页岩的孔隙作为页岩气的主要储集空间,其结构特征直接决定了气体的赋存状态、富集程度及渗流机制.然而页岩气储层中有机孔的结构十分复杂,非均质性强裂,制约了储层的精细评价和动态开发.为明确我国南方下古生界页岩储层有机孔的三维结构特征,本文以黔北地区寒武系牛蹄塘组和志留系龙马溪组两套富有机质页岩为研究对象,针对这两套热成熟度差异显著的页岩层系,本研究综合运用有机质提取、低温氮气吸附以及聚焦离子束扫描电镜(FIB-SEM)三维重构技术,系统表征了两套页岩有机孔的微观结构特征,并且基于氮气吸附和FIB数据,分别利用FHH和盒子计数模型评估了不同尺度下有机孔的孔隙结构复杂程度.研究结果表明,中等热成熟度的龙马溪组页岩(等效镜质体反射率Ro=2.1%～2.8%)有机孔十分发育,孔隙形态以气泡状、海绵状团簇为主,孔径主要介于200～450 nm,具有较高的比表面积(133.9～159.5 m2/g)和孔体积;与之相比,过成熟的牛蹄塘组页岩(Ro=3.0%～3.8%)有机孔的孔径较小(r=10～140 nm),其形态多呈不规则状或狭缝状,比表面积(30.9～31.4 m2/g)和孔体积较低.三维孔隙网络建模显示,在龙马溪组页岩中,有机孔孤立分布,连通性差,大孔(r>140 nm)贡献了总孔隙体积的 70%.牛蹄塘组页岩由于热演化程度较高,有机质发生缩聚形成微裂缝,提高了大孔(r>150 nm)之间的连通性;而小孔呈孤立分布,孔隙之间的连通性差,小孔贡献了总孔隙体积的64%.另外,两套页岩的有机孔分形表征显示,牛蹄塘组页岩的大孔具有更高的分形维数(D2=2.37～2.78),表明其孔隙结构更为复杂;而龙马溪组页岩有机孔的形态相对规则,这主要受有机质热演化程度控制.该研究不仅为不同热成熟度页岩的三维孔隙结构演化机制提供了系统认识,还为黔北地区页岩气储层评价和渗流开发提供了理论依据.

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.