The Ordos Basin is the second-largest sedimentary basin in China and constitutes a part of the North China Craton. During the Carboniferous-Permian period, the North China Craton existed as a sizable island situated near the ancient equator of the Pan-Mediterranean Sea (Torsvik and Cocks, 2004). Over the Paleozoic era, the coal-bearing basins of North China underwent a gradual transition from marine-transitional to terrestrial sedimentation (Li et al., 2019). This epoch of warm and humid climatic conditions, coupled with extensive coastal platforms, provided conducive environments for the formation of peat swamps and subsequent coal deposition. Consequently, the Upper Paleozoic coal resources within the Ordos Basin are abundant and widely distributed. Notably, both the #8 coal at the summit of the Benxi Formation in the Upper Carboniferous and the #5 coal of Shanxi Formation in the Lower Permian serve as regional marker beds. In the eastern sector of the basin, the #5 coal is relatively shallowly buried, thereby facilitating its exploitation (Figure 1).

The Upper Paleozoic gas source rocks in the Ordos Basin exhibit characteristics of “extensively-covered hydrocarbon expulsion” and “cumulative gas accumulation”: Organic matter maturity in the majority of the central basin has reached the stages of thermogenic condensate-rich gas generation and dry gas generation (Liu et al., 2007; Yang et al., 2012; Cao et al., 2013). The latest gas enrichment patterns suggest a closer affinity to a “quasi-continuous accumulation model” (Zhao et al., 2013; Zhao et al., 2017). Natural gas is primarily generated from the Middle Jurassic to Early Cretaceous. The main gas-bearing intervals in the Upper Paleozoic of the basin are the lithic quartz sandstones of the He 8 Member in the western area and the quartz sandstones of the Shan 2 Member in the eastern area. The analyzation of #5 coal in this study is directly deposited on top of the Shan 2 Member of Shanxi Formation.

Although there are a large number of exploration wells in the Eastern Ordos Basin, there are not many sampling wells to choose because one-quarter the drilled wells preserved cores that were only sandstone and did not coal seams. Thick and multiple sets of coal seams were selected and sampled, which were in order to analyze not only vertical trends in single well but also planar trends about #5 coal. Samples were distributed throughout the study area as much as possible, which helps to reduce sample selection bias and brings the samples closer to the overall coal seam characteristics. Six coal samples from comparatively thick coal seams with micro-fractures that can be visualized, which were selected as primary targets for three-dimensional reconstruction testing. So 76 samples from 35 wells in the Eastern Ordos Basin were selected, which combined analysis with their maceral group composition, minerals, porosity and permeability, a multi-scale and multi-parameter joint characterization of the coal seam micro-pore and micro-fractures structure in the Upper Paleozoic was conducted accordingly (Figure 1).

Various coal tests were conducted at the State Key Laboratory of Continental Dynamics (Northwest University) by using several instruments, including a transmission light-fluorescence microscope (Leica 6M), fluorescence spectrometer, confocal microscope with white light (Leica TCS SP8), adsorption analyzer (Quantachrome Quadrasorb), micro-CT (ZEISS Xradia 520 Versa), and mercury porosimeter (Quantachrome poremaster). These tests followed the procedures and technical requirements outlined in standard methods, Include the GB/T 16773-2008 (Method of preparing coal samples for coal petrographic analysis), GB/T 12937-2008 (Terms relating to coal petrology), GB/T 8899-2013 (Determination of maceral group composition and minerals in coal), GB/T 30732-2014(Proximate analysis of coal -Instrumental method), and MT/T 507-2019 (General rules of coal petrographic analysis).

Systematic testing to quantify coal industry components at first, which can acquire the carbon content and ash value. Based on the coal component data, half of the samples were selected for XRD and microscopic composition testing, which could further quantify the mineral composition and micro macerals and minerals. On the basis of the micro-image analysis, the samples in which micro-fractures were observed have been subjected to laser confocal testing to analyze their width and trapped period.

Semi-quantitative measurements and analysis about micro-fractures from microscopic images at first. The nanopores were quantitative analyzed by low-temperature adsorption (N₂ +CO₂) because these nanopores could not be observed by microscopic images Subsequently. Then quantify the nanopores and micro-fractures by high-pressure mercury intrusion test at the same time, and establish the correlation analysis between the micrometer-scale and nanoscale test results, thereby explaining the presence of bimodal features of pore throat radius in coal by multiple tests.

The macroscopic composition of the #5 coal in Shanxi Formation is predominantly semi-bright coal and bright coal, with relatively fewer semi-dark coals. Overall, there is an increasing proportion of semi-bright coal (Figures 2A–D) and bright coal (Figures 2E–I) from Shenmu to Jingbian, accompanied by a decreasing trend in semi-dark coal and dull coal. Macroscopic observations indicate that the main body of the #5 coal consists of intact coal, with a small amount of fractured coal observed in the southern Jiaxian area. Cleats are well-developed within the coal across different well locations.

