Influence of igneous intrusion on coal and gas by means of element geochemistry and vitrinite reflectance method: a case from the Huainan coalfield, northern China

Abstract

ABSTRACTMaterial composition changes of coal and gas under igneous intrusion have been one of the hot research fields in global geology. In this study, through in-depth discussion of a case from Huainan coalfield, North China, some important characteristics of thermal altered coal (TAC) are expounded by using the methods of element geochemistry and vitrinite reflectance (Rmax, Ro, and Rmin) calculation. The results show that the profile of coal reflectance/thermal metamorphic temperature (TMT) vs. distance from the intrusion indicates that the thermal aureole range reveals by the thickness of 2.4 m intrusion is approximately 3.24 m. Close to intrusion, the carbon (Cdaf) of coal increases significantly, while hydrogen (Hdaf) and oxygen (Odaf) decreases significantly, and the nitrogen (Ndaf) change is very irregular. After acid treatment, the volatile matter (VMacid) and ash yield (Ashacid) losses of coal have reached 5× and 4× of the ambient zone. The atomic ratios O/C and H/C are parallel to the changes of VMacid. Based on Rmax-Rmin diagram, the pressure effect of TAC is analyzed. In the actual geological model, the coal is mainly subjected to the bearing stress perpendicular to the contact surface instead of the vertical gravity of the magma body, and the applicable conditions of the theoretical model are proposed. Summing up the previous calculation methods of TMT, the exponential function method can be used when the Rmax is <1.0. On the contrary, the previous method can be used. The empirical formula for calculating the carbon isotope (δ13 CCH4) of gas by using Rmax of TAC shows a negative result. The δ13 CCH4 of the gas near the contact is relatively positive, which proves to some extent that 13 C of the thermal residual coal and 12 CH4 of the gas are subjected to carbon isotope fractionation. Here, the regression model between the Rmax of TAC and δ13 CCH4of gas in the case can be expressed as δ13 CCH4 = 16.777Ln(Rmax)-49.859.