Hydrous pyrolysis of asphaltenes and polar fractions of biodegraded oils

Abstract

Asphaltenes and polar fractions separated from oils and heavily biodegraded asphaltic bitumens, when heated under hydrous pyrolysis conditions produced pyrolysates that contained newly generated saturated hydrocarbons. The distributions (or fingerprints) of hydrocarbons in the pyrolysates showed many similar features to those in the free hydrocarbon fraction of the oils (Boscan, Venezuela; Douk Daka, Congo; Gaujacq, Aquitaine, France) but were not identical. The regular distributions, including their maturity-indicating isomerisation ratios, in the pyrolysates were very similar to those in the free hydrocarbon fractions, although some increase in the C 27:C 29 ratios was apparent. Oils that contained gammacerane as a free hydrocarbon also produced it in their asphaltene and polar fraction pyrolysates: oils that contained free 28,30-bisnorhopane did not. A C 28 hopane (29,30-bisnorhopane) and a series of hexacyclic hopanoid alkanes, tentatively identified in the free hydrocarbon fraction of an oil were also evident in its asphaltene and polar fraction pyrolysates. Similarities between the hydrocarbon distributions in asphaltene and related polar fraction pyrolysates, in all the samples analysed, confirm a genetic link between the two fractions. The pyrolysis of asphaltenes and polar fractions from heavily biodegraded asphaltic bitumens (Val de Travers and Les Espoisats, Switzerland) also generated new hydrocarbons. Differences in the hydrocarbon fingerprints and compound ratios (e.g. tricyclic terpanes:hopanes, hopanes:steranes) between the different bitumens indicated that such pyrolysis techniques may be used to distinguish bitumens from different sources, even when their free hydrocarbon distributions contain little information due to severe alteration by biodegradation. Hydrous pyrolysis of whole bitumens (asphalts), and maltene fractions, appears to be of limited use for correlation work since the microbially altered hydrocarbon distributions present in the degraded bitumens obscured many of the distributions of the newly generated hydrocarbons.