Microscopic pore structure characteristics of tight limestone reservoirs: New insights from Section 1 of the Permian Maokou Formation, Southeastern Sichuan Basin, China

Recently, industrial gas flows have been produced from the vertical well acidification tests of Wells JS1 and YH1 in Section 1 of the Maokou Formation in the southern region of eastern Sichuan Province, China. This result proves that Section 1 contains a group of excellent source rocks and gas-bearing reservoirs. In this study, the pore structure characteristics and primary causes of the nanopore structure were explored using scanning electron microscopy, X-ray diffraction of total rocks, total organic carbon, liquid nitrogen adsorption, and other experimental methods to determine the pore development features of nodular limestone reservoirs in Section 1. The results substantiate the presence of complex pore structures in the nodular limestone reservoirs in Section 1. Nanopores are dominant and mainly comprise micropores and mesopores approximately 4 nm in diameter. The pore forms are particularly schistose clay silt pores consisting of rigid particles and flask-shaped pores. N2 adsorption analysis showed that the specific surface area of the carbonate reservoir was 4.8842–8.1594 m2/g (mean = 6.0439 m2/g), and the pore volume was 0.008422–0.015098 cm3/g (mean = 0.043212 cm3/g). Mesopores were positively related to the total pore volume and total specific surface area, whereas macropore content had poorer correlations. Mesopores contribute more to the total pore volume and total specific surface area than macropores do. Spaces among the clay mineral layers (especially talcum gaps) predominantly contributed to the carbonate pore volume and specific surface area of the reservoir, and were positively related to the clay mineral content. The rocks have highly developed calcite pores, and their pore volumes and specific surface areas are negatively related to calcite pore content. Moreover, low organic content and low pore development degrees were observed. The organic content was weakly correlated with the pore volume and specific surface area, suggesting a low effect on reservoir performance. This assessment of the pore structure characteristics of Section 1 of the Maokou Formation yielded essential geological data that could inform the formulation of exploitation measures.