**2. Geological Background**

The Sichuan Basin is a structurally complex, superimposed basin that comprises an area of over 18 × 10<sup>4</sup> km2. The Sichuan Basin and the surrounding areas contain many gas fields (Figure 1). The two largest shale gas fields in the Sichuan Basin, Weiyuan and Changning, are located on the south and east borders of the basin, respectively. The organic-rich shales of the Wufeng–Longmaxi Formation (Ordovician–Silurian), one of the most important hydrocarbon source rocks in China, are the source of the shale gas generated ftom these two shale fields. The Silurian shales have equivalent vitrinite reflectance (EqVRo, %) values ranging from 2.4% to 3.8% [24–26], indicating that they are largely thermally over-mature and in a dry gas generation stage [24,27]. One possible external heat source for the maturation of organic-rich shale is the Emei Large Igneous Province (LIP), located in the southwestern part of the basin [28,29].

**Figure 1.** Geological sketch map of the Sichuan Basin, showing the locations of the main gas sampling sites, isolines of Ro values, and shale thickness of the Longmaxi Formation.

The Sichuan Basin is in the transition zone between the Palaeo–Pacific tectonic area and the Tethys–Himalayan tectonic area [30]. The Caledonian, Hercynian, Indosinian, and Yanshanian orogenies and Himalayan movement have all been recorded in the Sichuan Basin (Figure 2). There were two major tectonic evolution stages in the history of the Sichuan Basin: an early cratonic depression during the Palaeozoic era, followed by a foreland basin stage in the Triassic era [30–33]. This could have generated a large number of faults and unconformity surfaces, leading to diverse hydrocarbon migration and gas preservation [34]. The Lower Silurian Longmaxi Formation experienced deep burial in the Yanshan period, causing gas generation to reach its peak. Subsequent repeated uplift and erosion caused hydrocarbons to migrate out of the formation [35,36].


**Figure 2.** Schematic diagram showing the stratigraphy system of the Sichuan Basin, as well as the main tectonic events.

The Weiyuan (WY) area extends over 2700 km<sup>2</sup> on the southeastern edge of the Leshan–Longnvsi paleo uplift in the southwestern part of the Sichuan Basin (Figure 1) [37]. Silurian strata are present in the southeastern part of the WY area, but are missing from its northwestern part. Silurian Longmaxi Formation shale is characterized as graptolite shale, mainly organic type I with an EqRo range from 1.80% to 2.24% [38,39]. It is primarily found on the southeastern flank of the Weiyuan anticline at a current burial depth of 1600–3200 m. The Changning (CN) area is located southwest of the Changning anticline in a gentle flank zone. The CN has an overall area of nearly 4000 km<sup>2</sup> (Figure 1). The CN area is closer to the core zone of Emei Large Igneous Province (LIP) than the WY area (Figure 3). In this area, the Longmaxi shale mainly belongs to type I-II1 organic matter, with a thermal maturity Ro of 2.8% to 3.3% [40,41].

**Figure 3.** Schematic map of the Emeishan large igneous province (ELIP) and its thermal effect (Jiang, 2017, modified from Sun et al., 2010). The dashed lines show the boundaries of the inner, intermediate, and outer zones of the ELIP, as defined by He et al. (2003). For the interpretation of the references for color in this figure legend, the reader is referred to the web version of this article. The red column represents the Permian heat flow and the green column represents the present heat flow.

#### **3. Di**ff**erences in Gas Geochemical Characteristics**
