## 2 Target Overview  
Commercial gas production has been achieved from the Lower Silurian Longmaxi shale in the eastern Sichuan Basin (ESB) [4; 25]. In this paper, the research target is the Lower Silurian Longmaxi shale in the WHH fold belt, adjacent to the ESB (Figure 1a). Structurally, the WHH fold belt is composed of a series of anticlinoria and synchionria formed by multistage tectonic deformation. Compared with eastern Sichuan Basin, the WHH fold belt belongs to a tectonically active area, which results in the strong heterogeneity of shale gas distribution [26]. The Longmaxi Formation in both the WHH and ESB areas is integrated on top of the nodular limestone of the Ordovician Baota Formation and under the yellow-green shale of the Luoreping Formation, which indicates their sealing capacities to be comparable [27]. The sedimentary environment in both WHH and ESB areas in the early Silurian was shallow water semi-closed retention basin, which made that the WHH and ESB Longmaxi shales have similar mineral composition, such as clay minerals accounting for 34.7% and 34.6% on average, feldspar accounting for 7.3% and 8.3% on average, and quartz accounting for 56.5% and 44.4% on average, respectively [25; 27]. Field investigations and laboratory measurements have confirmed that the hydrocarbon generation ability and organic matter maturity of the Longmaxi shale in the WHH area are comparable to those in the ESB area [27]. The thicknesses of black Longmaxi shales with total organic carbon (TOC) greater than 2% range from 15 to 30 m in the WHH area, and these in the ESB area range from 20 to 50 m. The average kerogen reflectance of Longmaxi shales in the WHH and ESB areas are 2.68% and 2.69%, respectively, which indicates that they are both in the high maturity stage. In addition, the burial depths of the Longmaxi Formation in the WHH area range from 0 to 3300 m, which is basically in line with the favored burial depths (100-3500 m) for shale gas production [28]. Therefore, Longmaxi shale gas in the WHH fold belt is of great potential to exploration. Specifically, the study area (blue rectangle in Figure 1a), covering an area of about 1189.7 km\({}^{2}\), is located between the Yidu-Hefeng anticlinorium and the Sangzhi-Shimen synclinorium. Figure 1b is the elevation plan of the bottom interface of Longmaxi Formation in study area. Only the target formation in the northwest and middle of study area is retained due to erosion. The retained formation is characterized by well-developed faults and high-steep structures (elevation range \(-\)2500-0 m). Owing to erosion and fluctuating terrain, the surface elevation of study area varies dramatically between 0 and 1300 m, which is not conducive to three-dimension (3D) seismic acquisition. Now the study area is covered by twelve 2D seismic lines (dashed lines L1-L8, C1-C4 in Figure 1b). Six wells are drilled through the Longmaxi Formation in the study area, of which S-3 is a gas-free well, S-1 is a poor gas well, and the other four are gas wells (circles with different fill colors in Figure 1b). Deviated well SX-1 and vertical wells S-2, S-3, and S-4 are located near the seismic lines. Figure 2 displays the logging data of the vertical wells S-2, S-3, and S-4. The thickness of target formation is about 100 m, and its lithology is mainly carbonaceous shale with intercalations of argillaceous siltstone in the Longmaxi Formation. Here, shale is characterized by high acoustic time difference (AC) and high natural gamma (GR). Conversely, siltstone is characterized by low AC and low GR. As marked by red rectangles, the gas zones are located at the positions with high TOC value (about >2%). Therefore, TOC data can used as an indicator of a potential gas enrichment zone. TOC data from the vertical wells implies that the vertical gas enrichment zone is located near the bottom of Longmaxi Formation. Comparing the TOC data of three wells, we can also see that the lateral distribution of shale gas enrichment zones is different.