## 2 Geological Settings  
The Guxu mining area of SSC is administratively subordinate to the Gulin County and Xuyong County of Luzhou City, Sichuan Province (Figure 1a,b), and it is divided into eleven blocks, such as Lianghe, Gulin, Dacun and Heba. The Guxu mining area is tectonically located in the Xuyong-Junlian superimposed fold belt on the southern margin of the Sichuan foreland basin, and it is mainly composed of Gulin anticlinorium and several secondary folds (Figure 1c). The faults with a main NE strike are well developed in the hinge zone and two limbs of Gulin anticlinorium, and most of the faults are reverse faults. The study area, Dacun Block, is, on the whole, a synclinal structure (Dacun syncline). The axis of the Dacun syncline is nearly SN at the southern section, and NE-SW at the middle and north sections. The dip angle of the formation in the Dacun Block is generally 30-50\({}^{\circ}\) and can maximally reach 85\({}^{\circ}\) in the local areas. The secondary folds are not developed in the Dacun block; the faults are mainly developed in the northwest and south parts of the block (Figure 1c); it is proved through investigations that most faults in the Dacun Block do not cut coal seams. The strata outcropped in the Dacun Block include Silurian, Permian, Jurassic, and Quaternary from the bottom to the top. The coal-bearing stratum is the Upper Permian Longtan Formation (P\({}_{3l}\)), which is deposited in an interactive marine and terrestrial environment. The lower part of P\({}_{3l}\) is deposited in a lagoon-tidal flat environment, and the middle and upper parts are deposited in a delta-tidal flat environment [33]. The thickness of P\({}_{3l}\) ranges from 65.60m to 118m, with an average of 82.79m (Figure 2). The P\({}_{3l}\) is mainly composed of mudstone, sandy mudstone, muddy siltstone, siltstone, fine sandstone and coal, and carbonaceous mudstone. The P\({}_{3l}\) in the Dacun Block has a good coal-bearing property with a large number of coal layers and a stable thickness of main coal seams. The coal seams usually have a simple structure with 0-3 layers of gangues. More than 20 coal seams occur in the P\({}_{3l}\) of study area, and among them, 9 coal seams can be compared regionally and are numbered C\({}_{11}\), C\({}_{13}\), C\({}_{14}\), C\({}_{16}\), C\({}_{17}\), C\({}_{21}\), C\({}_{23}\), C\({}_{24}\), and C\({}_{25}\) from top to bottom. Among them, C\({}_{13}\), C\({}_{14}\), C\({}_{16}\), C\({}_{17}\), C\({}_{23}\), C\({}_{24}\), and C\({}_{25}\) are commercial coal seams. The total thickness (excluding gangue) of coal seams in the P\({}_{3l}\) varies from 4.35m to 19.76m (avg. 10.99 m), accounting for 6.35-21.47% (avg. 13.42%) of the total thickness of P\({}_{3l}\). Figure 1: **(a)** Location of Sichuan Province in China. **(b)** Location of Guxu coalfield in Sichuan Province. **(c)** The major structures in the Guxu coalfield and the location of the Dacun Block in the Guxu coalfield. The macrolithotypes of coals in the Dacun block are dominated by semi-dull coals (including C\({}_{13}\), C\({}_{14}\), C\({}_{16}\), C\({}_{17}\), and C\({}_{23}\)), followed by semi-bright coals (including C\({}_{21}\), C\({}_{24}\), and C\({}_{25}\)). The true and apparent densities of coals range from 1.20 to 1.98 g/cm\({}^{3}\) (avg. 1.60 g/cm\({}^{3}\)) and 1.35 to 1.83 g/cm\({}^{3}\) (avg. 1.54 g/cm\({}^{3}\)), respectively. The organic macroals of coals are dominated by vitrinite (vol. 59.12-88.74%; avg. 73.35%), followed by intertinite (vol. 11.26-40.88%; avg. 26.65%), and exinite is generally rare. The minerals in coals account for 10.80-44.60% (avg. 25.48%) of the total volume and are mainly composed of clay minerals, followed by calcite and sulfide. Proximate analysis shows that the moisture contents (M\({}_{ad}\)) of coals vary from 0.25 to 5.27% (avg. 1.30%); ash yields (A\({}_{d}\)) at 8.87-39.55% (avg. 23.92%), volatile yields (V\({}_{d}\)) at 4.96-30.32% (avg. 7.80%); and fixed carbon content (FC\({}_{d}\)) at 12.95-83.24% (avg. 68.29%).