**4. Results**

#### *4.1. Mineralogical and Petrophysical Properties*

Porosity, permeability, and mineral components of nine tight rock samples are listed in Table 1. The results display that tight gas rock samples are primarily composed of quartz, feldspar, and clay minerals, and a small amount of siderite and calcite was also found. Quartz and feldspar are the predominant constituents (Table 1), with contents of 30–63 wt.% and 13–64 wt.%, respectively. The content of clay minerals is between 2 wt.% and 24 wt.% (Table 1), and further analyses show that clay minerals mainly contain I/S, chlorite, and illite, corresponding to contents of 0–16.56 wt.%, 0.26–5.5 wt.%, and 0.24–3.36 wt.%, respectively. Due to the grea<sup>t</sup> burial depth of the studied samples, the proportion of smectite in I/S is mainly ≈15% (Table 1), which is in an evolution process from R1 I/S (35% smectite) to R3 I/S (10% smectite), corresponding to a temperature of 170–180 ◦C [41,42]. Almost no kaolinite was detected in these samples, which may be attributed to the chemical reaction between kaolinite and K-feldspar at a relatively high formation temperature of >100 ◦C to form illite and aqueous silica [32,43,44], and the latter may precipitate to generate authigenic quartz aggregates or quartz overgrowth, when the formation conditions change. In the study area, the geothermal gradient is ≈4.4 ◦C/100 m based on Zhou et al. [45], and the average surface temperature is determined as ≈5 ◦C. Thus, the formation temperature obviously exceeds 100 ◦C when the burial depth of these selected

samples is mainly >3000 m (Table 1). Noticeably, the average clay mineral content of the Denglouku Formation is generally lower than that of the Shahezi Formation, which is probably related to their different sedimentary environments.

The single crystal of illite in studied samples is present as ribbon or featheriness with a relatively regular arrangemen<sup>t</sup> (Figure 3A,B), generally between 0.15 and 0.5 μm, whereas the shape of their aggregates is mostly lamellar or fibrous being attached to pore walls, extending far into or completely across pores. In addition, the illite often coexists with feldspar, as a result of chemical reaction between feldspar and organic acid fluids [46]. The single crystal of chlorite is mainly needle-shaped with a dimension from 2 to 3 μm, while their aggregates appear as pompon-, flake-, or rose-like, and wrap the particle surface forming chlorite film (Figure 3C,D). This is favorable for the preservation of storage space in tight reservoirs by inhibiting the overgrowth of quartz [47]. I/S commonly occurs as honeycomb or cotton, primarily distributed in pores with discrete particles (Figure 3E,F).

**Figure 3.** Morphologies of various types of clay minerals and pore types in the tight gas reservoirs of the Xujiaweizi Rift. InterC. = intercrystalline; InterG. = intergranular; I/S = mixed-layer illite/smectite. (**A**) Sample #6; (**B**) sample #6; (**C**) sample #8; (**D**) sample #8; (**E**) sample #1; (**F**) sample #2.

Under a confining pressure of ≈30 MPa, the helium porosities of tight gas rock samples are relatively low, mainly ranging from 6.1% to 10.0%, and the nitrogen permeability primarily varies from 0.0352 × 10−<sup>15</sup> to 2.35 × 10−<sup>15</sup> m2. What calls for special attention is that no obvious positive correlation between porosity and permeability exists in studied samples (Table 1), unlike conventional sandstone reservoirs, generally characterized by better positive correlation between porosity and permeability [48]. Furthermore, we also found that the lower content of clay minerals agrees with a higher permeability (Table 1).
