Petrographic and geochemical effects of sill intrusions on coal and their implications for gas outbursts in the Wolonghu Mine, Huaibei Coalfield, China

Since the early 1980s, fifteen outbursts have occurred in the Huaibei Coalfield of China. These outbursts were reported to be associated with sills. To study the effect of sill intrusions on coal seam and their relationship to methane outbursts, eleven samples from the No. 10 coal seam were taken from the Wolonghu Mine at various distances from a diorite sill. Comparisons were made between unaltered and heat-affected coals using petrographic and chemical data, micropore characteristics, adsorption properties of coal, and gas outburst indexes from field. Approaching the intrusion, vitrinite reflectance levels increased from 2.74% to 5.03%, and the thermal aureole of the sill ~ 60 m (from the sill boundary to sample 9). Three zones along this gradient were identified as corresponding to (1) thermal evolution zone No. 1 (0–5 m from sill), (2) thermal evolution zone No. 2 (5–60 m from sill), and (3) unaltered zone. The methane adsorption capacity of coal samples in the thermal evolution zone No. 2 was generally higher than in the two other zones, and the unaltered zone higher than the thermal evolution zone No. 1. It is concluded that the contact-metamorphism decreased the adsorption capacity of coal and the thermal evolution of sill increased it. The trap effect of sill, combined with the mudstone and siltstone roof and floor of the No. 10 coal seam, provided a seal for the formation of a gas pocket. Abnormally high formation pressures at the No. 10 coal seam led to two outbursts.

► Approaching the sill, Ro levels increased from 2.74% to 5.03%, thermal alteration ~ 60 m.
► Contact-metamorphism decreased the adsorption capacity of coal (0–5 m from the sill).
► Thermal evolution of sill increased adsorption capacity of coal (5–60 m from the sill).
► Abnormally high formation pressures at the No. 10 coal seam led to two gas outbursts.