*4.2. Storage Spaces*

The morphology of pores in tight gas reservoirs is extremely complicated due to intense mechanical compaction and cementation. As exhibited in SEM images (Figures 3 and 4), the storage space of tight gas reservoirs in the Xujiaweizi Rift can be divided into three categories: intergranular pores, dissolution pores, and intercrystalline pores. Intergranular pores are primarily distributed between rigid particles (Figures 3E and 4A), such as quartz and feldspar, which have good resistance to compaction. In a polished SEM image, these pores are commonly presented as triangle or polygon with straight and

smooth edges (Figure 4A), and they can be easily formed into residual intergranular pores due to the filling of argillaceous and siliceous cement, such as grain-coating chlorite and authigenic quartz (Figure 4B). Dissolution pores are principally related to unstable components (Figure 4C,D), including feldspar and carbonates, which are susceptible to organic acids. Compared with intergranular pores, these pores have the characteristics of a fairly anomalous pore shape and relatively good connectivity due to the presence of dissolution channels. Intercrystalline pores are mostly found within clay minerals (Figure 3), including I/S, chlorite, and illite. This kind of pores are numerous but are generally below 2 μm in size. These pores can contribute to a certain amount of storage space, but they contribute rather less to the permeability of tight gas reservoirs.

**Figure 4.** Pore types of tight gas reservoirs in the Xujiaweizi Rift, identified by SEM images. InterC. = intercrystalline; InterG. = intergranular; Diso. = Dissolution. (**A**) Sample #1; (**B**) sample #3; (**C**) sample #6; (**D**) sample #6.

#### *4.3. Pore Structure Derived from N2GA and RMIP Analyses*

As shown in Figure 5, nitrogen adsorption isotherms of nine tight rock samples are attached to Type IV isotherm based on the classification scheme proposed by Brunauer et al. [49], which is a symbol of mesoporous materials (2–50 nm). The hysteresis loops produced by capillary condensation in pores larger than 2 nm are suitable for Type H3 according to IUPAC (International Union of Pure and Applied Chemistry) classification [50], which are usually interpreted to slit-shaped pores given by aggregates of ductile plate-like particles, mainly clay minerals, as exhibited in Figure 6. The above features are consistent with the observations of tight rocks from the Yanchang Formation in the Ordos Basin [19]. The pore volumes and specific surface areas of nine tight rock samples from the N2GA analyses present a wide range, as listed in Table 2. Sample #8 shows the highest surface area at 3.41 m<sup>2</sup>/g, while sample #7 shows the lowest at 0.213 m<sup>2</sup>/g, with a mean of 1.23 m<sup>2</sup>/g. The values of specific surface area are quite similar with the tight gas siltstone samples studied by Clarkson et al. [11]. Sample #8 corresponds to the largest pore volume at 0.010213 cm<sup>3</sup>/g, whereas sample #3 has the smallest value of 0.002324 cm<sup>3</sup>/g.

**Figure 5.** Nitrogen (N2) adsorption–desorption isotherms of nine tight rock samples in the Xujiaweizi Rift.

**Figure 6.** The slit-shaped pores related to the aggregates of plated-like clay minerals.

**Table 2.** Pore structure properties derived from N2GA and RMIP experiments for nine tight rock samples in the Xujiaweizi Rift.


SSA = specific surface area; *Stotal* = total mercury intrusion saturation; *Spore* = mercury intrusion saturation in pores; *Sthroat* = mercury intrusion saturation in throats; *RPTa* = ratio of pore to throat radius; *ra* = average throat radius; *rd* = maximum connected throat radius corresponding to displacement pressure; *rm* = mainstream throat radius; *Pd* = displacement pressure.

The RMIP curves of the nine studied tight sandstones are displayed in Figure 7. At the initial period of mercury intrusion, the shape of total intrusion curves is more consistent with that of throat intrusion curves, and this coincident trend will be more evident with decreasing permeability (Figure 7). The total mercury intrusion saturation derived from the RMIP method ranges from 26.25% to 65.70%, averaging 52.67% (Table 2), and the mercury intrusion saturation in throats mainly ranges from 24.60% to 54.27% (38.74% on average), evidently higher than that in pores corresponding to 1.65%–28.13% (13.93% on average). Owing to the relatively low mercury intrusion pressure (≈6.2 MPa), a large proportion of pores below 0.12 μm in radius cannot be well revealed by this method.

**Figure 7.** RMIP curves of total (pores + throats), pores and throats for the studied tight rock samples in the Xujiaweizi Rift.

As shown in Figure 8, the pore size of tight rocks obtained from N2GA method mainly ranges from several nm to ≈200 nm (red columns). For the RMIP method, all of the tight rock samples exhibit similar pore size distributions primarily ranging from 200 to 600 μm in dimensions (blue columns), and the pore volumes show a sharp decrease with increasing clay mineral content (Figure 8), whereas their throat size distributions witness considerable differences (green columns), mainly between 0.24 and 6 μm (Figure 8). The average *RPTa* is between 81.10 and 332.32 (Table 2), exhibiting a positive correlation with clay mineral content. Based on Figure 8, we also found that with increasing clay mineral contents and decreasing permeability, the throat distribution curves (black solid lines) are narrower, and the values of throat size drop gradually. In addition, several relevant throat radius

parameters, such as *ra* (average throat radius), *rd* (maximum connected throat radius corresponding to displacement pressure), and *rm* (mainstream throat radius), are positively correlated with permeability, as Figure 9 exhibits, which further indicates that the throat radius rather than pore radius controls the flow properties of tight rocks.

**Figure 8.** Pore size distributions of nine tight rock samples with various contents of clay minerals and permeability analyzed by nitrogen gas adsorption (N2GA) and RMIP experiments. The number inside the parentheses is the content of clay minerals.

**Figure 9.** Correlations between permeability and *ra* (average throat radius), *rd* (maximum connected throat radius corresponding to displacement pressure), *rm* (mainstream throat radius) obtained from the RMIP method.
