Hydrocarbon speciation in ancient sediments studied by stepwise high-temperature supercritical carbon dioxide extraction

Results on the application of high-temperature supercritical fluid extractions (HT-SFE) in the study of the speciation of hydrocarbons (HCs) in geological samples are presented. Two different shale samples were extracted by stepwise SFE at temperatures between 50 and 350 degrees C. Highest relative extraction yields for aliphatic and aromatic hydrocarbons were obtained primarily at both end members of the range of temperatures used. This observation indicates differences in the speciation of hydrocarbons in ancient sediments; those HCs easily extracted at lower temperatures can be considered ''free HCs'', while those released only at elevated temperatures are ''trapped'' within, or ''loosely bound'' to the matrix. Aliphatic HCs were observed to be mainly associated with the ''free'' fraction, whilst the aromatics were more abundant in the ''trapped'' fraction. The release of trapped HCs was confirmed by the inversion of molecular thermal maturity parameters of the hopanes for the highest temperatures. Rock-Eval pyrolysis determinations of HT-SFE treated samples showed that significant pyrolysis was not observed for the experimental conditions used. (C) 1997 Elsevier Science Ltd.