Inter-strata oil migration and its implications of hydrocarbon accumulation in the lacustrine shale system of the Lucaogou Formation, Jimusaer Sag, Junggar Basin

Traditionally, shale petroleum systems are considered to possess self-generating and self-storing characteristics. However, with the advancement in understanding of these systems, researchers have discovered that hydrocarbon migration is a common phenomenon within them. Given that the scale of hydrocarbon migration in shale petroleum systems is significantly smaller compared to conventional systems, identifying such migration within shales remains a challenging endeavor. Moreover, few studies have systematically discussed the impact of hydrocarbon migration on petroleum accumulation in shale systems. This study focuses on the Lucaogou Formation in the Jimusar Sag of the Junggar Basin. Building upon geochemical data reported by previous research, we have reinterpreted and further explored geochemical data, presenting a novel approach for quantitatively identifying various types of hydrocarbon migration within shale using pyrolysis and molecular geochemical parameters, which are commonly generated in the petroleum industry. Additionally, it explores the influence of hydrocarbon migration on oil enrichment. The findings reveal that various forms of hydrocarbon migration have contributed to the paradoxes in the pyrolysis (the mismatch between TOC and S₁ contents) and molecular geochemical features (anomalies of the C₂₉ αββ/(αββ+ααα) and C₂₉ ααα 20S/(20S + 20R) ratios) of the Lucaogou Formation. Based on the hydrocarbon migration status, four distinct types of reservoirs are identified within the Lucaogou Formation shale: lateral migration, carrier bed, inter-layer migration, and in situ retention. According to exploration practices, the Lucaogou shale can be divided into different sections (upper section sweet spot, middle section mudstone, and lower section sweet spot), each exhibiting distinct differences in hydrocarbon migration types. Lateral migration is commonly observed in the upper section sweet spot, whereas in the middle-section mudstone and the lower section sweet spot, the occurrence of lateral migration is limited, with inter-layer migration and in situ retention being more prevalent. In shale systems, extensive lateral hydrocarbon migration facilitates the formation of localized, high-quality exploration sweet spots, although these sweet spots bear more resemblance to conventional structural-lithologic reservoirs. Future exploration and development strategies for shale petroleum systems should consider the patterns of hydrocarbon migration when formulating plans. This study offers a framework for the detailed characterization of hydrocarbon migration types within shale petroleum systems and highlights the impact of hydrocarbon migration on shale oil enrichment.