On the Atypical Petroleum-Generating Characteristics of Alginite in the Cambrian Alum Shale

Abstract

A series of seven Alum Shale samples has been analyzed by pyrolytic, petrologic, and spectroscopic methods. All contain alginitic kerogens at various levels of alteration. Kerogens from four samples appear immature to early mature, whereas three samples are more mature, especially one which is very overmature. Decreasing pyrolysate yields per unit weight of carbon characterize the series. Alum Shale alginite is unlike any other alginite we have studied. Aliphatic carbon is present in all but the highly overmature kerogen, yet only a small proportion of aliphatic moieties is released during high-temperature pyrolysis. Furthermore, all the pyrolysates are very aromatic. The unusual structure of Alum Shale alginite might have been brought about by uranium-related alteration effects. Alternatively, or additionally, its structure might have been directly inherited from biopolymeric or carotenoid-derived precursors in algae and bacteria. Mainly gas and condensate are evolved from Alum Shale kerogen during simulated maturation, and a high proportion (more than 30%) of its originally labile components are progressively and cumulatively converted to “dead carbon”. Available data indicate that the Alum Shale behaves similarly during natural maturation.