Investigation of three natural bitumens from central England by hydrous pyrolysis and gas chromatography-mass spectrometry

Abstract

Bitumen samples from three hydrothermal mineral deposits (Windy Knoll, Derbyshire; Staunton Harold and Mountsorrel, Leicestershire) have been studied partly in an attempt to resolve the continuing controversy over whether they are biogenic or abiogenic in origin, and partly to investigate a new approach to studying such materials. A more conventional method was first employed, involving analysis of alkane fractions isolated from the bitumens using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). This showed that the Staunton Harold and Mountsorrel bitumens contained abundant biomarkers (tricyclic terpanes, steranes and hopanes) but no n-alkanes, acyclic isoprenoids and n-alkylcyclohexanes, whereas the Windy Knoll bitumen lacked biomarkers other than acyclic isoprenoids. Hydrous pyrolysis of the insoluble residue from the Windy Knoll bitumen, and of the asphaltenes from the other two bitumens, produced alkanes in the pyrolysates which were analysed using GC and GC-MS. The pyrolysates contained abundant n-alkanes, n-alkylcyclohexanes and acyclic isoprenoids in addition to the polycyclic alkanes found in the original bitumens. The distributions of steranes and hopanes in the original Staunton Harold and Mountsorrel bitumens and their pyrolysates are briefly discussed. Our results suggest that the bitumens are of biogenic origin and have been subjected to varying degrees of biodegradation and thermal maturation.