Shale pore structure characteristics and shale gas occurrence pattern of the Lower Cambrian Qiongzhusi Formation in the southern Sichuan Basin
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.
万方数据
维普
