Abstract: The sedimentary and diagenetic evolution of the deep tight gas reservoir in the East China Sea Basin is complex.The diagenetic evolution of different lithofacies reservoirs is key to revealing the sedimentary-diagenetic coupling, to understand the genetic mechanism of "sweet spot" in tight reservoirs, and also to lay a sound foundation for "sweet spot" prediction.A case study of the typical tight gas reservoir in the Huagang Formation located in the deep central structural inversion belt of the Xihu Depression, East China Sea Basin, reveals the diagenetic evolution of various lithofacies reservoirs through core observation, SEM, casting thin section, granularity and X-ray diffraction analyses.The control of deposition on diagenesis is revealed by our diagenetic evolution models for different lithofacies reservoirs, which are based on petrological characterization, classification of main lithofacies types, and analysis of diagenetic evolution sequence and burial-thermal evolution history.The results show that the sediments in underwater distributary channels constituting the main body of the Huagang Formation reservoirs in the braided-river delta front, have mainly developed four lithofacies types, namely massive medium-to-coarse grained sandstones, gravel sandstones, medium-to-fine grained sandstones with parallel bedding and muddy gravel sandstones.The massive medium-to-fine grained and gravel sandstones is high in rigid quartz content, but low in clay content, with dominant thin chlorite and siliceous cements.However, the best reservoir quality is observed in massive medium-to-coarse grained sandstones due to their relatively complete diagenetic process and growth of primary and secondary solution pores at the later buried stage.In contrast, the gravel sandstones are prone to generate tight reservoirs characterized by greater compaction and intense siliceous cementation due to their poor sorting.In addition, the medium-to-fine grained sandstones with parallel beddings, mainly associated with thick chlorite, I/S mixed layer and illite, feature low content of rigid quartz but high clay content.Clastic particles tend to arrange directio-nally in lamina due to sedimentary differentiation:the fine-grained laminae favor stronger compaction at later stage of diagenesis that helps the preservation of local primary porosity in the coarse-grained part, generating reservoir pockets.Muddy gravel sandstones contain lots of pelinite clasts as well as argillaceous matrix, and they have undergone stronger compaction at the eogenetic stage, resulting in destruction of porosity and reservoir quality.

Key words: sweet spot prediction, lithofacies, diagenesis, diagenetic evolution, tight reservoir, tight sand gas reservoir, Huagang Formation, Xihu sag, East China Sea Basin