A large number of high-angle fractures, typically developed perpendicular to bedding, are observed, often filled with minerals such as white calcite, forming a network of calcite-filled fractures (Figure 2, indicated by blue arrows). Statistical observations from core analysis demonstrate a gradual increase in cleat density and intensity from north to south.

The microscopic composition of the #5 coal in Shanxi Formation is predominantly vitrinite, ranging from 56.6% to 93.78%, with a mean value of 75.45%. Within the vitrinite, the majority comprises desmocollinite, followed by telocollinite and telovitrinite (Figures 3A–C). The content of inertinite varies significantly, ranging from 0.02% to 31.33%, with a mean value of 11.29%, while exinite is relatively less abundant, with a mean value of 2.6%.

The content of inorganic minerals ranges from 1.33% to 23.65%, with an average value of 10.90%. The predominant inorganic component is carbonate rocks, with small amounts of clay. The inorganic minerals exhibit a banded distribution under the microscope, with some grains semi-oriented along their long axes (Figure 3C). Under blue light excitation, exhibits no significant fluorescence (Figure 3A), displays a weak tawny-yellow fluorescence, while calcite cements exhibit yellow-green fluorescence. Early non-directional fractures, resulting from hydrocarbon generation, typically show relatively wide and curved surfaces, whereas later fractures, filled with carbonate, tend to appear straight and narrower.

At the microscopic scale, no obvious pores were observed in #5 coal in the study area, but microfractures were well developed, with multiple sets of microfractures exhibiting a vertical intersecting relationship (Figure 3B). Laser confocal quantitative analysis was conducted on seven coal microfractures, and the statistics showed that the shortest length of the microfractures ranged from 20∼40 µm, with the majority having lengths between 90∼300 µm; early fractures had widths mainly between 50∼110 µm, while later fractures had narrower widths, mostly less than 50 µm (Figure 4; Table 1). Significant differences can be quantitatively compared through microscopic scanning. The early microfractures length and width were relatively large simultaneously, which morphology also were uneven, those mainly caused by abnormal high pressure during organic matter hydrocarbon generation. The later microfractures width were relatively small, which morphology were straight and interconnected, that mainly caused by structural stress during deeply burial stage. These microscopic features indicate that Shanxi Formation #5 coal has favorable conditions for cleat generation, which is beneficial for hydraulic fracturing optimization.

The industrial composition of coal mainly consists of four parameters: fixed carbon, ash, volatile matter, and moisture, which are also important indicators for evaluating coal quality (Crosdale et al., 1998). The industrial test data of Shanxi Formation #5 coal indicates significant differences, with fixed carbon content ranging mainly from 45∼85%, with a mean of 66.16%; ash content ranging mainly from 10∼40%, with a mean of 23.55%; volatile matter ranging mainly from 3∼12%, with a mean of 7.51%.By analyzing the ash of the burned #5 coal through X-ray diffraction (XRD), the results show relatively abundant carbonates and quartz, along with small amounts of clay minerals and feldspar, and varying degrees of pyrite. Comparing the trends of fixed carbon and inorganic minerals on a planar scale, it is observed that fixed carbon content decreases from north to south and from east to west, which indicate that the deltaic depositional environments have changed and thus a deterioration in the quality of the coal bed. The quartz increases from north to south, while the carbonate content gradually decreases correspondingly, which trends were associated with the transportation distance, that indicate the sediments originated from the Yinshan uplift in the northern Ordos Basin (Figure 5).

During the process of metamorphism, coal rank indicators such as vitrinite reflectance (Ro), carbon content (C), and calorific value (Q) are used to determine the degree of coal metamorphism or coalification (Sang et al., 2005). For the #5 coal seam, the vitrinite reflectance Ro range is from 0.514% to 2.019%, indicating that the majority of coal seams in the area belong to bituminous coal and are in the middle to high metamorphic stages. The thermal evolution degree gradually increases from north to south (Figure 6), indicating that the overall coal seams are in a stage of significant hydrocarbon generation. There is no obvious correlation between the thermal evolution degree and burial depth in the present state of the #5 coal in the eastern Ordos basin, which is caused by the obvious uplift difference since the Early Cretaceous.

The adsorption space of coal seam is primarily composed of micropores, accompanied by a small number of small, medium, and large pores. They respectively constitute capillary condensation and diffusion regions, areas of slow gas permeation, and intense laminar flow regions (Can et al., 1972; Qin et al., 1995; Ju et al., 2005; Cheng and Hu, 2023). The desorption branch of the isothermal curve of the #5 coal exhibits a stable desorption within a relatively large range of relative pressure (P/P0≈1), with a narrow or absent hysteresis loop, indicating poor development of micropores and mesopores in the #5 coal.

