Microscopic Occurrence Characteristics and Sweet Spot Prediction of Shale Oil in Fengcheng Formation, Northwest Junggar Basin

The Fengcheng Formation (P₁f) is the main source rock in the northwest Junggar Basin, and is the key horizon for shale oil exploration and development in the future. Previous studies mainly focus on the sedimentary environment, petrology, reservoir pore structure and physical properties of P₁f but the relevant research on the microscopic occurrence characteristics of shale oil is relatively weak. To reveal the oil content and micro-occurrence characteristics of shale oil in different lithofacies of P₁f in northwest Junggar Basin, this paper used 81 mud/shale samples from 4 wells to effectively avoid the errors and accidents caused by single experiments through the joint application of field emission scanning electron microscope (FE-SEM), nuclear magnetic resonance (NMR) and stepwise extraction experiments. And systematically characterizes the occurrence state across six lithofacies of shale oil in the study area. The experimental results show that the combined application of SEM and NMR method can effectively identify shale oil occurrence states. Free oil exhibits a linear distribution of T₂＞2ms, T₁/T₂≈10, mainly residing in pores and microfractures ＞50nm. Adsorbed oil shows a zonal distribution of T₂＜2ms and T₁/T₂≈100, occurring as oil films on clay mineral surfaces and within pyrite aggregates. These findings can provide technical support for distinguishing sweet spot shale oil. Combined with the application of NMR method and three-step light/heavy hydrocarbon extraction, and combined with the development characteristics of different lithofacies of shale oil reservoirs, it is clearly pointed out that the calcareous mudstone/shale is the most favorable lithofacies of P₁f, in the study area, followed by dolomitic mudstone/shale (with the strongest oil capacity) and the dolomitic tuff, Among them, the third member of P₁f in the area of Well Hashenxie 1 has high oil content (TOC 1.5%-5.13%, S₁ 1.2-4.02mg/g, free oil accounting for 20%-40%), high porosity (porosity 4.2%-6.58%, development of intragranular pores, intergranular pores and micro-fractures), high fracability (brittle mineral content 55%-71%, hardness 450-650, good compressibility) and other characteristics, which are the most favorable target section.