Geological Control of Fold Structure on Gas Occurrence and Its Implication for Coalbed Gas Outburst: Case Study in the Qinan Coal Mine, Huaibei Coalfield, China
A systematic analysis on gas occurrence in geological structure zone is crucial to coalbed gas exploitation, gas disaster prevention and outburst risk prediction. To study the geological control of fold structure on gas occurrence and its contribution on coalbed gas outburst, a typical case taken from Qinan coal mine was investigated by laboratory tests, theoretical analysis and on-site exploration. Thereby, a comprehensive understanding was made from the influence of fold structure on tectonic stress, coal structure and gas occurrence based on integrated factors outburst hypothesis. The results indicate that burial depth from 470 to 675 m has a slight influence on the fundamental properties of 72 coal seam, whereas the existence of Wanglou anticline is more relevant to affect vitrinite reflectance, gas adsorption and emission capacity with a positive correlation. By comparing the field data of gas content, pressure and emission quantity, it can be discovered that gas occurrence in 81 mining area appears to be primarily controlled by fold structure. Similar trends can be found in drilling cuttings index, indicating that geological stress increases with the distance approaching Wanglou anticline. In this case, a detailed description of tectonic stress in studying area is conceptually presented. Based on physical, pore structure and surface analyses, further knowledge on structural transformation by fold structure demonstrates adsorption and seepage pores are well developed with complex and disordered surface morphology, providing an evidence on the geological formation mechanism of tectonic coal in Wanglou anticline zone. Moreover, a conceptual description is proposed to explain the generation, migration and preservation mechanism of gas occurrence in the fold zone, which is consistent with roof and floor lithological properties as well as gas content data. Additionally, the locations of six gas outburst accidents were investigated to verify the relationship with fold structure. Based on the above results, theoretical analysis for coal mine outburst is discussed, which highlights the effect of tectonic stress on gas occurrence and coal structure as well as their contributions to gas outburst.