Interparticle pores occur between rigid particles or ductile particles and are commonly well connected, forming effective pore networks. Examples of ductile particles are clay flocculates, mica, and organic matter; examples of rigid particles are quartz, feldspar, and pyrite.

Through the observation of samples collected from outcrop section and drill, it can be found that pores between rigid particles are mainly pores between quartz (Figure 5A; Figure 3B), pores between quartz and feldspar (Figure 5B), pores between quartz and pyrite (Figure 5B) and so on. Similarly, pores between ductile and rigid particles include pores between quartz and clay minerals (Figures 5C,D,F), pores between quartz and organic matter (Figure 5A,E), pores between pyrite and organic matter (Figure 5A,C), pores between pyrite and clay minerals (Figure 5B) and so on. Pores between ductile particles are pores between clay minerals (Figure 5C,D) and pores between clay minerals and organic matter (Figure 5A,E).

Interparticle pores are complex in shape and appear not to show a strong preferential orientation. These pores range from nanometer scale to micrometer scale, commonly in micrometer scale which are concentrated around rigid grains such as quartz and pyrite (Figure 5). From the very beginning, interparticle pores are abundant and in micrometer scale in majority. During the time, ductile grains are distorted and flow into interparticle pores between rigid grains, adding to the loss of interparticle pore space, so most pores became nanometer scale (Figure 5F).

Figure 5A shows interparticle pores occur commonly between quartz and organic matter, have lengths in the 1-μm range, and are in elongated shape. Figure 5B shows abundant interparticle pores between rigid grains. Figure 5C shows effective connectivity between illite taking the shape of thin slice. In Figure 5D, it can be found that compared to outcrop samples, core samples have much less interparticle pores because of compaction and cementation. Usually these pores are scattered with no strong preferential orientation. Sometimes, parts of the pores are triangular in shape which interpreted to be the remaining pore space between compacted and cemented rigid grains (Loucks et al., 2012). Figure 5E shows some interparticle pores between organic matter and quartz or clay minerals which are in micrometer scale and present elongated shape. These pores guarantee the channel between organic-matter pores and mineral pores and make the gas produced by organic matter transported to mineral pores which are of great importance to the migration and production of shale gas.

Intraparticle pores within mineral grains range from simple to complex in shape and with different origins. A small amount of the pores are primary formed at beginning, while the majority was formed in diagenetic processes. The main types of intraparticle pores are cleavage-plane pores within clay minerals, intercrystalline pores within pyrite framboids, and pores formed by partial or complete dissolution (Figures 6, 7).

Undergoing a long geological history and mechanical compaction, most primary pores are destroyed or cemented (Rouquerol et al., 1994). However, abundant dissolution pores formed in the latter time (Figure 6). Actually, dissolution pores are intraparticle pores resulted from the process of carbonate, apatite and feldspar dissolution and repreciptation. In this paper, pores resulted from the dissolution of feldspar are very abundant in most observed samples. These pores are irregular in shape and different in size (Figures 6A,B). Some of the dissolution pores are filled by illite, others are open and with the size of 5 μm which are more common in core samples compared with the outcrop samples (Figures 6C,D). In addition, pores resulted from the dissolution of quartz are also been found with relatively small size which is about 1 μm (Figures 6E,F).

In core and outcrop samples, there is abundant intraparticle space within pyrite framboids, especially when pyrite crystals were detached by outside forces (Figures 7A,B). The space within pyrite framboids contains clay or residual organic mucus, or nothing.

Research shows that the shape of intraparticle pore is generally controlled by its origin (Kanitpanyacharoen et al., 2012) which is suitable to cleavage-plane pores within clay minerals. In clay particles, especially illite, intraparticle pores are sheetlike and are parallel to one another with the size of smaller than 200 nm. In addition, residual organic mucus is commonly found around some clay minerals (Figures 7C,D,E,F).

Pores within organic matter are called organic-matter pores (OM pores) in this article. Compared with core samples, outcrop samples have less organic matter and organic-matter pores because of weathering, although there are considerable organic-matterpores in both core samples and outcrop samples (Figures 8, 9). Organic-matter pores commonly exist in samples which come from drill cuttings (Figure 9). Usually with the drilling depth goes deeper, the number of organic-matter pores shale increase. In addition, some organic mucus is found around pyrite framboids as said above (Figure 7B; Figure 8B).

Some organic-matter pores occur as absorbed coatings around clay grains and form abundant pores which are smaller than 100 nm scale. Published experimental and observational studies indicate that although these pores are not connected to each other in two-dimensional space, while in three-dimensional space their connectivity is perfect and has high value to shale gas reservoir (Nie and Zhang, 2011).

Microfractures and microchannels have a significant effect on shale gas system (Pitman et al., 2001; Gale and Holder, 2010), especially in hydrocarbon production. The scale of microfractures rangsfrom nanometer to micrometer and the fractures can have a strong influence on the impermeability of tight rocks. The existence of some cemented fractures may deeply effect induced fracture propagation.

In the process of hydrocarbon production, with internal pressure increases gradually there emerges more microfractures. When the numbers of microfractures reach sufficient quantity, these microfractures offer storage space and permeability pathways for hydrocarbon to through the shales (Slatt and Brien, 2011).

