Determining the Valid Petrophysical Threshold of Interbedded Sandstone Reservoirs in Shale: The Role of Source–Reservoir Assemblage and Divergence from Conventional Reservoirs

Shale oil stands as a critical growth engine for global unconventional hydrocarbon exploration and production. Sandstone interlayers within shale successions have become the primary reservoir units and key development intervals for interbedded shale oil plays due to their superior storage and seepage capacity relative to the shale matrix. Nevertheless, conventional petrophysical cutoff standards for effective reservoirs are mostly derived from independent conventional sandstone reservoirs, largely ignoring the controlling effect of source–reservoir configuration on hydrocarbon charging. This has resulted in a widespread paradox in production practices and severely constrained accurate resource assessment and efficient sweet spot delineation. To cover this gap, this study integrates thin section analysis, organic geochemical experiments, core petrophysical measurements, and geophysical logging data to systematically characterize the geochemical features of source rocks and the petrophysical properties of sandstone reservoirs. Then, a quantitative source–reservoir coupling index is established by integrating key controlling factors, including total organic carbon content, free hydrocarbon content, vitrinite reflectance, source–reservoir pressure difference, source–reservoir distance, and configuration relationship. According to the index, source–reservoir coupling relationships are categorized into strong, moderate, and weak grades. The corresponding petrophysical lower limits for effective sandstone reservoirs are determined hierarchically. The results indicate that threshold values increase with decreasing coupling intensity: strongly coupled systems have lower limits of 6.5% porosity and 0.12 mD permeability, while moderately coupled systems require higher thresholds of 8.5% porosity and 0.23 mD permeability. This study breaks through the limitations of traditional empirical methods that define reservoir cutoffs solely by porosity and permeability. A method and corresponding criteria for evaluating the petrophysical cutoffs of sandstone reservoirs were established for the study area. This enables effective identification of relatively low-quality sandstone reservoirs that can still form commercial oil accumulations under robust hydrocarbon supply conditions. This also significantly improves the accuracy of effective reservoir recognition and sweet spot prediction in interbedded shale oil systems.