Coal-bed methane geology of the No. 2 coal seam in Fengfeng Coalfield, North China

Abstract

The Fengfeng Coalfield is an important coal production base in the Hebei Province of North China. It has been reported that higher coal-bed methane (CBM) contents exist in this area showing good prospects for the exploration of CBM. However, there are few studies on CBM investigation in Fengfeng Coalfield. In order to better understand the geological controls on CBM in the Fengfeng Coalfield, geological surveys and laboratory experiments were conducted that included coal petrology analysis, proximate analysis, mercury intrusion, low-temperature liquid nitrogen adsorption, coal permeability analysis, and methane adsorption/desorption. The results show that the organic compositions are dominated by vitrinites, followed by inertinites, and minor liptinites. The CBM contents are range between 0.83 and 11.52 m3/t, with an average of 4.72 m3/t. The moisture contents are range from 0.76 to 2.54%, with an average of 1.21%, and the ash yields are range from 11.46 to 24.5%, with an average of 17.07%. Low-temperature nitrogen gas adsorption indicates the micropores provide most of the pore specific surface and the pore volume is mainly determined by small pores and mesopores. The geologic structures, coal-bearing strata, and the coal depositional environments were studied through field geological research and laboratory tests. The coals were deposited in a river-dominated shallow delta environment to form the mudstones or siltstones of the coal roof and floor that were used to seal the CBM. The CBM were mainly generated during stages of the Hercynian–Indosinian and the Early-Middle Yanshanian orogeny. Underground water flowed to the Dashucun Mine and its adjacent mines in the northeast of the Gushan Anticline to form a semi-closed hydrodynamic system that created the CBM emission and preservation. The coal rank increases from the southern to the northern areas. Then, it can be speculated that the CBM formation and preservation conditions in the north are better than in the south. The best prospective targets for CBM are in the northern areas. The results of this study may be useful for CBM exploration in North China.