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 samples. Hydrocarbons extracted at the lower temperatures (e.g., 50°C) are presumed to be part of the freely extractable fraction, while those recovered at the high temperatures (e.g., 300 and 350°C) are those ‘trappedʹ within the macromolecular organic matrix and are therefore, resistant to desorption. The latter are released from the matrix after this undergoes thermally induced structural rearrangements. However, the question still remains if and to what extend, pyrolysis of the organic matrix can contribute to this fraction. This study shows, based on the characteristics of the sample matrix of two different shale samples subject to HT–SFE, that pyrolytic contributions at elevated extraction temperatures are only minor under the experimental conditions used, and that thermally induced structural changes in the macromolecular organic matrix are only partially irreversible.