Pore System Diversity and Control Factors of High-Overmature Shale-Taking the Lujiaping and Longmaxi Formation Shales as an Example

Pore structure plays an important role in the preservation and migration of shale gas. The pore structure of shale samples from the lower Palaeozoic Lujiaping and Longmaxi Formations in northeastern Chongqing was characterized by image analysis and fluid injection, and the porosity, homogeneity, and roughness were studied by using various fractal methods. The results indicate that compared with the shale facies of the Longmaxi Formation, the Lujiaping Formation has more developed organic matter pores, smaller macropore (>50 nm) volumes, smaller macropore specific surface areas, smaller mesopore (2-50 nm) volumes, larger mesopore specific surface areas, larger micropore (<2 nm) volumes, and micropore specific surface areas. The shale samples of the Lujiaping Formation are in the stage of micrographitization, lack a rigid skeleton, and are obviously influenced by other external factors such as compaction; there is no obvious correlation between pore structure and R-O, TOC (total organic carbon), and clay and quartz of the Lujiaping Formation. However, the maturity of the shale samples of the Longmaxi Formation is in the high-maturity stage, which has not yet reached the organic carbonization stage or has a low carbonization degree and is less affected by compaction. The mesopore and micropore volumes and specific surface area have obvious negative correlations with R-O, TOC, and quartz minerals and positive correlations with clay minerals. In addition, compared with the Longmaxi Formation shale, the Lujiaping Formation shale has larger fractal dimensions of macropores, mesopores, and micropores, which indicates that it has stronger heterogeneity, stronger pore roughness, and more complex morphology. At the same time, the fractal dimension of mesopores and micropores has a negative correlation with R-O, TOC, and quartz. The main reason for the poor prospects of shale exploitation in the Lujiaping Formation may be pore shrinkage in the micrographitization stage.