Abstract: The deeply buried sandstone in the Xihu Sag of the East China Sea Basin bears great possibility to form the low-permeability and high density gas reservoir according to their geological conditions. The large-area of deep-buried coal-measure source rocks there continues to generate and provide gas as a solid material basis for gas accumulation. The thick clean sandstones of transitional-facies formed in shallow-water are widely developed as reservoirs, which keep a sandwish-contact with source and cap rocks and remain effective to provide space for gas migration and preservation. In addition, the abnormal high pressure, which developed commonly in the deep strata, creates favorable conditions to help the accumulation process mentioned above. To conclude, sufficient hydrocarbon source, continuously superimposed thick sandstones and abnormal pressure are the key elements for gas accumulation in the deep low-permeability and high density sandstone in the Xihu Sag.