1. Key Laboratory of Coalbed Methane Resource & Reservoir Formation Process (China University of Mining & Technology), Ministry of Education, Xuzhou 221008, China
2. Jiangsu Design Institute of Geology for Mineral Resources, Xuzhou 221006, China
3. Department of Earth Sciences, University of Yaoundé I, P.O. Box 3412, Messa, Yaounde, Cameroon