## 2 Geological Setting and Sampling  
The research area is located in the Dazu block of the western Sichuan Basin (Figure 1). Multiple sets of black gas-bearing shale strata have developed in the Sichuan Basin. Among them, the Wufeng-Longmaxi Formation is currently the main target strata for achieving commercial exploitation of shale gas. In the past decade, the utilization of middle-shallow shale gas resources in the Sichuan Basin achieved rapid development. In recent years, the exploration targets for shale gas have shifted towards deep shale. The Dazu block is a region that has successfully achieved commercial development of deep shale gas. In the study area, the Z-3 well is a representative borehole with high production capacity. The Z-3 well is located in the middle of the Puluchang syncline, between the low-gentletectonic area of the central Sichuan uplift and the southern Sichuan low-steep fold belt. The sedimentary environment of the Wufeng Formation and the S\({}_{1}\)I\({}^{1\text{-}1}\) sub-layer in the study area is mainly deep-water continental shelf facies. The lithology is mainly composed of black siliceous shale, black carbonaceous shale, and gray-black silty shale.  
In total, 16 representative samples were selected from the S\({}_{1}\)I\({}^{1\text{-}1}\) sub-layer of the Long-maxi Formation from the Z-3 well of the Dazu block in the western Chongqing area for experimental testing and analysis. The S\({}_{1}\)I\({}^{1\text{-}1}\) sub-layer of the Longmaxi Formation can be partitioned into four sub-layers based on the sequence stratigraphic wavelet analysis, reservoir petrological characteristics, and sedimentary facies markers [21; 22]. Representative samples were selected from these four sub-layers and the underlying Wufeng Formation, as well as the overlying S\({}_{1}\)I\({}^{1\text{-}2}\) sub-layer (Table 1). There are significant differences in mineral composition, total organic carbon (TOC) content, and physical features among the different layers, with extremely significant heterogeneity [23]. The interval differences in reservoir development within the sequence framework of the Longmaxi Formation can serve as a basis for reservoir evaluation and exploration. Sample Q0 was selected from the Wufeng Formation. Samples Q1 and Q2 were selected from the bottom of the Longmaxi Formation, with the lithology of black carbonaceous shale. The horizontal beddings of Samples Q0-Q3 are highly developed, and the bedding planes are rich in graptolite fossils. Samples Q3-Q5, Q6-Q9, and Q10-Q12 were selected from the lower, middle, and upper parts of the S\({}_{1}\)I\({}^{1\text{-}1}\) sub-layer, respectively. Within the S\({}_{1}\)I\({}^{1\text{-}1}\) sub-layer, the content of silty minerals in the samples relatively increases, while the development of horizontal bedding deteriorates. Q13-Q15 were selected from the S\({}_{1}\)I\({}^{1\text{-}2}\) sub-layer. The lithology of the Q13-Q15 samples is mainly silty mudstone and muddy siltstone. In addition to differences in mineral composition, sedimentary structure, organic matter content, and physical properties, the mechanical brittleness of the Z-3 well was proven to have interlayer differences. The TOC content of the 16 samples in the Z-3 well ranged between 0.23% and 4.76%, with an average value of 1.97%. The TOC content of the Wufeng-Longmaxi Formation sh  
Figure 1: Research area and lithological columnar diagram of Q\({}_{3}\)w-S\({}_{1}\)I\({}^{1\text{-}1}\) in the Z-3 well. **(a)** The location of the Chongqing Area and the Z-3 well. **(b)** Lithological column of the Wheng-Longmation Formation in the Sichuan Basin, China.  
from the deep to shallow layers. The samples were rich in quartz, ranging from 23.5% to 70.1%, which increased with the TOC content and burial depth in the Longmaxi formation.