Hydrocarbon speciation studies in ancient sediments by high temperature supercritical carbon dioxide extraction

摘要

Results on the application of high-temperature supercritical carbon dioxide extraction (HT-SFE) to the study of hydrocarbon (HC) speciation in geological samples are presented. Ancient sediments were treated by stepwise extractions, and by using a fresh sample each time. SFE temperatures ranged between 50 and 350°C. Individual analytes showed to be speciated in different ways throughout the solid matrix because different fractions are extracted under different energy conditions. Aromatic HCs appear to have a stronger association with the matrix than the aliphatics due to their higher polarity and molecular planarity. No evidence of geosynthesis of alkylaromatics during HT-SFE was obtained. Tri- and mono-aromatic steranes were totally extracted at relatively low temperatures while hopanes and steranes show a small fraction strongly interacting with the macromolecular organic structure possibly due to u22trappingu22 processes. The release of u22trappedu22 HCs was confirmed by the u22maturity inversionu22 effect observed for the molecular distribution of hopanes at the highest extraction temperatures. Branched compounds like pristane and phytane showed to have a weaker association with the organic matrix if compared with straight chain analytes. Fresh sample extracted by SFE at different temperatures confirmed the results obtained for the stepwise procedure. Rock-Eva! Pyrolysis and Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS) from SFE pre-extracted sediments showed that HCs extracted at the highest temperature levels are not dominated by pyrolysis products and may have been released from trapped conditions, through irreversible thermally induced structural changes of the matrix and/or rearrangements of the macromolecular organic matter possibly via sulfur bond cleavage. No significant changes of the matrix were apparent after HT-SFE as suggested by Rock-Eval Pyrolysis and Py-GC/MS experiments, as well as from Electron Scanning Microscopy (ESM) determinations.