The N₂ adsorption experiment is one of the most important methods for characterizing the pore structure of shale, as it covers the major pore size range present in such sediments. The goal of this work is to better understand both the mechanisms and application of low-pressure nitrogen adsorption experiments in pore structure characterization. To achieve this, the N₂ adsorption molecular simulation method, low-pressure N₂ adsorption experiments, total organic carbon (TOC) analysis, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and a total of 196 shale samples from the Wufeng–Longmaxi formations in the Sichuan basin have been employed in this study. Based on the analytical data and the simulations, two parameters, the connectivity index and the large pore volume index, are proposed. These parameters are defined as the connectivity of the pore system and the volume of large nanopores (>10 nm) respectively, and they are calculated based on the N₂ adsorption and desorption isotherms. The experimental results showed that TOC content and clay minerals are the key factors controlling surface area and pore volume. However, in different shale wells and different substrata (divided based on graptolite zonation), the relative influences of TOC content and clay minerals on pore structure differ. In three of the six wells, TOC content is the key factor controlling surface area and pore volume. In contrast, clay minerals in samples from the W202 well are the key factors controlling pore volume, and with an increase in the clay mineral content, the pore volume increases linearly. When the carbonate content exceeds 50%, the pore volume decreases with an increase in carbonate content, and this may be because in the diagenetic process, carbonate cement fills the pores. It is also found that with increasing TOC content the connectivity index increases and SEM images also illustrate that organic pores have better connectivity. Furthermore, the connectivity index increases as quartz content increases. The large pore volume index increases with quartz content from 0 to 40% and decreases as quartz increases from 40% to 100%. By comparing the pore structure of shale in the same substrata of different shale gas wells, it was found that tectonic location significantly affects the surface area and pore volume of shale samples. The shale samples from wells that are located in broad tectonic zones, far from large-scale faults and overpressure zones, have larger pore volumes and surface areas. On the contrary, the shale samples from shale gas wells that are located in the anticline region with strong tectonic extrusion zones or near large-scale faults have relatively low pore volumes and surface areas. By employing large numbers of shale samples and analyzing N₂ adsorption mechanism in shale, this study has expanded the application of N₂ adsorption experiment in shale and clarifies the effects of sedimentary factors and tectonic factors on pore structure.

To better determine the sweet spot in the vertical profile of the Longmaxi Formation, the shale heterogeneity was systematically investigated. A series of experiments were conducted on 40 shale samples collected from the Lower Silurian Longmaxi Formation in the Weiyuan shale gas field. The results indicated that total organic carbon (TOC), the mineral composition, methane adsorption capacity, and porosity of the four sub‐layers in the Longmaxi shale varied significantly. In terms of the TOC, the mid‐lower Long1₁¹ had the highest value, followed by Long1₁³, while the TOC values of Long1₁² and Long1₁⁴ were quite low. As for mineral composition, the mid‐lower Long1₁¹ had the highest quartz content, Long1₁² and Long1₁³ had equivalent quartz, carbonate, and clay mineral contents, and Long1₁⁴ had higher clay mineral and carbonate contents, but a lower quartz content. Shale porosity and methane adsorption capacity were the highest in the mid‐lower Long1₁¹, followed by Long1₁³, Long1₁², Long1₁⁴, and the upper Long1₁¹. The micro‐heterogeneity represented by the fractal dimension ranged from 2.590 to 2.750, with an average of 2.670. The mid‐lower Long1₁¹ had the largest fractal dimension, followed by Long1₁³, Long1₁², Long1₁⁴, and the upper part of Long1₁¹ had the smallest fractal dimension. The sedimentary environment controls the macro‐heterogeneity in the vertical profile. The micro‐heterogeneity depends on diagenesis, which can be investigated by the different effects of minerals on micropore development. The strong micro‐heterogeneity results in better preservation conditions for shale gas. The mid‐lower Long1₁¹ was rich in gas generation material (TOC) with enough storage space and is characterized by good preservation conditions, leading to the highest gas content of the four sub‐layers. In addition, the high brittle mineral content is conducive to fracturing and the formation of a fracture network. Thus, the middle ‐Long1₁¹ is the “sweet spot” in the vertical profile for the shale gas development.

