Pore structure characteristics of Taiyuan Formation coal measuresshale in south central Qinshui Basin

The study of the pore structure and its controlling factors of shale reservoir has important guiding significance for further exploration and exploitation of shale gas. Geochemical and low-temperature gas adsorption experiments were carried out on the transitional coal measures shale of Taiyuan Formation in Qinshui Basin. The geochemical characteristics,mineral macter composition characteristics and pore features were analyzed. On this basis,the controlling factors of pore development were discussed. The results show that the total organic carbon (TOC) content of the transitional shale of Taiyuan Formation ranges from 1.55% to 5.21%,with an average of 3.53%,the type of organic matter is mainly type III,and most of them are in a stage of highly to over mature stage. In addition to a small amount of feldspar and carbonate minerals,the two main components of this shale are 55%～69% of clay minerals and 27%～40% of quartz. Among the clay minerals,the content of kaolinite is the highest (ranging from 23% to 65%,with an average of 48%),followed by an illite mixed layer,illite and chlorite. Mesopores are the major contributor to pore volume (between 35.6% and 53.7%),followed by macropore (between 25.7% and 40.8%). The contribution of micropores to pore volume is the smallest,but the contribution of specific surface area is the largest (between 72.2% and 88.2%),while the contribution of macropores to specific surface area is almost negligible,accounting for only 1.5～2.2% of the total specific surface area. The TOC content has a positive effect on the development of micropores and mesopores,but has little effect on macropores,and thermal evolution is not conducive to the development of pores. Quartz inhibit the development of pores,while the total amount of clay minerals is beneficial to the development of pores,especially to mesopore,followed by micropores and weak effects on macropores. In clay minerals,kaolinite and chlorite are beneficial to the development of shale pores,while the illite mixed layer and Illite inhibit the development of pores in the transitional shale.