Quantitative analysis of nitrogen adsorption hysteresis loop and its indicative significance to pore structure characterization: A case study on the Upper Triassic Chang 7 Member, Ordos Basin

Low-temperature nitrogen adsorption can form a “hysteresis loop”, whose geometry and area can effectively reflect the pore structure of porous media and its retention effect on adsorbed gas. However, the role of the “hysteresis loop” in quantitative characterization of shale pore structure has often been ignored. The study aims to clarify whether shale can form “hysteresis loop” in low-temperature nitrogen adsorption-desorption experiment and the determinants on hysteresis loop’s area with experiments on the 7^th member of the Upper Triassic Yanchang Formation shale (Chang 7 shale) in the Ordos Basin. Various measures are applied in the study, including qualitative observation of pore structure under field-emission scanning electron microscopy (FE-SEM) and quantitative characterization of pore structure by low-temperature nitrogen adsorption test, hysteresis-loop quantitative analysis, total organic carbon (TOC) content analysis, pyrolysis and X-ray diffraction (XRD) experiments. The following results are obtained. First, whether shale can form a hysteresis loop in the low-temperature nitrogen adsorption-desorption experiment has an apparently positive correlation with the specific surface area, specific pore volume, clay mineral content, and pore structure fractal dimension, and an evidently negative correlation with the TOC content, while no apparent correlation with the average pore size, pore surface fractal dimension, highest pyrolysis peak temperature, and content of brittle minerals. Second, the hysteresis loop area depends on the development degree of the cylindrical pores with both ends open, ink-bottle pores, or parallel plate pores, the proportion of which to the total pores can be quantitatively evaluated by the hysteresis loop area. Third, the open-ended cylindrical pores, ink-bottle pores, or parallel plate pores in the samples from Chang 7 shale are mainly of the intergranular pores in clay minerals. Therefore, there is an apparently positive correlation between the hysteresis loop area and the content of clay minerals.