黔西北龙马溪组页岩微观孔隙结构及储气特征

为了深入研究黔西北地区海相页岩储层微观孔隙特征,选取下志留统龙马溪组页岩进行了氩离子抛光-场发射扫描电子显微镜、氮气吸/脱附实验、微孔特征分析以及相关地球化学分析等实验,探讨了微观孔隙对页岩储气特征的影响。实验结果表明:黔西北龙马溪组页岩存在粒间孔、粒内孔、晶间孔、溶蚀孔、化石孔、有机孔、微裂缝7种孔隙类型,其中粒间孔和有机孔最为发育;孔隙结构类型可根据氮气吸/脱附曲线分为3种,即以两端开口的圆筒孔、狭窄的平行板孔和四面开口的锥形平板孔等开放型孔隙为主;微观孔隙孔径分布范围为2~64 nm,峰值集中于2~6 nm。微孔(小于2 nm)孔径分布范围为0.4~1.8 nm,0.4~1.0 nm的微孔对微孔孔容贡献最大。孔容中以中孔(2~50 nm)为主,所占比例为83.1%,微孔和中孔贡献了主要的孔比表面积,微、中孔所占比例分别为20.1% 和79.3% 。有机碳含量是纳米孔隙发育的主控因素。不同孔隙类型具有不同的气体赋存特点和运移特征,说明纳米孔隙为页岩中气体的赋存和微观运移提供了有利条件。

Characteristics of microscopic pore and gas accumulation on shale in Longmaxi Formation,northwest Guizhou

To deeply study micro-pore characteristics of shale reservoirs in Longmaxi Formation, northwest Guizhou, the shale in Longmaxi Formation was selected for experiments including the Argon Ion milling-Field Emission Scanning Electron Microscopy, nitrogen sorption-desorption experiment, micro-pore characteristic analysis and related geochemical experiments. Meanwhile, the influences of micro-pores on shale gas accumulation characteristics were discussed. The experimental results show that there are seven kinds of shale micro-pore types in Longmaxi Formation, northwest Guizhou, i.e., inter-granular pore, intra-granular pore, intra-crystalline pore, dissolved pore, fossiliferous pore, organic pore and micro-cracks, of which inter-granular pore and organic pore are most developed. The pore structures can be divided into three types according to nitrogen sorption-desorption curves, i.e., cylindrical pore with both ends open, narrow parallel-plate pore and conical plate pore with four sides open. The micro-pore diameters are generally ranged in 2-64 nm, and the peak is distributed in 2-6 nm. The diameters of micro-pore (<2 nm) are ranged in 0.4-1.8 nm, which the 0.4-1.0 nm of micropore diameters dominate the mostly micropore specific surface. The pore volume is mostly occupied by meso-pores (2-50 nm) with the proportion of 83.1%. The micro-pores and meso-pores make a major contribution to pore specific surface area with the proportion of 20.1% and 79.3% respectively. The content of organic carbon is the main controlling factor of nano-pore development. Different pore types have different characteristics of gas occurrence and migration, proving that nano-pore provides favorable conditions for the occurrence and micro-migration of shale gas.