Quartz dissolution and its mechanisms in the Pinghu Formation, Baowu area, Xihu Sag, East China Sea Shelf Basin

This study aims to determine the diagenetic environments, characteristics, and mechanisms of quartz dissolution in the Pinghu Formation within the Baowu area, Pinghu slope zone, Xihu Sag, East China Sea Shelf Basin (ECSSB)and to assess the impact of quartz dissolution on reservoirs. By integrating data from rock thin-section observation, scanning electron microscopy (SEM), X-ray diffraction analysis, and heavy mineral analysis, along with the geochemical parameters of source rocks, we explore the quartz dissolution-related diagenetic environments and their evolutionary patterns. The research results indicate the occurrence of acidic and alkaline quartz dissolution with significantly different mechanisms in the study area. In the acidic digenetic environment, quartz dissolves through complexation with organic acids in the presence of alkali metal ions, while in the alkaline digenetic environment, quartz reacts directly with OH^-to form HSiO₃^-. The evolution of diagenetic environments affects quartz dissolution, which can be classified into two types based on the characteristics of dissolution boundaries: metasomatic and porosity-enhancing types. The metasomatic dissolution by kaolinite occurs through organic acid dissolution in the presence of alkali metal ions, whereas the metasomatic dissolution by illite, chlorite, and carbonate minerals is driven by alkaline dissolution. The porosityenhancing dissolution occurs in both acidic and alkaline environments. The degree of quartz dissolution varies significantly with the diagenetic environment, decreasing in the order of alkaline, acidic, acidic-alkaline transitional, and weakly acidic environments. Furthermore, this degree is positively correlated with the number of kinking crystal planes, temperature, and alkali metal ion content. The porosity-enhancing quartz dissolution includes kinking plane dissolution, flat plane dissolution (raindrop imprint-like), and flat plane dissolution (honeycomb-like), leading to the partial missing of quartz grains. The metasomatic quartz dissolution involves kaolinite, carbonate minerals, illite, and chlorite. Additionally, grain-edge dissolution, secondary overgrowth rim dissolution, and authigenic quartz dissolution improve pore structures and increase reservoir permeability. Notably, the metasomatic dissolution of quartz grains by clay minerals leads to the formation of intergranular and intercrystalline dissolution pores, significantly increasing the reservoir porosity.