Shale pores serve as the primary reservoir space for shale gas, whose structural characteristics directly determine the gas occurrence state, enrichment degree, and flow mechanisms. However, the complex structure and strong heterogeneity of organic pores in shale gas reservoirs significantly constrain precise reservoir evaluation and dynamic development. To clarify the three-dimensional structural characteristics of organic pores in the Lower Paleozoic shale reservoirs in South China, this study focuses on two organic-rich shale successions in the northern Guizhou: The Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation shales, which exhibit significantly different thermal maturities. An integrated approach was employed, combining organic matter extraction, low-temperature nitrogen adsorption, and focused ion beam-scanning electron microscopy (FIB-SEM) three-dimensional reconstruction techniques to systematically characterize the microstructure of organic pores in these two shale successions. Based on nitrogen adsorption and FIB data, the Frenkel-Halsey-Hill (FHH) and box-counting models were respectively applied to evaluate the complexity of organic matter pore structures across different scales. The results show that the moderately mature Longmaxi Formation shale (equivalent vitrinite reflectance Ro=2.1%~2.8%) contains well-developed organic pores, predominantly exhibiting bubble-like and sponge-like cluster morphologies with pore sizes (r) mainly ranging from 200 nm to 450 nm, along with high specific surface area (133.9~159.5 m2/g) and substantial pore volume. In contrast, the overmature Niutitang Formation shale (Ro=3.0%~3.8%) contains smaller organic pores (r=10~140 nm) with irregular or slit-shaped geometries, showing lower specific surface area (30.9~31.4 m2/g) and reduced pore volume. Three-dimensional pore network modeling further reveals distinct connectivity patterns between these two shale successions. In the Longmaxi Formation shale, organic pores are primarily isolated with poor connectivity, and large pores (r>140 nm) contribute approximately 70% of the total pore volume. The Niutitang Formation shale, however, shows enhanced connectivity among large pores (r>150 nm) through thermal-induced microfractures formed during organic matter condensation, while small pores (r<150 nm) remain largely isolated yet account for 64% of the total pore volume. Fractal dimension analysis highlights additional structural differences. The Niutitang Formation shale exhibits higher fractal dimensions for large organic matter pores (D2=2.37~2.78), indicating greater structural complexity, whereas the organic pores of the Longmaxi Formation shale display relatively regular geometries with lower fractal dimensions. These variations are mainly controlled by differences in thermal maturity. Our study provides systematic understanding of three-dimensional pore structure evolution in shales with different thermal maturities, and offers theoretical foundations for shale gas reservoir evaluation and development strategies in northern Guizhou.