The low-temperature N₂ adsorption indicates that the pore radius is primarily between 10∼120 nm, with a mean value of 88.6 nm according to the BET model, and 15.3 nm according to the DFT model (Figure 7A). The low-temperature CO₂ adsorption reveals a pore radius between 1.0∼2.0 nm, with a mean value of 1.82 nm according to the BET model (Figure 7B). Both N₂ and CO₂ adsorption results indicate that micro-mesopores smaller than 40 nm are predominant in the #5 coal, with a small number of macropores ranging from 50∼100 nm. The adsorption test results suggest that micropores are the primary contributors to specific surface area and pore volume.

The mercury intrusion data indicates that the porosity ranges from 3∼8%, with a mean value of 5.32%. The permeability is primarily distributed between (0.05∼3) µm² × 10⁻³ µm², with a median value of 0.37 µm² × 10^-3 µm², with few discrete permeability anomalies affected by fractures. The pore-throat radius distribution exhibits a bimodal feature, with half of the median pore-throat radius being less than 0.1 µm, while one-third of the pore-throat radius falls between 0.6∼10 µm (Figure 8; Table 2). The small pores smaller than 0.1 µm are mainly organic matter pores and intergranular pores of clay minerals, which quality were controlled by the carbon content and its thermal evolution. Recognized as a microfractures when its pore-throat radius is greater than 1 µm correspondingly, because large pores are difficult to form and preserve in coal bed. The bimodal characteristics of pore throat radius indicating the coexistence of few micrometer-sized fractures and massive nanometer-sized mesopores within the coal, which are significantly different in connectivity and complexity compared with sandstone.

The advantage of three-dimensional CT scanning method is the fast and non-destructive scanning and imaging of rock samples in all directions and over a wide area. The micro-CT results of Shanxi Formation #5 coal show the significant volume differences between micro-pores and micro-fractures. Among these, fractures with an equivalent diameter greater than 600 μm account for approximately 86% of the total pore and fracture volume, followed by those with equivalent diameters ranging from 10 to 600 μm, which represent approximately 10% of the total pore and fracture volume. Statistical analysis indicates that pores and fractures with an equivalent diameter less than 10 μm constitute 78% of the total number, yet they only contribute to 5.5% of the total pore and fracture volume (Figures 9A–D; Table 3).

The Micro-CT scanning result reveals significant differences in the development of micro-fractures. The number of fractures with a volume less than 0.1 mm³ is the highest, but their volume ratio is relatively low. Conversely, the number of fractures with a volume greater than 10 mm³ is the lowest, but their volume ratio is relatively high. Those graphic shows the rock skeleton, all the micro-fractures and micropores are identified as blue, and connected micro-fractures are displayed in the same color about four sample (Figures 9A–D). The correlation is more significant between micro-fracture porosity, micro-fracture frequency and permeability, compared to porosity (Figures 9E, F), which quantitative data indicates that permeability were mainly controlled by micro-fractures rather than micro pores. They exhibit a clear positive correlation: fractures’ porosity and permeability, and the number of fractures and permeability, which indicate the fracture system is the main factor of connectivity of the coal bed, which is a supplementary explanation for the bimodal characteristics of pore throat radius system of mercury intrusion data.

Micro-pores can provide a large number of adsorption sites for coalbed methane adsorption and occurrence. Meanwhile, micro-fractures can provide a large amount of storage space and provide space conditions for deep coalbed methane enrichment. Analysis of various test results were adopted to conduct full-scale quantitative characterization of nano-scale pore and micron scale cracks in the pore and fissure structure of coal reservoir, and comprehensively evaluate the pore and fissure structure characteristics of different scales. Multiple test results analysis and evaluation can provide a more scientific basis for judgment in the adsorption capacity and development potential of deep coalbed methane respectively.

The porosity and permeability ranges are essentially similar between #5 and #8 coals, but the monolayer and cumulative thicknesses of #5 coals are significantly thinner. Because of the #8 coals were deposited in the stable swamp sedimentary environments, which is conducive to organic matter enrichment and form stable coal seam structure.

The analysis of coalbed methane logging and hydrocarbon logging, combined with the structural characteristics and heterogeneity of coal reservoirs, demonstrates a positive correlation between coalbed gas measurements and the structure of coal reservoirs. The lithology, pore-fracture structure, and the #5 coal bed thermal evolution determine the heterogeneity of the coal reservoir and the porosity, thereby influencing the gas composition of the coal reservoir.