Hydrocarbon compositions of bitumens associated with igneous intrusions and hydrothermal deposits in Britain

Abstract

The possible modes of formation and thermal history of various Palaeozoic and Precambrian bitumens associated with hydrothermal deposits and igneous intrusions have been evaluated by molecular organic geochemistry. The aliphatic hydrocarbon compositions of extracts of such bitumens from a number of localities have been determined by GC and GC-MS procedures. The stereochemical configurations and low abundance of their steranes, hopanes and degraded hopanes, resemble those characteristic of various mature, weathered and biodegraded crude oils. Comparisons of the distributions of tricyclic terpanes, hopanes, demethylated hopanes, 8,14-secohopanes and steranes reveals a close correspondence between all the bitumens from the Welsh Borderlands; correlations suggestive of a common source rock. The occurrence and abundance of 8,14-secohopanes in the bitumens is consistent with their generation as thermal degradation products of hopanes. A similar mode of formation is proposed for the other, unidentified secohopane in a bitumen from the Midland Valley of Scotland. Overall, the hydrocarbon compositions of the bitumens are compatible with their ultimate biological origins, and subsequent thermal experience.