Laminated shale oil system mobility and controlling factors of the Paleogene Shahejie Formation: Evidences from T1-T2 NMR experiments, multi-temperature pyrolysis and confocal laser scanning microscopy

Characterizing the mobility of shale oil is crucial when attempting to understand petroleum accumulation, oil recovery enhancements, as well as petroleum resources assessments for the unconventional reservoirs. That study especially in the lacustrine environment remain, however, limited and are least understood due to laminae development heterogeneity. A total of 12 core samples from the third member of the Shahejie Formation of the Bohai Bay Basin were employed to study the shale oil mobility occurred in the lacustrine strata through nuclear magnetic resonance (NMR), Rock-eval pyrolysis, and multi-temperature pyrolysis experiments. Laminas consist of bright laminae that contain high contents of quartz, feldspar, calcite, and dolomite, which are frequently intercalated with dark laminae dominated by clay and organic matter. The movable oil contents estimated by NMR (3.96-7.05 mg/g) is higher than that defined by both Rock-eval pyrolysis (0.57-3.29 mg/g Rock) and multi-temperature pyrolysis methods (0.36-5.89 mg/g Rock). This divergence is likely due to the volatility of light hydrocarbons, resulting in the NMR estimations being considered the most reliable. The movable oil content that exists within bright laminae are typically higher than that to which resides within dark laminae due to the oils generated in dark beds migrating from the dark laminae segments to bright beds. This in turn leads to the increase of the heavy components, asphaltenes, in the kerogen of the dark laminae and that of light components in the bright laminae. Combination with the reservoirs characterizations results, it suggests that the pore sizes and structures may play a critical role in the aforementioned differences in movable oil content between dark and bright beds, where bright laminae is characterized by type H2 with the diameters of 100-500 nm, while dark laminae belong to type H3 with a smaller diameter of 20-200 nm. As a result, higher levels of movable oil were trapped within inorganic pores with a diameter of 100-500 nm in bright laminae, while heavier compositions with the characteristic of low mobility were absorbed in the kerogen. These findings have a significant impact on existing knowledge around the mechanisms of shale oil enrichment and the exploitation of sweet spots.