Change in diagnostic ratios in expelled oils and residual extracts during semi-open pyrolysis experiments of an organic-rich shale

Abstract

In order to investigate the effectiveness of diagnostic ratios in polycyclic aromatic hydrocarbon (PAH) source discrimination, semi-open pyrolysis experiments have been performed on an organic-rich, immature shale from the Winnipegosis Formation in southeastern Saskatchewan, Western Canada Sedimentary Basin. The concentrations and distributions of PAHs in expelled oils and residual extracts change drastically with increasing pyrolysis temperatures. The difficulty and inconsistency commonly encountered by using diagnostic ratios for PAH source identification in environmental samples seem to be rooted in the great variation of the diagnostic ratios themselves under different formation temperatures. No single diagnostic ratio allows a simple segregation of PAHs into petrogenic or pyrogenic sources. Some diagnostic ratios such as anthracene/phenanthrene and benz[a]anthracene/chrysene compound pairs are mostly effective for low-temperature pyrolysis, whereas indeno[1,2,3-cd]pyrene/benzo[ghi]perylene, aromatic hydrocarbon ring number distribution and degree of alkylation are mainly valid for high-temperature pyrolysis. The diagnostic ratios based on fluoranthene/pyrene, benzo[bk]fluoranthene/benz[a]pyrene compound pairs enjoy limited validity over a narrow pyrolysis range, whereas parameters derived from aromatic hydrocarbon ring number distribution, degree of alkylation and 1,7-/(2,6- + 1,7-dimentylphenanthrene) may be undistinguishable between petrogenesis and low-temperature pyrolysis. The apparent temperature-related variability must be taken into account when using the diagnostic ratios for source identification purposes. Multiple molecular markers need to be carefully selected to confirm the results obtained with PAH diagnostic ratios. Mechanical use of diagnostic ratios most likely leads to misinterpretation of environmental samples.