Evolution of Composition and Methane Occurrence of Bituminous Coal after Igneous Intrusion: A Case Study of Daxing Coal Mine, Tiefa Basin, China

Abstract

Since 1987, there have been 10 coal and gas outbursts in Daxing Coal Mine, Tiefa Basin. It was reported that these outbursts were related to igneous intrusion. To study the influence of igneous intrusion into bituminous on coal bed methane (CBM) occurrence, eight coal samples were collected at different distances (0.2−10 m) from an intrusion in Daxing Coal Mine. The petrographic and chemical data, pore characteristics, and adsorption properties of unaltered and heat-affected coals were tested and compared. Approaching the dike, vitrinite reflectance (R o ) increases from 0.56 to 1.14%, while moisture decreases from 6.4 to 3.1%. According to R o , with the thermal evolution of igneous rocks, the temperature of heat-affected coal seams generally increases from ∼78 to ∼169 °C. The bituminous coal contained high moisture content before the intrusion, and after the igneous intrusion, partial heat may be used for moisture gasification; the other heat was used to improve coal metamorphism, resulting in little increase in the coal metamorphism as expected. The synergistic effect of igneous thermal evolution and moisture content change improves the adsorption capacity of coal. Affected by the igneous intrusion, the gas content of the heat-affected coal seams No. 4 (above the sill) and No. 7 (below the sill) is higher than that of the unaltered No. 12 coal seam, and the gas content of No. 7 coal seam (beneath the sill) is higher than that of No. 4 coal seam. The low permeability sill has a sealing effect on the gas in No. 7 coal seam. Therefore, the evolution process of CBM occurrence in the Daxing Coal Mine can be speculated, that is, after magma intrusion, the temperature increased, the metamorphic degree of coal increased, and water gasification occurred at the same time. The original adsorption site occupied by moisture was vacated, which improved the methane adsorption capacity of the coal and provided favorable conditions for CBM occurrence.

Influence of magma intrusion on coal geochemical characteristics: a case study of Tiefa Daxing coal mine

“…2-1, 2-2, 2-3, 3-3, 4-2, 7-2, 8, 9, 9-2, 9-3, 10-1, and 10-2 coals (Fig. 2) 17,18 . The magmatic activity in Daxing coal mine is relatively intense, and its activity form is consistent with the regional situation, with two types: eruption and invasion.…”

Effect of magma intrusion on pore structure and trace element migration of anthracite

“…2 − 1, 2-2, 2-3, 3-3, 4 − 2, 7 − 2, 8, 9, 9 − 2, 9 − 3, 10 − 1, and 10 − 2 coals (Fig. 2) 14,15 . Magmatic activity in the area mainly includes eruptions and intrusions.…”

An Experimental Study on the Effect of Magmatic Thermal Evolution on the Molecular Structure of Low-Rank Coal

“…Through an analysis of coal and gas outburst accidents in coal mines, it has been discovered that the proximity of magma intrusion areas is the leading cause of outburst incidents [6][7][8]. The intrusion of magma alters the molecular structure of coal, consequently influencing the physical and chemical properties of the coal [9][10][11]. Typically, coal is primarily characterized as a porous material consisting of complex organic polymers and inorganic compounds.…”