Research on the controlling effects of a layered sill with different thicknesses on the underlying coal seam gas occurrence

•Samples under different thickness of the sill are collected for analysis.•Coal physical parameters and gas occurrence variations under the sill are analysed.•Combined effects on the underlying coal seam of the sill are interpreted.

Regional thermal effect plays a major role in coal metamorphism in China. Igneous intrusion provides a high-pressure and high-temperature environment for coal seams, which promotes the thermal evolution of coal seams, accelerates the generation of gas and causes enormous changes in coal metamorphism, pore structure and the adsorption characteristics. Magmatic activities are widespread in the Haizi coal mine with an average thickness of 120 m of igneous sill, which has resulted in 11 gas outbursts. Fifteen samples of the No. 10 coal seam were collected from the surface wells (10 samples) and the underground boreholes (5 samples) respectively, and the multiple coal physical parameters, gas occurrence, pore structure characteristics and outburst indexes were analysed to study the effects of igneous sill with different thicknesses on the underlying coal seam gas occurrence. In addition, rock samples of different lithology were collected to obtain the rock mechanical properties and permeability characteristics. The results demonstrate that the distribution of the sill with low permeability has significant effects on the coal rank and gas occurrence of the underlying No. 10 coal seam, which results in coal ranks ranging from gas coal to lean coal. With an increasing thickness of the overlying igneous sill, the coal pore structure is developed, the gas adsorption capacity is enhanced, and the gas content and the outburst indexes are increased. Under the combined effects of intrusion stress, entrapment and thermal evolution of the igneous sill, the underlying coal is easier to become soft coal, thus increasing the outburst proneness.