Abstract:

Recently, significant breakthroughs have been achieved in exploring tight reservoirs within the Chang 7 Member of the Yanchang Formation in the Ordos Basin.The lower limit of reservoir filling material properties is crucial for assessing the potential for tight oil exploration and development in this area.In this study, we analyzed the lower limit of physical properties for filling materials in tight reservoirs across different regions and layers using pore penetration analysis, oil saturation measurement, and high-pressure mercury injection.Additionally, we examined the lower limit of physical properties for filling materials in tight reservoirs across different regions and layers using the physical oil test method, empirical statistical analysis, and pore throat radius method.We also explored the controlling factors and their influence on the oil and gas enrichment.The results indicate that in the Longdong-Jiyuan area, dense sandstone is primarily composed of quartz, with an average reservoir pore penetration of 8.17% and 0.15×10^-3μm²; in contrast, the dense sandstone in north Shaanxi is predominantly feldspar, exhibiting an averages reservoir pore penetration of 8.20% and 0.21×10^-3μm².In the Longdong-Jiyuan area, the lower limit of physical properties for dense reservoirs in the Chang 7₁ submember is 4.93% and 0.034×10^-3μm², while in the Chang 7₂ submember, it is 4.36% and 0.027×10^-3μm².In north Shaaxi, the lower limit of the physical properties for dense reservoirs in the Chang 7₁ submember is 5.90% and 0.048×10^-3μm², while in the Chang 7₂ submember, it is 5.60% and 0.043×10^-3μm².The lower limit of reservoir filling is influenced by three factors: the physical properties of the reservoir, the hydrocarbon generation capacity of source rocks, and the distance between the sources and reservoirs.The lower limit of reservoir filling is lower in the areas with favorable reservoir properties, high hydrocarbon generation capacity of source rocks, and minimal distance between sources and reservoirs.For example, in the Longdong-Jiyuan area, where the thickness of hydrocarbon source rocks is greater than in north Shaanxi, the lower limit of reservoir filling is lower.The lower limit of reservoir filling is lower in the Chang 7₂ submember, where the distance between sources and reservoirs is smaller compared to the Chang 7₁ submember.A lower limit of reservoir filling is associated with higher oil saturation.For example, in the Longdong area, the lower limit of reservoir filling is relatively low, the average oil saturation is 31.28%.In contrast, the average oil saturation innorth Shaanxi is 28.40%, which is significantly lower than in Longdong.Similarly, the oil saturation in the Chang 7₂ submember reservoir is higher compared to the Chang 7₁ submember reservoir.

Key words: Ordos Basin, Chang 7 Member, shale series, tight reservoir, lower limit of physical properties, control factor, oil enrichment