Oil saturation is an important parameter in shale-oil reservoir evaluation. However, due to complex wettability and pore construction, we find that conventional resistivity and nuclear magnetic resonance (NMR) methods do not perform well in calculating oil saturation in shale-oil reservoirs. Hence, we have developed a practical NMR-based method to calculate the oil saturation of the Lucaogou shale-oil Formation, Permian, in Jimusar Sag, Junggar Basin, China. First, we analyze the relationships among the wettability, oil saturation, and T2 distribution based on the theoretical formula and core analysis data. Results indicate that the ratio of the surface area wetted by water and oil is approximately equal to the ratio of water saturation and oil saturation. So we conclude that oil is mainly stored in relatively bigger pores and the surface relaxivity of the oil-wet surface is lower than that of the water-wet surface, resulting in long relaxation signals, that is, the long relaxation signals of NMR T2 spectra of shale-oil reservoirs are primarily attributed to oil signals. We have made a series of NMR measurements of as-received samples and confirm this point. Thus, we propose a T2 cutoff for water and oil to calculate the oil saturation, and we determine 6 ms as the T2 cutoff based on the oil saturation analysis of cores and NMR logs. Finally, we verify and make application of our method and acquire good results.

The Niuchang ancient cavern is a rare red sandstone cavern in southern China, located within the Longyou Grottoes in Zhejiang Province, China. Featuring a unique inclined roof structure combined with fishtail-shaped support pillars, it has maintained stability for millennia, embodying valuable ancient engineering wisdom. To investigate the mechanical behavior of this inclined roof and fishtail-shaped pillar system, this study first conducted a detailed investigation and analysis of severe deformation and failure phenomena in the cavern based on engineering geological conditions. Subsequently, using FLAC 3D software, three-dimensional models of the ancient cavern complex, one with fishtail-shaped pillars and another with hypothetical rectangular-section pillar, were constructed to simulate displacement and stress fields under self-weight conditions. Key findings indicate: (1) Long-term weathering (particularly biological weathering) and the excessively thin roof composed of gently inclined red beds are identified as the primary causes of significant deformation and failure in the Niuchang cavern; (2) Compared to a hypothetical “flat roof” cavern model, the inclined roof exhibits smaller roof displacement and experiences lower triaxial stress states in all three principal directions. (3) Compared to rectangular-section pillars, the fishtail-shaped pillars effectively reduce settlement and mitigate stress concentrations within the cavern complex, demonstrating the inherent stability and mechanical superiority of their original design. Conclusively, this study confirms that the ancient builders’ structural design, combining an inclined roof with fishtail-shaped pillars, provides enhanced long-term stability for the cavern complex compared to conventional flat roof structures with rectangular pillars.