The influence of diabase on the occurrence of coalbed methane in Zhangjiakou Xuandong mine

Zhangjiakou Xuandong mine is a high gassy mine. For finding out the occurrence regularity of coal mine methane and its cause of formation, this paper consults a large number of data exposed by investigation boreholes and roadways, studies intensively the intrusion age, occurrence characteristics and spatial distribution of diabase, analyzes the varying pattern of coal seam, coal quality and coal bed gas, discusses the influence of diabase on coal bed gas. Interrelated analyses conclude that Xuandong Diabase intrudes into Jurassic Xiahuayuan coal measure strata in the form of bed rock, resulting in abnormal geothermal fields and regional thermal metamorphism of coal bed in the study area. The thermal metamorphism of coal is the direct cause for the secondary hydrocarbon generation. This kind of hydrocarbon generation is the most important factor for current gas content, playing decisive role on the generation of coalbed gas. The distribution characteristics of coal and coal bed gas in this area are highly consistent with the occurrence characteristics of diabase bed. Diabase bed is a good caprock which blocks the escape route of the gas. The bed rock provides conditions for the protection and enrichment of the gas.