焦石坝地区五峰组-龙马溪组页岩孔隙结构特征及其主控因素

Characteristics and the Main Controlling Factors of Micro-pore Structure of the Shale in Wufeng Formation-Longmaxi Formation in Jiaoshiba Area

孔隙结构是评价页岩储气能力、渗流能力以及是否具有商业开采价值的关键。以重庆焦石坝JY1井五峰组-龙马溪组底部富有机质页岩为研究对象,选取了15块页岩样品开展了有机碳含量、X射线衍射、压汞、氦气孔隙度测定,低温氮吸附、氩离子抛光电镜观察,并结合沥青反射率测定、天然气碳同位素等资料表征页岩孔隙体积、大小和分布特征;从外部和内部两方面探讨了孔隙结构的主要控制因素。研究表明:1JY1井五峰组-龙马溪组页岩孔隙类型主要为无机孔(黏土矿物晶间孔、粒间孔和粒内孔)、有机质孔和微裂缝;2压汞和吸附实验显示页岩孔隙结构相对较复杂,以孔径小于50nm的孔隙为主,微孔提供了大部分的比表面积(约占65%),中孔提供了大部分的比孔容(约占57%),且以四面开放的平行板状孔隙为主,兼有多种其他形态的孔隙;3自白垩纪以来的多次挤压抬升剥蚀过程中,构造应力或温压的变化可能形成了大量微裂缝,沉积环境的差异制约了富有机质页岩发育的厚度、分布以及有机碳的富集程度;4相关性分析表明,微孔、中孔的比表面积和比孔容与有机碳质量分数关系密切,其中中孔体积和微孔的比表面积表现最明显;黏土矿物和石英质量分数对孔隙结构的影响呈现此消彼长的效果;当Ro<3.0%时,微孔和中孔的比表面积、比孔容与Ro值呈弱的负相关,反之呈现增大趋势,这与过高热演化阶段,生气速率变慢反而制约了纳米级孔隙发育,导致微孔数量减少有关。

Micro-porestructure is the key element influencing the storage capacity, permeabxhty and aeter- mining whether the shale is of commercial value. Taking 15 organic-rich shale samples from the Wufeng Formation-Longmaxi Formation of JY1 well as a target, this research studies the shale＇s pore volume, size and distribution by using total organic carbon, X-ray diffraction, high pressure mercury injection, helium porosity, low temperature N2 adsorption and desorption technology and argon-ion polishing scanning elec- tron microscope （SEM） combined with the the bitumen reflectance and natural gas carbon isotope date. Then this research also discusses the main controlling factors of the pore structure from both external and internal aspects. The research indicates that the pore types in organic-rich shale in the formation are mainly inorganic pores （intergranular, intragranular and inside pores）, organic pores and microfractures. The re- suits of mercury injection and adsorption experiments show that the shale pore structure is relatively com- plex as pores sized less than 50 nm are predominant and micropores occupy approximately 65~ of the total surface area is provided by micropores, while about 57% of the specific pore volume is provided by mes- pores, dominated by parallel plate with all sides open exixting simultaneously with a variety of other forms of pores in the shale. During the process of extrusion, uplifting and erosion since Cretaceous, the change of tectonic stress or temperature and pressure may contribute to the formation of a large number of cracks. The thickness, distribution and the enrichment of organic cabon of the shale may he restrained by the difference of the sedimentary environment. Correlation analysis shows that specific surface area of micro- pores, mespores and volumes have strong relationship with total organic carbon content. The influence of clay minerals and quartz content on pore structure has reciprocal effect. When Ro is less than 3.0%,spe- cific surface area and volume present positive relationship, however, when it exceeds 3.0% the negative relationship is observed. It can be explained that the development of the nanoscale pore is restricted by thermal maturation, which results in the reduction of mieropores.