The effects of magma intrusion on localized stress distribution and its implications for coal mine outburst hazards

Magma intrusions into coal seams are common throughout the world. The high temperatures and pressures induced by magma intrusion can alter the properties of coal, including its rank, mechanical properties, cleat and pore structures and adsorption-desorption behaviors. Because the thermal properties of magma intrusions also affect localized in situ stress distributions, both the alteration of coal properties and changes in stress distribution can directly influence coal seam gas outburst disasters. This phenomenon was recently observed and confirmed by several outbursts in the Haizi coal mines of Huaibei, China. In this paper, the processes and mechanisms of this magma intrusion are comprehensively analyzed based on an integrated data set including previously published geological data, in-house laboratory tests, and field observations. The results demonstrate that there is an intrinsic relationship between the intrusive sill and the subsequent in situ stress distribution. These data suggest that the magma intrusion in the Haizi coalfield originated as a crust-mantle type controlled by magmatic underplating; later transported mantle-derived magma successively along the Tanlu Fracture Belt, the Subei Fault, and the Daliujia Fault; and finally intruded into the coalfield as a thick sill. By comparing the lithologies of the coal seam roof and floor, as well as the distances between key coal seams (No. 3 and No. 7), it was discovered that the magma intrusion that entered the No. 5 coal seam was also associated with erosional effects. The squeezing that occurred during this intrusion changed the in situ stress distribution within the region, which increased along with the increasing thickness of the overlying sill. Therefore, the increased in situ stress also caused an increase in the outburst risk of the underlying coal seam, a conclusion that was verified by measuring the quantity of drilling cuttings on working faces as well as onsite outburst events.