*2.2. Reservoir Characteristics*

The tight sandstones are mainly composed of litharenites, with small amounts of lithic quartz arenite and feldspathic litharenite. In terms of mineral content, the average amounts of quartz and feldspar are 69% and 8.2%, respectively [32]. The effect of compaction on the reservoirs is more significant than that of cementation. Figure 1 shows that the compaction process is responsible for the developed geological characteristics such as complex pore structures and network microcracks. The average secondary, primary, and microcrack porosities are 2.9%, 1.1%, and 0.3%, respectively. The grain size varies from fine to medium, and the sorting property varies from poor to good. Moreover, the reservoir exhibits low porosity, low matrix permeability, and small pore-throat radii [31]. The microcracks maintain the reservoir connectivity and facilitate the gas production [33].

**Figure 1.** Thin section of a tight sandstone showing microcracks and a complex pore structure.

#### *2.3. AVO Characteristics*

Amplitude variations with offset (AVO) discriminate lithologies and help to detect hydrocarbons [34]. Rutherford and Williams [35] categorized the AVO responses of mudstone/gas-bearing sandstone interface into the three types. Subsequently, Castagna and Swan [36] added a fourth AVO response type. We analyze the AVO characteristics based on the profiles of Well P, shown in Figure 2, where the target layer is indicated with dashed red lines. Table 1 shows the seismic properties of the sandstone model of the Xujiahe Formation based on the log data. To obtain the P- and S-wave velocities and density, a 60 m interval is selected. The model data are collected from the interval above the top interface of the target layer that has a relatively stable P-wave velocity. For the target layer, we consider average values.

Figure 3 illustrates the AVO of the top (red color) and bottom (blue color) interfaces. The characteristics of the curves agree with the fact that that the P- and S-wave impedances of the target layer are smaller than those of the overburden mudstone. The top and bottom responses are type IV and I AVO curves, respectively.


**Table 1.** Seismic properties, corresponding to the sandstone model of the Xujiahe Formation.

**Figure 2.** Log profiles of Well P. (**a**) P-wave velocity, (**b**) S-wave velocity, and (**c**) density.

**Figure 3.** (**a**) AVO curves and (**b**) synthetic seismic records of the target layer.