The Ordovician Wulalike Formation in the northwestern Ordos Basin is a new prospect for exploring marine shale gas in China, facing prominent problems such as unclear reservoir conditions and the distribution of enrichment areas. The types of reservoir space, fracture development, porosity composition, and physical properties of the lower Wulalike Formation are discussed through the multi-method identification and quantitative evaluation of reservoir space for appraisal wells. The Wulalike Formation in the study area contained fractured shale reservoirs with matrix pores (mainly inorganic pores) and permeable fractures. The fracture system of the lower Wulalike Formation is dominated by open bed-parallel fractures that are intermittent or continuous individually, with a width of 0.1–0.2 mm and spacing of 0.5–14.0 cm. The fracture-developed intervals generally exhibit bimodal or multimodal features on NMR T2 spectra and have a dual-track feature with a positive amplitude difference in deep and shallow resistivity logs. The length and fracture porosity of fracture-developed intervals varied greatly in different parts of the study area. In the Majiatan-Gufengzhuang area in the southern part of the study area, the fracture development degree generally decreased from west to east. In the Shanghaimiao area in the central part of the study area, fractures were extremely developed, the continuous thickness of the fracture-developed interval was generally more than 20 m, and the average fracture porosity was higher than 1.3%. In the Tiekesumiao area in the northern part of the study area, the fracture development degree was generally lower than that in the central and southern parts of the study area and also showed a decreasing trend from west to east. The lower Wulalike Formation had a total porosity of 2.46–7.08% (avg. 4.71%), roughly similar to the Longmaxi Formation in the Sichuan Basin, of which matrix porosity accounts for 34.0–90.0% (avg. 61.1%) and fracture porosity accounts for 10.0–66.0% (avg. 38.9%). From this, it could be inferred that the shale gas accumulation type of the lower Wulalike Formation in the northwest margin of the basin is mainly a fractured shale gas reservoir controlled by structure, and its “sweet spot area” is mainly controlled by tectonic setting and preservation conditions. This indicates that the Wulalike Formation in the northwestern Ordos Basin has good shale gas exploration prospects, and a large number of fault anticlines or fault noses formed by reverse dipping faults have the potential of favorable exploration targets.

The Lower Cambrian Qiongzhusi formation and the Lower Silurian Longmaxi Formation are the two most important shale strata. Although differences between these two shales have become the focus of current research, a comparative study of the depositional environments has not been performed. Using cores of both Longmaxi and Qiongzhusi formations of well W201, the in situ comparison of the sedimentary environment was realized, and the interference of other factors was eliminated, which made the results more reliable. In this study, 72 samples from both formations were collected from well W201, Weiyuan shale gas field, Sichuan Basin. A systematic study, including total organic carbon (TOC) content, mineral composition, and major/trace elemental analyses, was conducted to elucidate the paleoenvironments of the Qiongzhusi and Longmaxi formations. The results show both formations were deposited in non-sulfidic environments. The depositional conditions of the Longmaxi formation varied from reducing to oxidizing from bottom to top. The detrital flow happened during the deposition of the Qiongzhusi formation, which resulted in three stages of the redox conditions, from anoxic to oxic and then to anoxic from bottom to top of the Qiongzhusi formation. The anoxic conditions of the Qiongzhusi formation were considerably stronger than those of the Longmaxi formation. Both formations were deposited in warm and humid climates. Ratios of Eu/Eu*, Y/Y*, La_N/Yb_N, light rare earth element (LREE) and heavy rare earth element (HREE) revealed that the Longmaxi formation was primarily controlled by seawater, whereas the Qiongzhusi formation was jointly influenced by seawater and hydrothermal fluid. The organic matter enrichment for the Longmaxi and Qiongzhusi formations was controlled by paleoproductivity and redox conditions. Due to the slightly lower paleoproductivity and influence of detrital input, the degree of organic matter enrichment in the Qiongzhusi formation was lower than that in the Longmaxi formation.