1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, P. R. China
2. Unconventional Oil and Gas Science and Technology Research Institute, China University of Petroleum, Beijing 102249, P. R. China
3. Shale Gas Research Institute, PetroChina Southwest Oil & Gas Field Company, Chengdu 610056, P. R. China
4. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada