The Lower Cretaceous Shahezi shales are the targets for lacustrine shale gas exploration in Changling Fault Depression (CFD), Southern Songliao Basin. In this study, the Shahezi shales were investigated to further understand the impacts of rock compositions, including organic matters and minerals on pore structure and fractal characteristics. An integrated experiment procedure, including total organic carbon (TOC) content, X-ray diffraction (XRD), field emission-scanning electron microscope (FE-SEM), low pressure nitrogen physisorption (LPNP), and mercury intrusion capillary pressure (MICP), was conducted. Seven lithofacies can be identified according to on a mineralogy-based classification scheme for shales. Inorganic mineral hosted pores are the most abundant pore type, while relatively few organic matter (OM) pores are observed in FE-SEM images of the Shahezi shales. Multimodal pore size distribution characteristics were shown in pore width ranges of 0.5–0.9 nm, 3–6 nm, and 10–40 nm. The primary controlling factors for pore structure in Shahezi shales are clay minerals rather than OM. Organic-medium mixed shale (OMMS) has the highest total pore volumes (0.0353 mL/g), followed by organic-rich mixed shale (ORMS) (0.02369 mL/g), while the organic-poor shale (OPS) has the lowest pore volumes of 0.0122 mL/g. Fractal dimensions D₁ and D₂ (at relative pressures of 0–0.5 and 0.5–1 of LPNP isotherms) were obtained using the Frenkel–Halsey–Hill (FHH) method, with D₁ ranging from 2.0336 to 2.5957, and D₂ between 2.5779 and 2.8821. Fractal dimensions are associated with specific lithofacies, because each lithofacies has a distinctive composition. Organic-medium argillaceous shale (OMAS), rich in clay, have comparatively high fractal dimension D₁. In addition, organic-medium argillaceous shale (ORAS), rich in TOC, have comparatively high fractal dimension D₂. OPS shale contains more siliceous and less TOC, with the lowest D₁ and D₂. Factor analysis indicates that clay contents is the most significant factor controlling the fractal dimensions of the lacustrine Shahezi shale.

Organic matter (OM) pores are significant for shale gas accumulation and flow mechanisms. The pores of Wufeng-Longmaxi (W-L) shale in the Sichuan Basin, China have been extensively characterized, however, the proportion of OM pores in this shale have not been adequately discussed. In this study, the contribution of OM pores to the total pore volume of W-L shale was quantitatively studied through the analysis of OM isolation, field emission scanning electron microscopy (FE-SEM) and low-pressure CO₂ and N₂ adsorption (LPGA). FE-SEM images showed abundant OM pores, interparticle pores and intraparticle pores with various shapes and widths in the W-L shales. The pore size distribution (PSD) of the isolated OM from five shale samples showed a consistent, unimodal pattern. The pore volume of isolated OM was greater than that of the bulk shale samples, suggesting that OM is more porous than the inorganic compositions in shales. The average contribution of OM to the volumes of micropores, mesopores and macropores was 58.42%, 10.34% and 10.72%, respectively. Therefore, the pore volume of the W-L shale was dominantly related to inorganic minerals. This was probably due to the small weight ratio of OM in the shale samples (1.5 wt%–4.2 wt%). The findings of this study reveal the different effects of OM and minerals on pore development, and provide new insights into the quantitative contribution of OM pores to the total pore volume of the W-L shale.

Pore connectivity of lacustrine shales was inadequately documented in previous papers. In this work, lacustrine shales from the lower Cretaceous Shahezi Formation in the Changling Fault Depression (CFD) were investigated using field emission scanning electron microscopy (FE-SEM), mercury intrusion capillary pressure (MICP), low pressure gas (CO₂ and N₂) sorption (LPGA) and spontaneous fluid imbibition (SFI) experiments. The results show that pores observed from FE-SEM images are primarily interparticle (interP) pores in clay minerals and organic matter (OM) pores. The dominant pore width obtained from LPGA and MICP data is in the range of 0.3–0.7 nm and 3–20 nm. The slopes of n-decane and deionized (DI) water SFI are in the range of 0.34–0.55 and 0.22–0.38, respectively, suggesting a mixed wetting nature and better-connected hydrophobic pores than hydrophilic pores in the Shahezi shales. Low pore connectivity is identified by the dominant nano-size pore widths (0.3–20 nm), low DI water SFI slopes (around 0.25), high geometric tortuosity (4.75–8.89) and effective tortuosity (1212–6122). Pore connectivity follows the order of calcareous shale > argillaceous shale > siliceous shale. The connected pores of Shahezi shales is mainly affected by the high abundance and coexistence of OM pores and clay, carbonate minerals host pores.

To figure out porosity evolution mechanisms of marine shales inside and at the margin of the Sichuan Basin, South China, we measured samples selected from Lower Cambrian Qiongzhusi shales and Lower Silurian Longmaxi shales by a combination of X-ray diffraction, geochemistry measurement, focused ion beam milling and scanning electron microscopy imaging, and [ ] adsorption. It was suggested that shales of the upper Qiongzhusi Formation (a marine shallow continental-shelf setting) and the Longmaxi Formation (a marine abyssal continental-shelf setting) possessed larger pore volumes and larger pore surface areas than those of the lower Qiongzhusi Formation (a marine abyssal continental-shelf setting). Pores, in terms of pores of mineral frameworks, pores associated with clay minerals, dissolved pores, and organic matter (OM)-hosted pores, were all observed in the upper Qiongzhusi Formation and Longmaxi Formation, whereas none of the four types of pores developed well in the bottom of the Qiongzhusi Formation. Moreover, migrated OM is superior to depositional OM in the contribution of the pore space, in terms of pore productivity and pore protection. Good sealing abilities of the upper Qiongzhusi Formation and Longmaxi Formation allow more migrated OM and gaseous hydrocarbon retention than the lower Qiongzhusi Formation with an unconformity beneath acting as a channel of liquid and gaseous hydrocarbon migration, which bring about better pore properties. Finally, through the above analysis, the pore evolution mode has been established to gain insights for mechanisms of destruction, formation, and preservation of pores ranging from original sedimentary to metamorphic stage ([ ]–[ ]). Mechanisms of pore destruction contain mechanical compaction, chemical cementation, and OM carbonation. Mechanisms of pore generation comprise thermal pyrolysis of OM, transformation of clay minerals, and dissolution of soluble minerals. Mechanisms of pore preservation include mechanical stability of rigid grains, chemical stability of hydrophobic OM, and gas supporting through overpressure.