Because of the great potential for hydrocarbon generation, the Lower Cambrian Wangyinpu and Guanyintang Formations of the Jiangxi Xiuwu Basin have become the most important targets for shale-gas exploration in the Jiangxi province. We investigate the pore characteristics and main controlling factors of overmature shale using field emission-scanning electron microscopy, image-processing software (i.e., the Particles [Pores] and Crack Analysis System), X-ray diffraction, and gas-adsorption experiments. The results show that the shales have a high abundance of organic matter (OM), over maturity, and highly siliceous mineral content. The kerogen type is identified as type I. OM pores are the most developed, followed by interparticle (interP) pores and intraparticle (intraP) pores. We combine complementary image processing and gas-adsorption methods to reveal that micropores are mainly from OM pores; mesopores are from OM pores and interP pores; and macropores are from OM pores, interP pores, and intraP pores. Although the number of micropores is at a maximum, the total contribution of mesopores and macropores to the pore volume (PV) is larger than that of micropores. However, the specific surface area (SSA) is mainly from the micropores. OM content and maturity are the main controlling factors for the development of pore structures. Because of overmaturity, OM loses its potential for hydrocarbon generation and new pores cannot be produced. Gas loss leads to reservoir pressure drop, and the pores generated during the mature stage collapse and even disappear because they lack support. Therefore, PV, SSA, and porosity decrease when the OM content is more than 10%. When the OM content is less than 10%, most of the OM pores are preserved because they are protected by the skeleton particles.

The stock of shale gas in the Shahezi shale reservoir in the Changling fault depression, Songliao Basin, is believed to be worth exploring. In-depth study on the pore structure and fractal characterization of organic matter (OM) can help better understand the pore system of shale reservoir, which has implications for the exploration of lacustrine shale. To demonstrate the nanoscale pore structure and irregularity of the isolated OM, we collected a large number of samples and then conducted a series of laboratory experiments, such as X-ray diffraction, scanning electron microscopy, and CO2 and N2 adsorption experiments to determine the pore structure parameters and reveal their heterogeneity according to Frenkel-Halsey-Hill theory. As suggested by the experimental results, the pore size distribution curve of the N2 and CO2 exhibits similar pore peaks, whereas the trends of change in bulk shale and isolated OM samples are basically paralleled. The pore volume of the isolated OM ranges between 0.034 and 0.056 cm3/g, which is approximately 0.90–3.06 times that of bulk shale samples. As for the fractal dimensions D1 (2.594 on average) and D2 (2.657 on average) of bulk shale, they are larger as compared with isolated OM, indicating that inorganic minerals can make a significant difference in the heterogeneity of shale pores. The fractal dimensions (D1 and D2) of bulk shales indicate a close correlation with the parameters of pore structure, whereas there is no significant correlation observed between the dimensions of isolated OM and its parameters. In addition, thermal maturity and solid bitumen have only limited impact on the OM pore structure of isolated OM samples. Then, we conducted further research to reveal that the insoluble OM macerals derived from terrestrial higher plants can be used to explain the difference in pore structure and heterogeneity between isolated OM samples. Therefore, we arrived at the conclusion that the composition of macerals depends on the exact pore structure and fractal characteristics of isolated OM samples with similarity in thermal maturity.

To understand the pore structure and heterogeneity of pore size distribution (PSD) is essential for revealing fluid mechanics and evaluating the utilization of unconventional resources. In this study, there are multiple shale examples collected from the Chang 7 section in the Ordos Basin for the investigation was conducted on the basis of various experiments on total organic carbon (TOC), X-ray diffraction (XRD), and nitrogen gas adsorption, through scanning electron microscopy (SEM) and multifractal method. The multifractal characteristic parameters, including the width of singularity spectra (), Hurst exponent (), , and nitrogen gas adsorption, were used to find out about the characteristics of pore development and to quantify the complexity and heterogeneity of pore structure. Depending on the exact mineral composition, the Yanchang Formation of Chang 7 shales is classified into either silty mudstone (SM) or muddy siltstone lithofacies (MS). According to the investigative results, the Chang 7 lacustrine shale features a complex pore system with the pores ranging from 1.5 to 10 nm in diameter. Besides, mesopores contribute significantly to the total pore volume (TPV) and total surface area (TSA). As for TPV and TSA of the SM lithofacies in the samples under investigation, they are nearly 1.09–1.78 and 0.80–1.72 times greater as compared to the MS lithofacies samples. The dominant types of reservoir spaces include organic matter (OM) pore and interparticle pore which are related to inorganic minerals. The value of is higher for MS lithofacies than for SM lithofacies, indicating a greater heterogeneity of PSD in the MS lithofacies. The pore structure of MS lithofacies is determined mainly by TOC and siliceous mineral content, whereas the influencing factors for SM lithofacies are TOC and clay mineral content. There is a significant relationship between multifractal parameters and pore structure parameters for both SM and MS lithofacies. The TOC of SM and MS lithofacies exhibits a close correlation with , suggesting that the pores in organic matter are dominated by those nanopores with a complex and heterogeneous pore structure. The rock composition of the lithofacies can affect to a varying extent, which means that the minerals have an evident impact on the heterogeneity of MS and SM lithofacies.