Characteristics of deep Jurassic coalbed methane reservoir and its pore-fracture structure in the Junggar Basin

The physical properties of coal reservoirs are crucial for evaluating deep coalbed methane (CBM) potential. The Jurassic Xishanyao and Badaowan formations in the Junggar Basin are key CBM targets, characterized by thick, stable, and widely distributed coal seams. To compare their reservoir differences, samples were collected and analyzed using coal petrography and pore structure tests. Key findings: ① Coal composition: Xishanyao is dominated by inertinite, followed by vitrinite, while Badaowan shows the opposite trend. Xishanyao has higher vitrinite reflectance. ② Depositional environment: Xishanyao formed in dry-humid forest swamps with herbaceous-woody plants, whereas Badaowan developed in shallow-deep waterlogged swamps dominated by herbaceous plants. ③ Coal quality: Both formations consist of low-rank bituminous coal with low moisture, low-medium ash, and ultra-low sulfur. ④ Pore-fracture features: Xishanyao exhibits balanced microporosity with good connectivity, partial mineral filling, and complex fractures. Badaowan has stronger heterogeneity, simpler fractures, and higher mineral filling. ⑤ The porosity of middle-low rank coal in the deep Jurassic strata of the Junggar Basin follows a “U”-shaped trend with organic matter evolution (inflection point at R_o,max=0.7%), characterized by compaction-induced porosity reduction during the relatively low-maturity stage and hydrocarbon generation-induced porosity increase during the relatively high-maturity stage, with continuous development of microfractures. Coal facies analysis indicates that high TPI, low GI, moderate GWI, and high VI promote the development of porosity in deep coal seams.