Experiments on the effects of igneous sills on the physical properties of coal and gas occurrence

The violent igneous intrusion produced two layers of igneous cover sills and an annular dike and led to several gas dynamic accidents and repeated phenomena of unusual gas emission in the Yangliu Coal Mine in Huaibei Coalfield, China. Six coal samples were collected at different distances from the igneous sills for the experiments to study the physical properties of coal and gas adsorption/desorption properties. The results indicate that the vitrinite reflectance and ash and moisture contents increase irregularly while the volatile content decreased irregularly with decreasing distance from the igneous sills. The coal mass being researched was divided into three zones based on the degree of thermal erosion: strong thermal erosion zone, weak thermal erosion zone and no thermal erosion zone. The metamorphism of the coal in two adjacent zones changes gradually rather than suddenly. The thermal erosion caused by the igneous intrusion volatilized the organic matters, left a large number of pyrolysis stomata, greatly increased the volume and surface area of pores, especially micropores, and improved the gas adsorption and preserved abilities of the coal. The thermal erosion, gas trapping and tectonic stress effects of the igneous intrusion on the underlying coal mass increased the outburst risk of coal covered by igneous sills to a greater extent than that of coal without the effect of igneous intrusion. The regions covered by igneous sills are sites where the gas resource is enriched, and the engineering applications show that the gas extraction technology via surface wells can ensure the safe and high-efficiency co-exploitation of coal and gas and be of significant commercial value.