Accumulation characteristics and models of low-rank multi-source coal-rock gas in the Jurassic of Junggar Basin, China

The Junggar Basin is a large Jurassic low-rank coal-bearing basin in western China, mainly comprising the Xishanyao Formation and the Badaowan Formation, which are characterized by substantial coal measure thickness and stable spatial distribution. The coal-rock gas resources in the whole basin exceed 3 × 10 12 m 3 , and with a maximum daily gas production of 57 × 10 3 m 3 . However, the accumulation characteristics and enrichment laws of coal-rock gas remain insufficiently understood. Subsequent to this development, this paper comprehensively analyzes coal quality, reservoir characteristics, and gas-bearing properties of coal seams, and concludes that: (1) The southeastern margin of the Junggar Basin has superior conditions for coal-rock gas accumulation, characterized by wide distribution of Jurassic coal seams with large thickness, good coal quality, moderate to poor reservoir physical and adsorption properties, and rich coal-rock gas resources. (2) Reservoir formation is jointly controlled by multiple factors, including gas source conditions, structural conditions, and preservation conditions. The identified reservoir types include exogenous fault-occlusion type, exogenous anticlinal type, endogenous pore type, formation pinch-out type, and exogenous fracture type. (3) The enrichment of coal-rock gas shows distinct characteristics of plane distribution, being more favorable in the south than in the north and better in the west than in the east. The main controlling factors include coal quality, thermal evolution degree, coal seam roof and floor sealing and hydrogeological conditions. (4) Using the volumetric method, coal-rock gas resources in the weathered zone shallower than 5000 m across the basin were estimated. In the Urumquqi to Fukang area at depths shallower than 2000 m, the estimated resource volume is 1623.82 × 10 9 m 3 , while resources at depths between 2000 and 5000 m amount to 1316.36 × 10 9 m 3 . Based on an optimal selection index system of advantageous areas, Urumqi–Dahuangshan and Baijiahai are identified as advantageous exploration targets, covering a total area of 2630 km 2 with a predicted resource volume of 320 × 10 9 m 3 , indicating abundant resources and favorable reservoir-forming conditions.