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Petrographic analysis combined with thin section observation, fluid inclusions, burial history analysis, and core porosity and permeability, were used to determine diagenetic evolution and its effect on reservoir-quality of beach-bar sandstone reservoirs of Eocene Es4s interval in the Boxing sag, Bohai Bay Basin. Diagenesis is mainly characterized by strong compaction, undercompaction, cementation of carbonate, quartz, pyrite and clay minerals, dissolution of detrital grains and carbonate cements, lime-mud matrix recrystallization and hydrocarbon filling. Paragenetic sequence is determined as 'early compaction / precipitation of calcite, dolomite and framboidal pyrite → dissolution of feldspar grains and early-formed carbonate cements/precipitation of quartz → hydrocarbon filling → precipitation of ferroan calcite and ankerite/lime-mud matrix recrystallization → dissolution of ferroan carbonate cements'. In the same diagenetic environment, diagenetic processes for different lithofacies significantly varied and subsequently resulted in great heterogeneity for reservoir quality during progressive burial. Lithofacies for both bar body and beach edge subfacies which are far from sandstone-mudstone contacts, are well sorted and have a strong ability to resist compaction. Abundant primary and secondary porosities are well developed because of modifications by acidic dissolution and hydrocarbon filling. The present physical properties are relatively well and numerous oil beds are developed through shallow to deep burial depth. Lithofacies for both bar body and beach edge subfacies which are adjacent to sandstone-mudstone contacts, are typically characterized by extensive cementation of calcite and dolomite. The present physical properties are bad and dry beds are extensively developed for sandstone reservoirs in both bar body and beach edge subfacies which are adjacent to sandstone-mudstone contacts. Lithofacies for both bar edge and beach mat subfacies were deposited in weak hydrodynamic conditions with high contents of limemud matrix. The reservoir porosity decreases continually due to strong compaction and weak dissolution. The present physical properties are extremely poor and dry beds are commonly developed through shallow to deep burial except for minor oil beds developed in the shallow burial. [ABSTRACT FROM AUTHOR]