The Chang 6 reservoir in the Ordos Basin is originally characterized by coexisting oil and water. Combined with the influence of primary natural microfractures, the distribution of remaining oil becomes highly heterogeneous during waterflooding development, which makes it difficult to clarify its occurrence rules. To reveal the microscopic occurrence patterns and distribution mechanisms of remaining oil in the Chang 6 reservoir, micro-nano fabrication technologies including photolithography and etching were adopted to accurately replicate the microscopic geometric characteristics and permeability of rock samples on silicon wafers. Miscible fluids composed of crude oil and formation water with different proportions were prepared to simulate oil-water coexistence under different initial water saturations. A high-precision optical microscope was used to record the oil-water distribution throughout the microscopic displacement process. The occurrence rules of microscopic remaining oil were compared and analyzed, and typical areas of the chips were magnified for observation to directly display the microscopic oil-water occurrence under different initial oil saturations and wettability conditions. The results show that: (1) In water-wet chips, capillary force acts as the driving force for displacement. The water phase adheres to the channel wall, and the oil phase mainly exists in the form of oil droplets. Water presents high displacement efficiency and achieves a favorable effect on miscible fluids. (2) In oil-wet chips, capillary force serves as displacement resistance. The oil phase tends to attach to the chip wall and form oil films on both sides of pore spaces. After displacing the oil phase in the middle of pore throats, the water phase forms dominant flow channels, which hinders further oil production. (3) At low initial oil saturation, evenly distributed dispersed oil droplets dominate; while at high initial oil saturation, cluster structures are predominant with enhanced continuity of the oil phase. (4) Affected by the Jamin effect, three typical blocking mechanisms are observed in the chips: oil cluster blocking, single oil droplet blocking, and combined blocking of large and small oil droplets. (5) The waterflooding recovery factor is ranked in descending order: 76.7% for water-wet chips with an initial oil saturation of 85%, 41.2% for water-wet chips with an initial oil saturation of 45%, 28% for oil-wet chips with an initial oil saturation of 85%, and 9.1% for oil-wet chips with an initial oil saturation of 45%. Water-wet reservoirs can effectively improve the waterflooding performance on miscible fluids, and a higher initial oil saturation corresponds to a higher oil recovery factor.