Geochemical parameters for evaluating shale oil enrichment and mobility: A case study of shales in the Bakken Formation, Williston Basin and the Shahejie Formation, Jiyang Depression

Geochemical parameters serve as important indicators for shale oil enrichment and mobility evaluation. Using pyrolysis experiments of minerals/post-extraction shale residues mixed with oil/alkanes, as well as the comparison and forward modeling of pyrolysis parameters of oil-generating shales before and after shale extraction using organic reagents, we perform a case study of the oil-generating shales in the Bakken Formation of the Williston Basin and the Paleogene Shahejie Formation of the Jiyang Depression in the Bohai Bay Basin. Accordingly, the characteristics of shale oil enrichment and corresponding responses of geochemical parameters are analyzed to investigate the lower limits of the oil saturation index(OSI) and productivity index(PI) as indicators of shale oil enrichment and mobility. The results reveal that conventional Rock-Eval pyrolysis on oil in rock generates both free hydrocarbons(S₁) and pyrolyzed hydrocarbons (S₂), affecting the S₂ curve's peak style and peak temperature for hydrocarbon pyrolysis (T_max). Crude oil enrichment in shales, thereby, leads to anomalously high values of OSI and PI and anomalously low T_max values, with these three parameters in coordinated variation. Oil-rich shales with a low organic matter content exhibit more pronounced anomalies in pyrolysis parameters. In contrast, for organic-rich shales, their OSI and PI values tend to stabilize after the total organic carbon (TOC) content reaches a specific threshold. The liquid-solid interactions in shales affect the lower limits of indicators for hydrocarbon mobility. A systematic analysis of the Bakken Formation in one well in the Williston Basin and the Shahejie Formation in three wells in the Jiyang Depression indicates that the lower limits of OSI for the enriched shale oil and its mobility fall below 50 ~ 75 mg/g, corresponding to PI values of 0.12 ~ 0.20. The lower limit of OSI for the enriched shale oil and its mobility is closely associated with shale lithology and crude oil properties, with carbonate-rich shales exhibiting lower limits of OSI for shale oil mobility.