Pore Structure Characteristics of Marine-Continental Transitional Shale: A Case Study in the Qinshui Basin, China

Organic shales deposited in a marine-continental transitional environment are well developed in the Qinshui Basin, northern China. However, previous research concerning shales has predominantly focused on marine shales and barely on marine-continental transitional shales. In this study, geochemical and mineralogical analyses were performed on 23 marine-continental transitional shale samples obtained from four wells in a currently active shale gas play, Yushe-Wuxiang area in the Qinshui Basin. Furthermore, the complex pore structure of the transitional shales was well characterized by scanning electron microscopy (SEM), mercury intrusion, and low pressure gas physisorption. The results showed the abundance of organic matter (OM) with an average of 2.03% in the target shales. The dominant minerals in the shale were found to be clay and quartz, and the major clay minerals type is kaolinite and Illite/smectite. SEM images clearly exhibited that the pores in the shale matrix are mainly associated with clay minerals and OM. Results of mercury intrusion and low pressure gas physisorption indicated the presence of pore volumes (PVs) and specific surface area (SSA) with different scales. The pore size distribution analysis indicated that mesopores were dominant in the shale from the study area. The PV was mainly attributed to the presence of mesopores and macropores, and the SSA was mainly associated with the mesopores and micropores. Results of research on factors controlling pore structure development showed that it was principally controlled by clay mineral contents and total organic carbon content. The pore structure characteristics of the marine-continental transitional shale may have contributed to the preservation of shale gas in Yushe-Wuxiang area. This study provides important significance in gaining a comprehensive understanding of the transitional shale pore structure and the shale gas storage-seepage mechanism.