Research on the pore structure of Gufeng Formation shale in the Lower Yangtze area based on fractal theory

The complex pore structure system is the main controlling factor restricting the accumulation of shale gas, but there is less research on the pore structure of Gufeng Formation shale. In order to clarify the pore structure characteristics of the Gufeng Formation shale reservoir in the Lower Yangtze Region, we applied various experimental methods such as scanning electron microscopy, high-pressure mercury injection and nitrogen adsorption, to reveal the multi-stage fractal characteristics of the Gufeng Formation shale pores in combination with the pore fractal theory. This article establishes a classification scheme for pore size of Gufeng Formation shale, which consists of five categories with description of various development features: micropores (＜2 nm), micropores (2～20 nm), mesopores (20～50 nm), macropores (50～5 000 nm), and macropores (＞5000 nm). The results showcase diverse reservoir space of Gufeng Formation shale, including organic pores, mineral dissolution pores and micro-fractures. The distribution of the pore size in the shale shows that the Gufeng Formation are dominated by small pores (2～20 nm), whose volume fraction is 50.3% on average. In different types of pores, with the development of micropores below 20nm, the shale pore system became more complex and the adsorbed gas capacity strengthened, which is more conducive to enriching the shale gas and integrating reservoirs in the Lower Yangtze complex structure area. Therefore, the small pores-dominated Gufeng Formation shale has a good shale gas exploration prospect.