滇黔北地区筇竹寺组高演化页岩气储层微观孔隙特征及其控制因素

Microscopic pore structure and its controlling factors of overmature shale in the Lower Cambrian Qiongzhusi Fm,northern Yunnan and Guizhou provinces of China

页岩气钻探资料表明,滇黔北地区下寒武统筇竹寺组页岩储层富气状况明显不如下志留统龙马溪组页岩,由此严重影响其勘探部署决策,查明其原因是当务之急。为此,以页岩气钻井岩心为基础,采用环境扫描电镜、原子力显微镜、比表面积测量、低温液氮吸附等试验手段,分析了筇竹寺组页岩储层的微观孔隙类型、结构特征等。结果表明:①筇竹寺组页岩储层呈现出极为发育的以纳米级为主的微观孔隙结构特征,发育黏土矿物层间孔、有机质孔、晶间孔、矿物铸模孔、次生溶蚀孔等多类型的基质孔隙,具有比表面积小和面孔率大的特点;②TOC、干酪根类型、黏土矿物和Ro是控制筇竹寺组微观孔隙结构的主要因素,以Ro的影响最为明显,且在页岩达到过成熟状态后,其比表面积和孔体积随着Ro的增大而急剧减小。结论认为:已处于过成熟中后期的该区筇竹寺组页岩层,长期的地质作用过程和过高热演化程度严重制约了其微观孔隙发育,呈现微孔隙骤减和比表面积、孔体积明显较小的情形,不利于页岩气的吸附储集,由此导致该区筇竹寺组页岩气富集程度不如龙马溪组的结果。

Drilling data reveal that shale gas potential of the Lower Cambrian Qiongzhusi Fm is obviously poorer than that of the Lower Jurassic I.ongmaxi Fm in northern Yunnan and Guizhou provinces of China, the reason of which will be urgently discovered to make further exploration decision. Therefore, various testing methods such as environmental scanning electronic microscope （ES EM）, atomic force microscope （AFM）, the pore specific surface area measurement, and isothermal adsorption/desorption experi- ments were used to analyze microscopic pore types and pore structures of the shale cores. The following results were obtained. First, nanopores are well developed in the Qiongzhusi shale where the types of matrix pores include interclay mineral pores, organic pores, inter-crystalline pores, mineral moldic pores, and secondary dissolution pores, featured by a small specific surface area and large sur- face porosity. Second, the main factors controlling the microscopic pore structure there are organic content （TOC）, kerogen type, types and content of clay minerals, and thermal maturity （R,,）, among which thermal maturity is the most significant. At the over mature stage, the specific sur{ace area and pore volume decrease sharply with the increase of thermal maturity. In conclusion, the reason for the revelation from the drilling data in the study area is that after a long period of geological process and rather high ther mal evolution, microscopic pore structure characteristics such as a sharp decrease of micropores, the significantly reduced pore vol ume and pore specific surface area, etc. are unfavorable for gas adsorption and accumulation in the overmature Qiongzhusi shale.