## 4 Exploration Practices  
Through the above systematic study of the geological characteristics of the Chang 7, Chang 8, and Chang 9 shale layers in the Yanchang Formation in the study area, it was found that Chang 8 and Chang 9 shales are much smaller in thickness and distribution  
Figure 14: Reservoir parameters of Chang 7 shale in Well Y22. Figure 15: S\({}_{1}\) distribution from rock pyrolysis of shales in the Yanchang Formation. range than Chang 7 shale. However, their organic matter abundance, mineral composition, reservoir physical properties, and shale oil and gas contents are relatively similar to those of Chang 7 shale, so they also have good exploration potential. In addition, through hydrocarbon generation simulation experiments, organic matter maturity, and oil and gas content analysis, it was confirmed that the Chang 7, Chang 8, and Chang 9 shale layers in the Yanchang Formation have the characteristics of oil and gas symbiosis. In the past, the target layer for shale oil and gas exploration in the study area was mainly limited to Chang 7 shale, focused only on one aspect of shale oil or shale gas, and did not involve Chang 8 and Chang 9 shales. In order to achieve the integrated exploitation of multilayer shale oil and gas in the Yanchang Formation and obtain full utilization of the above three sets of shales, multilayer staged fracturing has been carried out on six vertical wells for the Chang 7, Chang 8, and Chang 9 shale layers in the past two years, achieving very high oil and gas production. Taking Well Y13 as an example, it can be seen from Figure 16 that the thickness of Chang 9 shale is 16 m, and the acoustic and resistivity curves show obvious box-shaped characteristics. The thickness of Chang 8 shale is about 13 m, and the siltstone and mud shale are interbedded in this shale, so the resistivity curve presents a multi-thin-layer finger shape. The thickness of Chang 7 shale is about 62 m. As shown in Figure 16, the above three sets of shales have obvious gasometric anomalies. During the initial exploration in 2019, only single-layer fracturing was carried out in the Chang 7 shale layer, and the average daily shale gas production rate was only 471 m\({}^{3}\). Meanwhile, in 2022, multilayer staged fracturing (i.e., re-fracturing) was implemented on the Chang 7, Chang 8, and Chang 9 shale layers (Figure 16); the average daily gas production rate increased significantly to 1574 m\({}^{3}\)/d, with a maximum value of 3024 m\({}^{3}\)/d; and the average daily oil production rate also grew to 1.3 t/d, with a maximum value of 2.5 t/d (Figure 17). Figure 16: Reservoir characteristics of three sets of shales in Well Y13 and the comparison of perforation sections of two kinds of different fracturing. As shown in Figure 18, for another example of Well Y27, the thickness of Chang 7 shale is about 55 m. From well-logging interpretation, its average TOC is 5.1%, its average porosity is 3.1%, and its average gas content is 3.3 m\({}^{3}\)/t. The thickness of Chang 9 shale is about 21 m, its average TOC is about 4.5%, the average porosity is 3.2%, and the average gas content is 3.2 m\({}^{3}\)/t. Chang 8 shale is not developed in this well. After multilayer staged fracturing was implemented on the Chang 7, Chang 8, and Chang 9 shale layers in 2023, the average daily gas production rate was 4745 m\({}^{3}\), with a maximum value of 5886 m\({}^{3}\)/d, and the average daily oil production rate was 1.6 t, with a maximum value of 3.2 t/d (Figure 19). Figure 17: Daily oil and gas production rates of Well Y13. Figure 18: Reservoir characteristics of three sets of shales in Well Y27. Table 1 compares the oil and gas production rates between multilayer staged fracturing of shales in the Yanchang Formation and single-layer fracturing of Chang 7 shale. It can be seen that the overall exploration of these three sets of shale formations in six vertical wells in the study area has achieved significantly increased oil and gas production rates.