin of xanthones in crude oils and source rocks is not known but they could be generated as diagenetic products, formed by oxidation of xanthenes in the reservoir, or originate by geosynthesis from aromatic precursors.