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Abstract:
Asphaltene is important for hydrocarbon generation and source correlation. In this study, we compared the pyrolysis yields and phase kinetic differences of a pair of Qianjiang shale samples previously classified as source-rock and hybrid source-reservoir samples, respectively. A novel technique, thermovaporization/pyrolysis two-dimensional gas chromatography time-of-flight mass spectrometry (Tvap/Py-GC×GC-TOF-MS), was developed for source correlation purposes. This technique identified significant differences in the overall composition of pyrolysis yields from asphaltenes, resins, and thermally extractable free petroleum; therefore, asphaltene pyrolysates could not be readily correlated to those of resins or free petroleum. However, closer examination of covalently bound steranes and terpanes from resin pyrolysates revealed a high correlation between the genetically paired reservoir and source-rock samples. Given that asphaltene is the petroleum fraction least overprinted by migrated oil, its covalently bound biomarkers are likely diagnostic of its real source input. Thus, Tvap/Py-GC×GC-TOF-MS can effectively identify nonindigenous petroleum. Because fractionation led to the migrated resins and asphaltenes being less polar, progressive cracking of the migrated petroleum in the reservoir occurred with greater thermal stress than expected in the source rock. More important, the hydrocarbon generation potential of polar fractions was greater than that of kerogen in the Qianjiang shale samples. Therefore, the use of whole-rock samples rather than solvent-extracted rocks is recommended for kinetic measurements of hypersaline source rocks.
