High temperature supercritical carbon dioxide extractions of geological samples: effects and contributions from the sample matrix

Stepwise high temperature supercritical fluid extraction (HT-SFE) has been suggested as a tool to study the speciation of hydrocarbons in geological s amples. Hydrocarbons extracted at the lower temperatures (e.g., 50 degrees C) are presumed to be part of the freely extractable fraction, while those recovered at the high temperatures (e.g., 300 and 350 degrees C) are those 'trapped' within the macromolecular organic matrix and are therefore, resis tant to desorption. The latter are released from the matrix after this unde rgoes thermally induced structural rearrangements. However, the question st ill remains if and to what extend, pyrolysis of the organic matrix can cont ribute to this fraction. This study shows, based on the characteristics of the sample matrix of two different shale samples subject to HT-SFE, that py rolytic contributions at elevated extraction temperatures are only minor un der the experimental conditions used, and that thermally induced structural changes in the macromolecular organic matrix are only partially irreversib le. (C) 1999 Elsevier Science Ltd. All rights reserved.