MULTISCALE HETEROGENEITY AND THEIR IMPLICATIONS FOR THE NANOPOROUS SYSTEM OF THE WUFENG-LONGMAXI SHALES IN JIAOSHIBA AREA, SOUTHEAST CHINA: RESPONSE TO DEPOSITIONAL-DIAGENETIC PROCESS

The organic matter-rich shales in Wufeng-Longmaxi Formation are showing a notable heterogeneity characteristic, which significantly controls the mineral composition and organic matter component in the adjacent sections of the shale reservoir. This study combines the multiple scale observation and detailed mineralogical and geochemical analysis to investigate the manifestation, origin, and affection of shale heterogeneity within the equivalent-time stratigraphic framework. The results indicate that polytropic deposition-environment lead to different sediment supplies in the depositional process. After that, these differences among shale sections are magnified through different diagenetic pathways and result in current shale heterogeneity. Section TST is controlled by global glacialism and regional volcanism, leaving an abundance of fine-grained intrabasinal silica and enrichment of organic matter. At diagenesis stage, authigenic quartz aggregates derive from siliceous organisms are spawned. They filled in primary interparticle pores, forming a rigid particle-bracing structure that effectively resisted later compaction and provided enough space for organic matter migrating and occluding. Finally, the production of pores in this migrated organic matter make an essential contribution to the total porosity of shale reservoir. Section EHST is strongly influenced by contour current, which brings about more extrabasinal index and impoverishes organic matter. In diagenesis stage, less siliceous supply could not produce enough authigenic quartz. Therefore, the rigid particle-bracing structure could only preserve in partial place. Primary interparticle pores significantly reduce owing to compaction, leaving less space for later organic matter migrating and occluding. As a result, the total porosity of shale reservoir decline in this section. In a rapid tectonic-uplifting background, section LHST is dominated by a rapid expansion of terrigenous clay minerals and further dilute organic matter. Ductile clay experiences strong compaction during diagenesis and then occupy most of the primary interparticle space. Rigid particles are wrapped by overwhelmed clays, leading non-existent of the particle-bracing structure. As a result, nanoporous system in shale could not be well preserved.