Solid bitumen and vitrinite reflectance suppression explored using hydrous pyrolysis of artificial source rock (2021)

The most commonly used parameter for thermal maturity calibration in basin modelling is mean random vitrinite reflectance (Ro). However, Ro suppression, or lower than expected Ro, has been noted in samples containing a high proportion of liptinite macerals. This has been empirically demonstrated using hydrous pyrolysis experiments of artificial source rock containing various proportions of thermally immature Wyodak-Anderson coal and liptinite-rich kerogen from the Parachute Creek Member of the Green River Formation. Analysis of samples pyrolyzed at 330°C for 72 h demonstrates that the Ro values of both vitrinite and solid bitumen are suppressed, where the degree of suppression increases with increasing amounts of liptinite. Solid bitumen Ro (BRo) is suppressed to a greater degree than Ro. Raman and micro-Fourier transform infrared (µ-FTIR) analyses were performed to investigate the mechanisms responsible for this suppression. Raman maturity proxies show decreased aromaticity in samples with suppressed Ro, particularly in solid bitumen, with aromaticity decreasing as the proportion of liptinite increases. The µ-FTIR proxy for aliphatic chain length and/or branching ratio is relatively static in solid bitumen yet increases slightly in vitrinite as the proportion of liptinite increases. These spectroscopic results suggest slightly different Ro suppression mechanisms for vitrinite and solid bitumen, with reduced C-C bond cleavage and marginally reduced aromaticity in vitrinite with suppressed Ro, and strongly reduced aromaticity and C-C bond cleavage in solid bitumen with suppressed Ro. These results support the hypothesis that the generation of free radicals during maturation slows aromatization and highlight the disadvantages of using solid bitumen Ro for maturity calibration in liptinite-rich samples. Furthermore, our results indicate using Raman data obtained from liptinite-rich samples may also result in suppressed maturity indicators, particularly if the macerals are not identified prior to analysis.