The pore structure of shale is an important basis for evaluating the porosity, permeability, and effectiveness of shale reservoirs. According to the research on the pore structure of different lithofacies of Longmaxi Formation shale in Fuling area (Wang et al., 2018b), argillaceous-rich siliceous shale of Longmaxi Formation contains a large amount of organic matter pores, but rare intrgranular and intergranular pores. Moreover, the organic matter pores are mostly round or elliptical shaped, with the pore diameter ranging from nanoscale to micron and good connectivity.
The calcareous-argillaceous-siliceous mixed shale lithofacies contains various types of pores. The diameter of organic matter pore is relatively smaller, and the inorganic pores are mainly clay mineral intercrystalline pores and primary intergranular pores among clastic particles. The pores of siliceous-rich argillaceous shale are dominated by intergranular pores of clay minerals, and the organic matter pores are not much.
According to the scanning electron microscopy (SEM) observation on pore structures of 25 samples from different lithofacies in Sinian Doushantuo Formation and Cambrian Niutiang Formation of Eyangye 1 Well, it was found that organic pores in siliceous rock (S) were flock-like distribution, and some pores were connected to form long strip-shaped pore fractures, which can be up to 120 nm in length (Fig. 11a). In the siliceous shale (S-2), the organic pores are densely developed, and the number of organic pores is the most among all the shale lithofacies (Fig. 11b), and then the calcareous-rich siliceous shale (S-1) (Fig. 11c). However, organic matter in argillaceous-rich siliceous shale (S-3) often forms intergranular crack with mineral particles, but the organic pores are hardly developed (Fig. 11d).
Figure 11. Organic pore characteristics of siliceous shale lithofacies of Eyangye 1 Well. (a) Organic pores, organic matter filling between pyrite crystals, Eyangye 1 Well, S lithofacies, 3 041.98 m; (b) organic pores, Eyangye 1 Well, S-1 lithofacies, 3 313.3 m; (c) organic pores, densely distributed, Eyangye 1 Well, S-2 lithofacies, 3 037.69 m; (d) no organic pores developed, Eyangye 1 Well, S-3 lithofacies, 3 016.88 m.
Cloddy shaped organic matter with a large number of organic pores was vastly distributed in the calcareous-siliceous mixed shale (M-1). These organic pores were nearly round in shape and the pore diameter was between 10-100 nm (Figs. 12a, 12b). The organic pores on the surface of the organic matter filled between flake or flock-like clay minerals are irregular in shape and have a large pore size ranging from 50 to 200 nm (Figs. 12c, 12d). In limestone facies (C), a small amount of organic matter in long strips is distributed among inorganic minerals. A small number of near-circular holes on their surface, with a large pore size of about 100 nm (Fig. 13a). Siliceous-rich calcareous shale (C-1) has a relatively high content of organic matter, which develops in wedge-shaped pores between inorganic mineral particles. A large number of organic pores can be seen on their surface with a pore diameter of about 30 nm (Fig. 13b). However, the shale samples in the calcareous shale (C-2) and argillaceous-rich calcareous shale (C-3) have very little organic matter content, and basically no organic pores (Figs. 13c, 13d).
Figure 12. Organic pore characteristics of mixed shale lithofacies of Eyangye 1 Well. (a) Organic pore, Eyangye 1 Well, M-1 lithofacies, 3 309.37 m; (b) organic pore, Eyangye 1 Well, M-1 lithofacies, 3 307.32 m; (c) organic pores, irregular in shape, distributed among flake-shaped clay minerals, Eyangye 1 Well, M-1 lithofacies, 3 307.32 m; (d) organic pores, distributed among flock-shaped clay minerals, Eyangye 1 Well, M-1 lithofacies, 3 308.09 m.
Figure 13. Organic pore characteristics of calcareous shale lithofacies of Eyangye 1 Well. (a) Organic pore, nearly circular, Eyangye 1 Well, C lithofacies, 3 309.36 m; (b) organic pore, Eyangye 1 Well, C-1 lithofacies, 3 311.96 m; (c) no organic pores developed, Eyangye 1 Well, C-2 lithofacies, 3 320.19 m; (d) no organic pores developed, Eyangye 1 Well, C-3 lithofacies, 3 324.30 m.
Intercrystalline pore, secondary dissolution pore and clay mineral pore are commonly developed in different lithofacies of Doushantuo Formation and Niutitang Formation shales. An intergranular pore is mainly the pore between pyrites, and pores among calcite particles and authigenic siliceous mineral particles formed by recrystallization. However, these pores mainly developed in the siliceous shale lithofacies (S) and mixed shale lithofacies (M), and rarely in the calcareous shale lithofacies (Fig. 14). The secondary dissolution pores mainly consist of intergranular dissolution pores and intragranular dissolution pores. The pore size of intergranular dissolution pores is relatively large, most of which are 1-10 micron, and the distribution is irregular and linear around the edge of the particles, with generally good connectivity. The pore size of intragranular dissolution pores is small, mainly less than 1 micron, and generally develops on the surface of mineral particles such as feldspar and calcite, showing a scattered distribution (Fig. 14). In the siliceous shale lithofacies, the dissolution pores are mainly in feldspar, while mainly in calcite in the calcareous shale lithofacies. In mixed shale lithofacies (M) dissolution pores both in feldspar and calcite are developed. The pores among clay minerals are mainly developed in the mixed shale lithofacies, but rare in the siliceous and calcareous shale lithofacies.
