Comparison of pore structures between Qiongzhusi Formation and Wufeng-Longmaxi Formation in Sichuan Basin
Shales in the Qiongzhusi Formation and Wufeng-Longmaxi Formation in the Sichuan Basin and surrounding areas are a major target for the exploration of marine shale gas in China,but their analyses have yielded significantly varying results thus far.To comprehensively investigate the characteristics of the pore structure and differential mechanisms of these two marine shale reservoirs,this study combines qualitative and quantitative methods(including field-emission scanning electron microscopy,low-pressure CO2/N2 adsorption,and multiple analyses)to accurately characterize and compare the pore structures of shales in the Qiongzhusi Formation and Wufeng-Longmaxi Formation,with the aim of revealing the differences in their mechanisms of formation.The results revealed significant variations in pore structure between shales of the Qiongzhusi Formation and Wufeng-Longmaxi Formation,including in the development,style,size distribution,and connectivity of micro-and meso-macro-pores as well as their heterogeneity.Shale in the Wufeng-Longmaxi Formation contained pores with a higher organic matter(OM)content,higher porosity,lower connectivity,greater heterogeneity,higher ratio of micropores,and a larger surface specific area and pore volume than the Qiongzhusi Formation shale.The differences in their pore structures can be attributed to the following distinct genetic mechanisms of the two shale reservoirs:(1)variations in the depositional environment,leading to differences in mineral composition that influenced primary porosity preservation;and(2)key diagenetic processes,including mechanical compaction,thermal maturation of OM,sealing conditions during thermal maturation,and intense burial-induced compaction,that led to disparities in the pore structure.The presence of biogenetic quartz,moderate thermal maturity of OM,sealed shale gas systems,shallow depth of burial,and relatively stable tectonic conditions were found to be conducive to the formation and preservation of residual inorganic and OM pores in the shales.
pore structuredifferences in genetic mechanismWufeng-Longmaxi FormationQiongzhusi FormationSichuan Basin and surrounding areas