Pore Structure Variation of the Coalbed Methane Reservoir caused by Igneous Intrusion in the Zijinshan Block of the Eastern Ordos Basin, North China

The abnormal thermal effect caused by igneous intrusion has an important influence on the exploration and development of deep coalbed methane. Compared with coals with normal burial depth, coals affected by igneous intrusion or magmatic hydrothermal fluids have obvious changes in reservoir properties of coalbed methane (CBM). The differences between coal reservoirs affected by igneous intrusion thermal effects and undisturbed normal reservoirs need to be investigated. In this work, eight coals from four CBM exploration wells at different distances from igneous intrusion in the Benxi formation were selected from the Zijinshan block of the eastern Ordos Basin. Results indicate that igneous intrusion significantly elevated coal maturity while causing irregular increases in moisture and ash content alongside decreased volatile matter. It further promoted secondary hydrocarbon generation, reduced dolomite content, and enriched hydrothermal minerals/elements. Furthermore, the relationship between the igneous intrusion and the pores in the coals was revealed: <2 nm super micropores contribute the most to the pore volume (PV) and specific surface area (SSA) of the total pore space, and igneous intrusion makes the pore size distribution of the super micropores more complex, which mainly leads to the development of micropores and small pores. Finally, the influence of igneous intrusion on the pore complexity of the coal was analyzed, and it was found that the influence of magmatic heat makes the fractal dimension of medium and large pores decreases with the closer distance to the igneous intrusion, but the fractal dimension of micropores and small pores increases significantly, which increases the surface area of the pores in the coal and leads to the pore surface to be rougher. This study may help understand the multiscale pore structure of deep coalbed methane reservoirs influenced by magmatic thermal effects, providing critical theoretical and technical guidance for optimizing natural gas production in high metamorphic coal regions.