Study on Reservoir Characteristic Differences between the Late Triassic Yanchang Formation and Early Jurassic Yan'an Formation in Ordos Basin: A Case Study from Zhenbei Oilfield in Southwestern Margin

As the Early Jurassic and Late Triassic reservoirs in the Ordos Basin enter stages of high water-cut and elevated recovery, it becomes essential to implement stratified development adjustments in mature oilfields. These adjustments should be guided by the principles of the "Anchor Project." The goal is to achieve an effective water shut-off control and optimize the injection-production process for commingled production. To improve reservoir differentiation and formulate sustainable production strategies efficiently, this study conducts a detailed comparative analysis of reservoir characteristics using the Yan 10 (Yan'an Formation) and Chang 3 (Yanchang Formation) reservoirs in Block M of the Zhenbei Oilfield along the southwestern margin of the basin. Through comprehensive core analysis and laboratory data, microscopic differences are revealed by integrating petrographic analysis, reservoir space, petrophysical properties, sensitivity, and waterflood seepage investigations. The findings demonstrate the following: Variations in interstitial material, with chlorite-dominated cements in Yan 10 and illite/kaolinite prevalent in Chang 3; Larger pore geometries and stronger microscopic heterogeneity in Yan 10, which results in higher porosity, while Chang 3 exhibits superior permeability; Wettability differences show that Yan 10 is weakly oil-wet, while Chang 3 is water-wet, with the latter displaying superior flow capacity and oil displacement efficiency, reflecting better pore-throat network connectivity. In conclusion, unlike the single pore-throat structure controlling the properties of Yan 10, the permeability of Chang 3 is more sensitive to pore-throat geometry and microscopic heterogeneity, while porosity is less affected. This research enhances theoretical comprehension for formulating strategies for reservoir-specific development modifications in mature oilfields.