Influence of lamprophyre sills on coal metamorphism, coalbed gas composition and coalbed gas occurrence in the Tongxin Minefield, Datong Coalfield, China

Abstract

Magmatic intrusions in coal-bearing strata can significantly change the physical and chemical properties of the intruded coal. Lamprophyre intrusions are extensive in coal seams 3–5 and 8 of the Carboniferous-Permian coal-bearing strata in the southeastern and southwestern regions of the Tongxin Minefield, Datong Coalfield, China. They appear primarily as sills of varying magnitudes, with thicknesses ranging from 0.15 to 23.51 m. In this area, coals can be classified as unaffected (normal geothermal gradient) and intrusion-affected. Unaffected coals are of high-volatile bituminous rank, with values of maximum vitrinite reflectance ranging from 0.7 to 0.9%, whereas intrusion-affected coals have values of maximum vitrinite reflectance ranging from 0.9 to 7.2%. A marked decrease in the degree of coalification can be observed with increasing distance from the lamprophyre intrusions. Near the intrusion-coal contacts, high-volatile bituminous coals have been thermally altered to form natural coke. The carbonate contents in intrusion-affected coals (0.2–46.1%) are significantly higher than those of the unaffected coal (0–2.0%). The coalbed gases of coal seams 3–5 and 8 are highly variable in both their molecular compositions and overall gas contents. Their CO2 volumes vary from 0.01 to 6.55 m3/t (daf); higher CO2 values are found in the coals affected by intrusions, and roughly decrease with increasing distance from the lamprophyre igneous intrusions toward the central region of the minefield. These variations in gas compositions are mainly related to magmatic activity. The coalbed gas contents in coal seams 3–5 and 8 range from 1.48 to 16.43 m3/t (on a dry, ash-free basis). The positive correlation observed between gas content and the ratio of CO2/CH4 + CO2 occurs only in intrusion-affected coals. Lamprophyre intrusions have negative effects on CH4 content in the study area. Since CO2 has a higher adsorption capacity than CH4, C2H6, and N2, the injection of gaseous CO2 derived from magmatic intrusions can effectively displace CH4-rich reservoir gas. In addition, there is a positive correlation between ash content and gas content on a dry, ash-free basis. Ash content is positively correlated with the ratio of CO2/CH4 + CO2 occurring only in intrusion-affected coals. Massive secondary carbonates that filled in the cleats act as small-scale impermeable seals, which provide a sealing effect both on gas contents and gas compositions during uplift.