By field emission scanning microscopy, we have the opportunity to observe microfractures in various sizes and shapes clearly in core samples and outcrop samples. These microfractures are commonly in micrometer scale and almost exist in all samples (Figure 10). Some of them are open and less than 500 nm in width with sinuous and discontinuous structure (Figures 10A,B). Some microfractures are cemented by clay minerals and exist between the space of rigid grains such as quartz (Figure 10E,F). Previous studies show that quartz is one of the factors which have strong influence on microfractures. Also, some microfractures are filled with bitumen with pyrite around (Figures 10C,D). All the microfractures above are wide enough to provide a permeability pathway for gas molecules which are of significant importance.

In the process of studying shale gas, some researchers (Loucks et al., 2009; Yang et al., 2022) suggest that there are two major unanswered questions: (1)where is the gas stored in the rock and (2) what pathway does the gas follow from the matrix to these induced fractures that allow it to flow into the well bore. To address these questions, we need to investigate pore structure present in this unit. As we all know, pore structure, which usually means the shape, size, distribution and microstructure of pore and pore throat, is of great importance to gas production and gas storage in shale gas exploration.

The images above indicate that the diameter of interparticle pores commonly range from several nanometers to a few micrometers (Figure 5). Intraparticle pores are subdivided into two categories from the perspective of pore size: dissolution pores are in micrometer scale (Figure 6) while cleavage-plane pores within clay minerals and intercrystalline pores within pyrite framboids are in nanometer scale (Figure 7). Organic-matter pores are in nanometer scale and smaller than intraparticle pores in size range (Figures 8,9). Therefore, shale-gas reservoir of the Longmaxi Formation in southeastern Chongqing is mainly nanometer pores with small amount of micrometer to millimeter pores.

In shallow-buried mudrocks, interparticle pores are relatively abundant in both rigid grains and ductile grains and have good connectivity. As rocks are buried and exposed over time to high temperature and pressure, they become lithified and have less pores because ductile grains are distorted and flow into interparticle pores between rigid grains, blocking pore space and throats and adding the loss of interparticle pores (Loucks et al., 2012). This phenomenon is very obvious from observed samples. Comparing the two sets of samples from Lujiao section and Well Yuye-1, we note that the outcrop samples from Lujiao section have much more interparticle pores than the core samples from Well Yuye-1 (Figure 5). This is because compaction is more clear in core samples than outcrop samples, leading to the change of interparticle pores in quantity and size (Jin, et al., 2011).

Although core samples do not have interparticle pores as many as outcrop samples, they own most intraparticle pores such as cleavage-plane pores within clay minerals, intercrystalline pores within pyrite framboids, and pores formed by partial or complete dissolution (Figures 6, 7) which is caused by special temperature and pressure. Also we found that most intraparticle pores are filled with organic mucus (Figure 7). Loucks et al. (2009) holds the opinion that these pores are isolated and disconnected from each other which is not good for shale gas reservoir. However, others (Curtis et al., 2012) suggest that organic mucus have a strong adsorbability to gas and clay minerals have a large internal surface area (Zhong, 2012) which can store lots of gas and are of great importance. In addition, this kind of pores looks disconnected in two-dimensonal but actually connected in three-dimensonal demonstrated by many researchers.

The maturity of Longmaxi samples is over 1.9% and TOC is over 1.6%, so organic matter pores is quite common in most samples (Figures 8,9). The research of organic-matter pores are gradually valued in the study of shale gas basins in America (Nix and Burkhardt, 2003; Loucks et al., 2009; Chalmers et al., 2012; Modica and Lapierre, 2012). Usually organic matter pores form like this: during the diagenetic processes, following the increasing temperature and pressure, organic matter becomes porous and these pores provide places for generated gases. In this paper, we observed organic matter pores in different shape and commonly are in nanometer scale. Organic-matter pores can form an effective pore system and the dominate flow pathway.

When studying the pore characteristics of the early Silurian Longmaxi Formation shale in northwestern Hunan, Yang (2022) believed that the development degree of micropores was mainly controlled by organic matter content. Mesopores with a pore size of 2–40 nm were mainly contributed by organic matter pores and clay minerals, while pores with a pore size greater than 40 nm were closely related to intergranular pores formed by quartz and other minerals. Quantitative analysis (Li et al., 2012; Huang et al., 2022) has shown that with increasing clay minerals, micropore and mesopores volumes relatively increase whereas macropore volumes decrease. An inverse relationship exists between clay minerals and quartz. With increasing quartz, volumes of micropore and mesopores relatively decrease whereas macropore volumes increase (Figure 11). These quantitative results are consistent with the observed images by field emission scanning electron microscopy.

Most interparticle pores are distributed between detrital minerals with the size of macroscale. So with the increasing of quartz, macropores increase. Interparticle pores usually exist between clay minerals except dissolution pores. Therefore, with the increasing of clay minerals, micro to meso pores increase.

In addition, research of the low to middle maturity shale abroad has identified that interparticle pores are mostly macropores and have a close relationship with quartz while micro to meso pores have a close relationship with clay minerals and organic matter (Slatt and Brien, 2011; Loucks et al., 2012; Chalmers et al., 2012). Thus, there are similarities and difference between low-middle maturity shale abroad and high maturity shale in South China.