Abstract: Analyses based on data of casting thin sections,SEM,porosity and permeability analysis,mercury injection test,vitrinite reflectance and acoustic emission test reveal that lithic sandstone with fine to medium grain sizes dominate the sandstone reservoirs in the Lower-Middle Jurassic in Amu Darya Basin.Their formation was controlled by various genesis processes including compaction,cementation,dissolution and fracturing,among which,the most predominant is the multi-stage cementation of carbonate,siliceous and clay minerals that filled up primary pores first and later secondary pores.Secondary pores formed by the dissolution of the unstable components such as feldspar,debris and calcite,contributed the most to the formation of the reservoirs.Reservoir space was composed of remanent intergranular pores,intergranular and intragranular dissolved pores,intercrystalline micropores and fractures,causing ultra-low porosity and permeability in reservoirs.The development of the reservoir was interfered by many factors: sedimentary microfacies determined the location of the reservoirs;sustained steady tectonic subsidence controlled the early and middle stages of diagenesis;compaction and cementation caused tight formations;pore fluid changes facilitated dissolution of unstable components and cements and formed secondary pores,and the precipitation of later kaolinite and illite;and facturing activities improved the permeability of the reservoirs.However,development of the reservoirs was confined to some extent.The reservoir may be grouped into three classes based on physical parameters and pore structures.Among them,classes I and II are poor in quality,assuming high development risk.

Key words: diagenesis, sandstone reservoir, Lower-Middle Jurassic, Amu Darya Basin