CHARACTERIZATION OF ASPHALTENES BY PYROLYSIS-FIELD IONIZATION MASS-SPECTROMETRY - SOME OBSERVATIONS

Pyrolysis-field ionization mass spectrometry (Py-FIMS) has been applied in an exploratory study to petroleum asphaltenes derived from (a) oils sourced from different source rock types, and (b) reservoir core extracts from tar mat containing petroleum reservoirs. Between 50 and 75% of the asphaltenes were volatilized under the conditions used (heating from 50 to 750-degrees-C at 1-degrees-C/s in a high vacuum of 10(-3) Pa). Weight average molecular weights of the asphaltene pyrolysates, as determined by Py-FIMS, range from 451 to 704 Dalton, number average molecular weights ranging from 394 to 599 Dalton. Molecular weights of asphaltene pyrolysates (from Py-FIMS) correlate weakly with molecular weight data of asphaltenes determined in solution by gel permeation chromatography. The Py-FIMS pyrolysate data were compared with elemental analysis, quantitative flash pyrolysis and gel-permeation chromatography molecular weight data for the isolated asphaltenes and with biomarker parameters determined from the saturated hydrocarbon fractions of the respective oils or core extracts. Very general inverse correlations between asphaltene pyrolysate molecular weight as determined by Py-FIMS and routinely determined biomarker maturity parameters were found, tentatively suggesting maturity dependencies of the molecular weights of asphaltenes and their pyrolysates, i.e. generally decreasing weight average molecular weight with increasing thermal maturity.