## 2 Geological Setting  
The Western Hubei region is in the Western Hunan-Hubei fold belt in the central Yangtze platform of the South China plate. Through numerous orogenies, including the Caledonian, Hercynian, Indosinian, Yanshanian, and Himalayan, this region developed with two sets of basin-controlling faults trending NNE and NW. The basin is actually a residual basin formed by the modification and superposition of several prototype basins [32; 33; 34; 35]. The Western Hubei region is adjacent to the Sichuan Basin in the northwest (with the Qiyueshan fault as its border), the Huangling anticline-Yichang slope belt to the northeast (with the Tianyangping-Jianli fault as its border), and the Jiangnan-Xuefeng overthrust belt to the southeast (with the Cili-Baojing fault as their boundary) [34; 36]. It contains four secondary tectonic units: the Lichuan complex syncline, Central complex anticline, Huaguoping complex syncline, and Yidu-Hefeng complex anticline (Figure 1c).  
In the Changxing period (late Permian), the South China plate was near the paleo-equator and adjacent to the Paleo-Tethys Ocean in the west and Panthalassa in the east, with numerous small land masses along its periphery and thus exhibiting a multi-island ocean basin framework overall [37] (Figure 1a). Most of the Yangtze region of the South China plate was a carbonate platform, and the Dalong Formation, dominated by deep-water siliceous facies, was distributed on the northern and southern edges of the Yangtze platform [38]. Late Permian strata in Western Hubei were deposited in a secondary rift basin within a platform on the northern edge of the central Yangtze region; this basin was surrounded by the platform on three sides and connected to the vast ocean to the north, with a semi-enclosed sedimentary environment (Figure 1b). Dalong Formation shales are mainly developed in the rift trough, which hosts deep-water deposits, whereas the Changxing Formation limestone is deposited on the platform around the trough [25]. Frequent sea-level fluctuations during the deposition of the Dalong Formation produced  
Figure 1: (**a**) Global paleogeographic map at the end of the late Permian (modified from [37]), (**b**) Paleogeography of late Permian lithofacies of the South China plate (modified from [30]), (**c**) Present-day tectonic map of Western Hubei.  
siliceous mudstones, carbonaceous mudstones, calcareous mudstones, siliceous rocks, and bioclastic limestones.  
In well ED-2 in the Enshi area of Western Hubei, the deposition of the Dalong Formation occurred in a sedimentary rift trough within the platform. There, the Dalong Formation is entirely composed of siliceous shales, and the inter-platform basin facies mainly control the shales' thicknesses, mineral compositions, and depositional environments [22; 39; 40; 41; 42]. From bottom to top, the stratigraphic column of well ED-2 can be subdivided into three intervals based on lithological changes (Figure 2): (1) Bioclastic limestones interlayered with grayish-black siliceous shales, (2) Grayish-black carbonaceous-siliceous shales interlayered with thinly bedded marks and dolostones, (3) Marls interlayered with thinly bedded grayish-black siliceous shales. The well logs show that the mudstone interval has high natural gamma ray (GR) values and low deep (RD) and shallow (RS) lateral resistivities; the limestone interval has low GR values and high RD and RS, and the resistivity of the siliceous mudstone interval in the middle gradually increases from deep to shallow.