A practical approach to quantitative group type analysis of kerogens using non-dedicated pyrolysis-mass spectrometry equipment: applications and comparison with other methods

Pyrolysis-mass spectrometry (Py-MS), based on a slight modification of a non-dedicated MS, has been applied to the quantitative determination of different groups of hydrocarbons and non-hydrocarbons present in kerogen pyrolysates. The Py-MS experiments were carried out by using conventional pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) equipment where the pyrolysate enters the ion source directly via an expansion chamber. A suite of kerogens were flash pyrolysed at 700°C for 20 s and the mass spectral data of teh pyrolysates were acquired at low electron voltage (20 eV), scanning from m/z 30 to 250. Elapsed analysis time is approximately 2–3 min for each kerogen sample. Medium resolution (2500 RP) was applied to reduce the nominal mass overlap and thus enhance the identification potential. Quantification of the pyrolysate compounds was achieved by adding an internal polymer standard to the samples, accompanied by the use of response factors for each compound class, established by analysis of reference compounds. The Py-MS results have been evaluated in comparison with data from other analytical methods (Rock Eval pyrolysis, combined microscale sealed vessel pyrolysis-gas chromatography (MSSV-Py-GC), elemental analysis, nuclear magnetic resonance spectroscopy, high resolution MS of pyrolysates). The Py-MS method appears to be a reproducible and rapid technique for evaluation of sedimentary organic matter. Py-MS data and results from conventional and other analyses mentioned above were correlated using multivariate analysis. Good correlations between pyrolysate yield parameters from Rock Eval pyrolysis, MSSV-Py-GC and atomic H/C content from elemental analyses of kerogens versus hydrocarbon yields measured by Py-MS, are observed. The Py-MS pyrolysate yields and the relative aromatic hydrocarbon content clearly differentiate between kerogens with the high (>1.4), intermediate (0.5–1.4) and low H/C ratios (<0.5), when multivariate processing was applied. Kerogens with atomic H/C ratios between 0.5–1.0 are separated from those with H/C ratios between 1.0–1.4, mainly by differences in the alkylthiophene and carbon dioxide yields as well as the relative amount of different classes of hydrocarbons. In addition, there was a good correlation between Py-MS parameters and a variety of parameters obtained by other pyrolysis techniques widely in use in the petroleum industry. Thus, gas to oil ratios determined by sealed vessel pyrolysis GC correlate negatively with e.g. the relative amount of cycloalkanes present in pyrolysates as measured by Py-MS. Finally, the mean alkyl chain length in the kerogens has been estimated from pyrolysate Py-MS data based on calibration of MS-fragmentation ratios ( ) againnst known alkyl chain lengths.