四川盆地南部下寒武统筇竹寺组页岩孔隙结构特征与页岩气赋存模式

Shale pore structure characteristics and shale gas occurrence pattern of the Lower Cambrian Qiongzhusi Formation in the southern Sichuan Basin

近年来,四川盆地南部地区(以下简称川南地区)下寒武统筇竹寺组页岩气勘探开发取得了实质性的突破,但是,关于筇竹寺组页岩气储层孔隙结构特征及其对页岩气赋存模式影响的研究还较为薄弱。为此,以川南地区W207井筇竹寺组页岩为例,以总有机碳含量(TOC)不小于1.0%为界,将筇竹寺组页岩划分出4个高有机碳页岩层段(H1—H4层),并基于大视域扫描电镜、流体注入法孔隙定量表征、三维分子结构建模与分子模拟等方法,研究了不同富有机质层段页岩孔隙特征,并着重分析了H3层页岩孔隙发育特征与页岩气赋存模式。研究结果表明:(1)筇竹寺组页岩纵向上孔隙发育差异较大,其中H3层页岩孔隙系统最为有利,发育粒缘缝—有机质纳米连通孔隙—有机质基质分子内孔隙3级孔隙网络,其矿物粒缘缝与有机质纳米孔隙提供了较大的游离气储集空间,页岩有机质分子结构中普遍发育微孔,提供了较大的吸附空间;(2)微孔对总吸附量的贡献随压力增高而降低,但在30.0 MPa时仍贡献了56%以上的总吸附气量;(3)高温高压下(30.0 MPa、70℃),页岩中游离气甲烷含量占总含气量的57%,当压力由30.0 MPa降低至16.5 MPa,游离气贡献总开采气量超过80%;(4)粒缘缝、有机质纳米连通孔隙和有机质基质分子内孔隙良好的配置关系为页岩气富集及开发提供了良好的连通体系,远离风化壳的超压层段有利于页岩孔隙发育。结论认为,川南地区筇竹寺组H3层页岩孔隙系统发育,具备页岩气高产地质条件,是筇竹寺组下一步重点关注层段,在此基础上寻找远离风化壳、孔隙发育的超压页岩储层是筇竹寺组选区选层的关键。

In recent years,substantial breakthroughs have been made in the exploration and development of the Lower Cambrian Qiongzhusi Formation shale gas in the southern Sichuan Basin,but the shale pore structure and shale gas occurrence pattern of the Qiongzhusi Formation reservoirs have been scarcely investigated.In this paper,the Qiongzhusi Formation shales of Well W207 in southern Sichuan Basin,as an example,are divided into four organic-rich intervals(H1-H4),with TOC=1.0% as the boundary.Based on the methods of large-field scanning electron microscope,quantitative pore characterization by fluid injection,three-dimensional molecular structure modeling and molecular simulation,the pore characteristics of shales in the organic-rich intervals are studied,and the shale pore structure and shale gas occurrence pattern of H3 are specifically analyzed.The results show that the Qiongzhusi Formation shales have greatly varying pore development vertically.H3 exhibits the most favorable pore system in shale,with a three-level pore network consisting of mineral grain boundary fractures,organic matter connected nanopores,and organic matter matrix pores.Among them,mineral grain boundary fractures and organic matter connected nanopores constitute a large storage space for free gas,and the widespread micropores in the molecular structure of shale organic matter contribute a huge adsorption space for shale gas.Micropores' contribution to the total adsorption capacity declines with the rising pressure.Such contribution exceeds 56% at 30.0 MPa.Under high temperature and pressure(30.0 MPa,70 ℃),the methane content of free gas in shale accounts for 57%.When the pressure drops from 30.0 MPa to 16.5 MPa,free gas contributes more than 80% of the total gas production.In addition,the good configuration of mineral grain boundary fractures,organic matter connected nanopores,and organic matter matrix pores provides a good connection system for shale gas enrichment and development.The overpressure interval far from the weathering crust is conducive to the development of shale pores.The conclusion is that the H3 interval of the Qiongzhusi Formation has a well-developed shale pore system,geologically supporting a high production of shale gas.H3 will be the focus of subsequent operations.On this basis,finding overpressure shale reservoirs far away from the weathered crust and with well-developed pores is a key task in selecting favorable areas/intervals in the Qiongzhusi Formation.