Enrichment model of continental shale oil in Zhanhua Sag of Jiyang Depression

The quest for enrichment model of continental shale oil in the Zhanhua Sag of the Jiyang Depression in the Bohai Bay Basin to provide reference for exploration and development requires a comprehensive approach. Therefore, this study employs rock pyrolysis, Scanning Electron Microscopy(SEM), X-Ray diffraction analysis(XRD), Nuclear Magnetic Resonance(NMR), and other experiments to analyze the conditions for shale oil enrichment and establish its patterns. The results show that favorable hydrocarbon generation potential and appropriate thermal maturation degree control “in situ enrichment”;while the storage capacity and the mobility of shale oil determine “migration enrichment.” In the process,the TOC governs the oil-generating capacity of shale with medium to large pores and microfractures serving as the main enrichment spaces and migration pathways for shale oil. Based on the deposition model, the study area can be divided into five lithofacies stages(Ⅰ-algal limestone, Ⅱ-laminated marl, Ⅲ-laminated recrystallized limestone, Ⅳ-laminated mudstones, and Ⅳ-blocky calcareous mudstones). Integrating the geochemical parameters into the sedimentary patterns makes it clear that the study area underwent two phases of hydrocarbon expulsion during the thermal evolution of source rocks(Stage Ⅱ: 3 060–3 120 m and Stage Ⅳ: 3 020–3 040 m). However, judging by the observed TOC(2% to 5.6%), thermal maturity(Ro>0.8%),S1(>2 mg/g) and OSI(>100 mg/g) as well as moderate basin size, climate, and quantity of terrestrial input,the blocky calcareous mudstones(Stage Ⅳ) have better oil-prone characteristics and potential to generate a substantial quantity of hydrocarbons at this stage. More so, with a brittleness index exceeding 60%, it exhibits favorable fracturability accounting for the main controlling factors and enrichment patterns of shale oil in the area. Hence, this study further enriches and develops the theoretical understanding of shale oil enrichment in the area, provides valuable insights for future exploration of continental shale oil in eastern China and other similar basin around the world.