Macroscopic Characterization and Microscopic Pore Structure of Permian Shale Reservoirs in Hunan–Guizhou–Guangxi Basin: Insights from NMRC, Fractal and Image-J Methods

Permian shale is the largest and most promising shale gas exploration target in southern China after Silurian shale. The fine evaluation of shale reservoirs is a prerequisite for large-scale exploration and development. Based on the fractal method, this study, through the combined technology of nuclear magnetic resonance cryoporometry (NMRC) and Image recognition software (Image-J), clarifies the pore size distribution of Permian shale in the HGG Basin. The purpose of this study is to characterize the macroscopic parameters of Permian shale and reveal the level of reservoir space development in Permian shale. The controlling factors of porosity and pore structure are demonstrated. It is suggested that Permian shales in the HGG Basin have organic carbon contents similar to marine shales. In the favorable interval of the Dalong Formation, the average organic carbon content is comparable to that of the LMX pay zone. The lower Longtan shales have the highest organic carbon and the greatest gas generation potential, followed by the Dalong shales. TOC is the primary control on porosity in the lower Longtan and Dalong formations, whereas clay minerals dominate the control in the upper Longtan. Abundant pores between grains and between layers within clay minerals account for most of the porosity in Upper Longtan shale. In the lower Longtan and Dalong formations, organic pores are pervasive, explaining the difference in the dominant controls on porosity between these intervals. Clay minerals are a key control on the development of Permian shale reservoirs. The fractal dimension of adsorption pores (DA) has no clear relationship with the total clay content, is negatively correlated with the illite content, and shows no clear relationship with the chlorite content. In contrast, the fractal dimension of flow pores (DS) shows a weak positive correlation with the total clay content, a clear positive correlation with the illite content, and a negative correlation with the chlorite content. When illite interacts with water, it tends to break down and plug pores, an effect that is especially pronounced in the smallest pores hosted by organic matter; this accounts for the negative correlation between DA and the illite content. In larger, flow-bearing pores, disintegrated illite roughens otherwise smooth walls between and within grains, increasing structural complexity and raising DS. By contrast, reactions between chlorite and pore fluids tend to smooth the walls of flow pores, reducing structural complexity and lowering DS.