Analysis of coalbed methane occurrence in Shuicheng Coalfield, southwestern China

This article is the result of authors' long-term efforts to evaluate the occurrence of coalbed methane (CBM) for CBM exploration and coal gas disaster prevention in Shuicheng Coalfield (SCC). A relatively large program of exploratory drilling to determine the content of CBM, geological surveys and laboratory experiments, including coal proximate analysis, coal physical properties analysis and methane adsorption were conducted. The results show that the coals from the SCC contain 21.42-41.34% volatile, 0.24-1.44% moisture and 6.27-38.1% ash yield, with coal porosity in the range of 5.97-14.89%. The type of coal structure ranges from II to IV. The sedimentary characteristics, tectonic evolution, geological structure, coal reservoir conditions and hydrodynamics were studied via geological surveys, field research and laboratory tests. The relationships between these factors and gas content are explained and analyzed in this present paper. The formation of the Shuicheng coal basin was formed under the control of the Ziyun fracture and the Luoping fracture. Ejective folds and synclines that evolved from inverted geological structures in the stage of Yanshan movement affect CBM occurrence. Folds generated in coal bearing strata play an important role in methane accumulation in the period of Himalayan movement. A positive relationship is observed between gas content and coal burial depth, coal thickness, and hydrodynamics. CBM easily concentrates because of the separation between coal bearing strata and Maokou enrich aquifer. We divide the SCC into three methane geological units, the Ertang syncline unit, the Dahebian syncline unit and the Xiaohebian syncline unit. Furthermore, gas content spatial distributions were predicted in the SCC. The probability of CBM exploration was evaluated based on coal permeability and gas flow attenuation coefficient. These research results could contribute to CBM exploration and coal gas disaster prevention in the SCC.