Quartz types,genesis and their geological significance within the Wufeng-Longmaxi Formation in north-western Hunan,China

Quartz is an important mineral component in the Late Ordovician–Early Silurian Wufeng-Longmaxi Formation with various forms and sources and has a significant impact on the properties of shale gas reservoirs.In this study,geochemical analysis,scanning electron microscopy(SEM)observation,and rock mechanics testing were performed on shale samples from the Wufeng-Longmaxi Formation in north-western Hunan,South China.Quartz is classified into four types based on morphological features and cathodoluminescence(CL)images under SEM–terrigenous detrital quartz,quartz overgrowths,biogenic skeletal quartz and microquartz.The quartz in Upper Longmaxi Formation is predominantly of terrigenous origin and contains a small amount of quartz formed by clay transformation.The quartz in the Wufeng-Lower Longmaxi Formation is predominantly biogenic.The biogenic quartz has a direct effect on organic matter(OM)abundance,pore structure and brittleness.It is indicated by the positive correlation with TOC content and biogenic Ba content that biogenic quartz-rich strata have high paleoproductivity.The rigid frameworks formed by biogenic quartz during the early diagenesis stage facilitated the preservation of the primary pores.The interparticle pores of biogenic quartz are the space for OM preservation and migration,creating conditions for the development of OM pores.Additionally,the calculated brittleness index(BI)shows a positive correlation with biogenic quartz content,indicating that layers rich in biogenic quartz are more conducive to fracture.Therefore,the Wufeng-Lower Longmaxi Formation has higher OM content,porosity and represents a more favorable exploration and development target.