Abstract: To study the controlling effect of extremely thick igneous rock on gas occurrence of underlying coal seam, five coal samples had been selected in igneous rock covered areas with different thickness in No.10 Coal Seam of Haizi Coal Mine. The changes of coal firmness coefficient, initial methane diffusion velocity, methane adsorption constants, outburst risk index value, etc, as the increasing of igneous rock thickness were tested in this paper. Meanwhile, the proximate analyses, micro porous measured by carbon dioxide, scanning electron microscopy and maceral were used to study the influence of igneous rock on the coal quality and pore structure. The results show that the distribution of extremely thick igneous rock plays a control role in the coal property and gas occurrence of underlying No.10 Coal Seam. The metamorphic degree of No.10 Coal Seam has been improved under the thermal evolution and thermal metamorphic effects of igneous rock. The underlying coal seam is gradually improved from long flame coal in without igneous rock covered area to lean coal under the extremely thick igneous rock. With the thickness increase of overlying igneous rock, the micro porous of No.10 Coal Seam developes, the ability of gas adsorption and reservoir enhances, and the gas content increases. The strength of underlying No.10 Coal Seam decreases under the squeezing action of igneous rock intrusion, and soft coal layer can be easily formed in local area. Thus, the risk of coal and gas outburst increases.

Key words: extremely thick igneous rock, thermal metamorphic effect, coal property, vitrinite reflectance, gas occurrence