Figure 14. Inorganic pore characteristics of each lithofacies of Eyangye 1 Well shale. (a) Pyrite intercrystalline pore, Eyangye 1 Well, S-1 lithofacies, 3 029.15 m; (b) pyrite intercrystalline pore, Eyangye 1 Well, M-1 lithofacies, 3 309.37 m; (c) dissolution pores in feldspar, Eyangye 1 Well, M-1 lithofacies, 3 309.4 m; (d) dissolution pores in calcite grains, Eyangye 1 Well, M-1 lithofacies, 3 306.88 m; (e) quartz intergranular pore, Eyangye 1 Well, C-2 lithofacies, 3 052.12 m; (f) intergranular dissolution pores of calcite, Eyangye 1 Well, S-1 lithofacies, 3 029.15 m; (g) intergranular pore of clay minerals, Eyangye 1 Well, C-2 lithofacies, 3 319.9 m; (h) intergranular pore of clay minerals, Eyangye 1 Well, C-2 lithofacies, 3 319.9 m
In general, the pore structure development characteristics of the three types of organic-rich shale lithofacies in the Doushantuo Formation and Niutitang Formation in Yichang area are as follows: organic matter and pyrite are developed in the siliceous shale lithofacies, and the pore types are dominated by organic matter and pyrite intercrystalline pores, followed by secondary dissolution pores, but pores among clay mineral are rare. The contents of organic matter and pyrite in mixed shale lithofacies are medium, and the pore types are mainly among clay minerals and secondary dissolution pores, followed by organic matter pores. The content of organic matter in the calcareous shale lithofacies is relatively small, and the pore types are mainly secondary dissolution pores of carbonate particles, followed by organic pores, but pores among clay mineral are rare (Table 1).
Lithofacies types Organic pores Inorganic pores Intercrystalline pores Intergranular dissolution pores Intragranular dissolution pore Pores among clay mineral particles Siliceous shale lithofacies Vastly developed, Nearly spherical and irregular Vastly developed, in strawberry pyrites Vastly developed, among quartz or feldspar particles, leptosomatic shape Developed, mainly dissolution pores in feldspar Relatively developed, leptosomatic shape, parallel to clay minerals Mixed shale lithofacies Relatively developed, organic matter among clay mierals Relatively developed, Nearly spherical Rare Developed, dissolution pores in feldspar or calcite Developed, forming flocculent shaped network structure, good connectivity Calcareous shale lithofacies Developed, small diameter and less amount Developed, in small strawberry pyrites Vastly developed, among calcite particles Vastly developed, dissolution pores in calcite Rare
Table 1. Comparison of pore characteristics of different lithofacies of Doushantuo and Niutitang formations
Based on the comparison of porosity and permeability among the Niutitang Formation and Doushantuo Formation shale in Yichang area, the Wufeng-Longmaxi Formation shale in Jiaoshiba area and the shale in north American (Table 2), it is concluded that the Niutitang Formation and Doushantuo Formation shale in Yichang area have their unique porosity and permeability characteristics. The burial depth of Niutitang Formation and Doushantuo Formation shale in Yichang area is larger, which means larger overlying formation pressure. It leads to the relatively undeveloped pore system in Niutitang Formation and Doushantuo Formation shale. The shale in Jiaoshiba area has higher content of clay minerals than that in Yichang area. In the process of organic matter evolution and gas generation, the clay minerals are more prone to form local fracture, and then increase the porosity. Therefore, the porosity of Niutitang Formation and Doushantuo Formation shale in Yichang area is much lower.
Formations Porosity (%) Permeability (10-6 μm2) Longmaxi 3.22-7.13 (4.48) 0.003-0.936 5 Wufeng 3.01-7.08 (5.02) 0.001 6-0.545 1 Niutitang 0.28-6.26 (2.25) 0.000 16-0.003 76 Doushantuo 0.28-3.41 (1.83) 0.000 47-0.003 7 Fayetteville 2-8 0.1-0.8 Barnett 4-5 0.073-0.5 Haynesville 8-9 0.05-0.8 Marcellus 9-11 0.1-0.7 Utica 3-6 0.8-3.5
Table 2. Summary table of the porosity and permeability from the different shale strata and typical shale in the United States
Compared with the Wufeng-Longmaxi Formation shale in Jiaoshiba area, the Niutitang Formation and Doushantuo Formation shale in Yichang area have lower transverse permeability. Besides siliceous mineral content impact, the shale in Yichang area is also affected by carbonate mineral content, which is mainly formed by chemical precipitation and then late filling in shale. These carbonate minerals are greatly influenced by the environment and in irregular shape, blocking the relatively complete and well-connected pores. Hence, these carbonate minerals decrease horizontal permeability in the case of constant porosity.
The porosity and permeability of the Niutitang Formation and Doushantuo Formation in Yichang area are not correlated with each other, which was caused by the greater contribution of microcracks rather than the pores on permeability. Eventually, it leads to different pore connectivity in different shale samples, and samples with largest porosity do not necessarily has the characteristics of the largest content of connected pores.
By comparison, the content of brittle minerals in Doushantuo Formation shale is similar to that in Barnett shale, but the content of clay minerals is relatively low. The results show that the high content of brittle minerals, such as quartz, is not only conducive to the formation of micro-fractures and the increase of storage space, but also beneficial to the reservoir fracturing and shale gas exploitation. The high calcite content of carbonate minerals is conducive to the development of dissolution pores. The mineral composition of shale is dominated by quartz. Quartz from different sources reflects different sedimentary environments and has different effects on the physical properties of shale reservoirs. Generally, the mineral components of shale are divided into organic and inorganic types. Based on the previous research results, two types of quartz are summarized, including detrital quartz and authigenic quartz. The authigenic quartz of biological origin show obvious correlation with organic matter content. The shale containing the highest content of authigenic quartz of biological origin shows the highest porosity and permeability, and the best physical properties, which are fully reflected by the specialty of Wufeng-Longmaxi Formation biogenic silica shale.
3.1. Microscopic Pore Structure Characteristics
3.2. Porosity and Permeability