# Estimation of Sedimentary Rock Porosity Using a Digital Image Analysis
## 3 Results and Discussion
### Scanning Electron Microscope (SEM) and Pore and Crack Analysis System (PCAS)  
Scanning electron microscope images of each rock specimen are given in Figure 6. Upon examination of these images, the voids and fissures can be clearly identified as distinct black areas. The following observations can be made for each rock type:  
* The SEM image surface of the sandstone contains a variety of mineral crystal particles such as kaolinite, calcite, and quartz, with a large number of intergranular pores and microcracks, mainly distributed around the mineral crystals. At a 500\(\times\) magnification (Figure 8a), the voids and quartz mineral grains can be clearly observed to be uniformly distributed over the entire sample surface. At a magnification of 10,000 (Figure 8d), the quartz grains appear to be of different sizes, with more visible kaolinite particles (flakes), and an increasing porosity space. Under a greater magnification of 30,000\(\times\) (Figure 8g), the kaolinite particles between the sandstone grains can be clearly identified. Moreover, it is evident that the kaolinite flakes are being sandwiched between the quartz minerals, and a small amount of calcite adhering to the kaolinite and quartz can be recognized. Figure 7: Experimental data plotted as the differential intrusion against the pore size diameters of sandstone (**a**), siltstone (**b**), and coal (**c**) (dashed line represents the corrected data). The siltstone is mostly made of larger-sized quartz grains and kaolinite particles that create intergranular pores with a large pore area. At a 500\(\times\) magnification (Figure 8b), a large number of kaolinite flakes can be clearly observed on the surface of the quartz, with a relatively large amount of voids formed between these flakes. The pore space appears to be evenly distributed. As can be seen in the image with the 10,000\(\times\) magnification (Figure 8e), the kaolinite particles/aggregates are located on the quartz grains. At the 30,000\(\times\) magnification (Figure 8h), the kaolinite flakes are attached to the smooth quartz surfaces. The voids in the siltstone consist mainly of gaps between the quartz particles, which can be observed at every level of magnification. * Many pores and cracks can be found on the surface of the coal specimen. At the magnification of 500x (Figure 8c), the micro cracks seem to be evenly distributed. At the magnification of 10,000\(\times\) (Figure 8f), a large number of voids and cracks are also evident. At the magnification of 30,000\(\times\) (Figure 8i), large micro voids can be clearly identified, which are not noticeable at the lower magnifications of 500\(\times\) and 10,000\(\times\). In summary, at the same magnification, the siltstone specimen displays significantly fewer fissures and voids than that of the sandstone. The coal specimen has a distinct pattern of voids and fissures compared to the sandstone and silt, with more evenly distributed voids. By increasing the magnification around the crack area, it is possible to study the microscopic fissures and mineral characteristics of the rock. By comparing Figure 8d,g and  
Figure 8: Scanning electron microscope images of sandstone (**a**,**d**,**g**), siltstone (**b**,**e**,**h**), and coal (**c**,**f**,**i**). Figure 8e,h, it can be determined that the minimum pore size of siltstone is significantly smaller than that of sandstone. The pore size of sandstone is estimated to be around 100 nm, which agrees with the MIP data. Figure 8f,i depicts the void distribution in the coal specimen. It is evident from this figure that the voids are uniformly distributed, a finding that is also consistent with the MIP test results. It is noted that, compared to a relatively small magnification (500\(\times\)), at larger magnifications such as 10,000\(\times\) and 30,000\(\times\), the image analysis seems to overestimate the specimen porosity, as some 'extra' pore space (which may not exist) can be created by shadows or due to the irregular shape of pores.